Order
Now
Product
Folder
Support &
Community
Tools &
Software
Technical
Documents
ADS1235
SBAS824 – OCTOBER 2018
ADS1235 Precision, 3-Channel, Differential-Input, 7200-SPS, 24-Bit
Delta-Sigma ADC for Bridge Sensors
1 Features
3 Description
•
The ADS1235 is a precision, 7200-SPS, delta-sigma
(ΔΣ) analog-to-digital converter (ADC) with an
integrated programmable gain amplifier (PGA). This
device also includes diagnostic features such as PGA
overrange and reference monitors. The ADC provides
high-accuracy, zero-drift conversion data for highprecision equipment, including weigh scales, strain
gauges, and resistive pressure sensors.
1
•
•
•
•
•
•
•
•
•
•
•
•
•
•
24-Bit, High-Precision ADC
– 120,000 Noise-Free Counts
(10-mV Input, 10 SPS)
– Offset Drift: 1 nV/°C
– Gain Drift: 0.5 ppm/°C
Three Differential or Five Single-Ended Inputs
Two Reference Inputs
Wide Input Voltage Range: ±7 mV to ±5 V
Low-Noise PGA, Gain: 1, 64, and 128
Data Rate: 2.5 SPS to 7200 SPS
AC- or DC-Bridge Excitation Option
Chop Mode for Zero Drift Operation
Simultaneous 50-Hz and 60-Hz Rejection Mode
Single-Cycle Settling Mode
Missing Reference Input Monitor
Signal Overrange Monitor
Temperature Sensor
Cyclic Redundancy Check (CRC)
5-V or ±2.5-V Power Supply
2 Applications
•
•
•
•
The ADC has signal and reference multiplexers that
support three differential signal inputs and two
reference inputs. The ADC also includes a low-noise
PGA that provides gains of 1, 64, and 128. The ADC
also has a 24-bit ΔΣ modulator and programmable
digital filter.
The high-impedance inputs (1 GΩ) of the PGA
reduce measurement error that is caused by sensor
loading.
The ADC supports ac-bridge excitation to remove the
drift errors from the sensor wiring. The ADC provides
the clock control signals for the ac-excitation
operation.
The flexible digital filter is programmable for singlecycle settled conversions, and provides 50-Hz and
60-Hz line cycle rejection at the same time.
The ADS1235 is available in a 5-mm × 5-mm QFN
package, and is specified across the –40°C to
+125°C temperature range.
Weigh Scales and Strain-Gauge Digitizers
Dynamic Weigh Systems
Pressure Measurement
Process Analytics
Device Information(1)
PART NUMBER
PACKAGE
ADS1235
VQFN (32)
BODY SIZE (NOM)
5.0 mm × 5.0 mm
(1) For all available packages, see the package option addendum
at the end of the data sheet.
Block Diagram
3 V ± 5 V (D)
5 V (A)
REFP0
REFN0
ADC Conversion Noise
Ref
Mux
Internal
Oscillator
Clock
Mux
10009.65
10 SPS
V IN = 10 mV
10009.6 V N = 0.084 PV P-P
CLKIN
Input
Mux
AIN1
AIN2
Temp
Sensor
Ref
Monitor
Buf
AIN3
AIN4
AIN5
PGA
GPIO
AC-Exc
24-Bit
û ADC
START
Serial
Interface
and
Control
RESET
PWDN
CS
DIN
Digital
Filter
DOUT/DRDY
SCLK
PGA
Monitor
DRDY
Conversion Data (PV)
ADS1235
AIN0
10009.55
10009.5
10009.45
10009.4
(A)
(D)
10009.35
0
10
20
30
Time (s)
40
50
60
D109
1
An IMPORTANT NOTICE at the end of this data sheet addresses availability, warranty, changes, use in safety-critical applications,
intellectual property matters and other important disclaimers. PRODUCTION DATA.
ADS1235
SBAS824 – OCTOBER 2018
www.ti.com
Table of Contents
1
2
3
4
5
6
Features ..................................................................
Applications ...........................................................
Description .............................................................
Revision History.....................................................
Pin Configuration and Functions .........................
Specifications.........................................................
6.1
6.2
6.3
6.4
6.5
6.6
6.7
6.8
8.5 Programming........................................................... 39
8.6 Register Map........................................................... 48
1
1
1
2
3
4
9
9.1 Application Information............................................ 59
9.2 Typical Application .................................................. 63
9.3 Initialization Setup ................................................... 65
10 Power Supply Recommendations ..................... 66
Absolute Maximum Ratings ...................................... 4
ESD Ratings.............................................................. 4
Recommended Operating Conditions....................... 5
Thermal Information .................................................. 5
Electrical Characteristics........................................... 6
Timing Requirements ................................................ 8
Switching Characteristics .......................................... 9
Typical Characteristics ............................................ 12
7
Parameter Measurement Information ................ 17
8
Detailed Description ............................................ 19
10.1 Power-Supply Decoupling..................................... 66
10.2 Analog Power-Supply Clamp ................................ 66
10.3 Power-Supply Sequencing.................................... 66
11 Layout................................................................... 67
11.1 Layout Guidelines ................................................. 67
11.2 Layout Example .................................................... 67
12 Device and Documentation Support ................. 68
12.1
12.2
12.3
12.4
12.5
12.6
7.1 Noise Performance ................................................. 17
8.1
8.2
8.3
8.4
Overview .................................................................
Functional Block Diagram .......................................
Feature Description.................................................
Device Functional Modes........................................
Application and Implementation ........................ 59
19
20
21
32
Documentation Support ........................................
Receiving Notification of Documentation Updates
Community Resources..........................................
Trademarks ...........................................................
Electrostatic Discharge Caution ............................
Glossary ................................................................
68
68
68
68
68
68
13 Mechanical, Packaging, and Orderable
Information ........................................................... 68
4 Revision History
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
2
DATE
REVISION
NOTES
October 2018
*
Initial release.
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
ADS1235
www.ti.com
SBAS824 – OCTOBER 2018
5 Pin Configuration and Functions
REFP0
REFN0
AIN0
AIN1
AIN2
AIN3
AIN4
AIN5
32
31
30
29
28
27
26
25
RHB Package
VQFN-32
Top View
NC
1
24
NC
CAPP
2
23
NC
CAPN
3
22
NC
AVDD
4
21
NC
AVSS
5
20
NC
NC
6
19
NC
PWDN
7
18
CLKIN
RESET
8
17
DVDD
10
11
12
13
14
15
16
CS
SCLK
DIN
DRDY
DOUT/DRDY
BYPASS
DGND
START
9
Thermal Pad
Not to scale
Pin Functions
PIN
NO.
NAME
TYPE
DESCRIPTION
1
NC
—
2
CAPP
Analog output
No connection; float or connect to AVSS
PGA output P; connect a 4.7-nF C0G dielectric capacitor across CAPP and CAPN
3
CAPN
Analog output
PGA output N; connect a 4.7-nF C0G dielectric capacitor across CAPP and CAPN
4
AVDD
Analog
Positive analog power supply
5
AVSS
Analog
Negative analog power supply
6
NC
—
7
PWDN
Digital input
Power down, active low
8
RESET
Digital input
Reset, active low
9
START
Digital input
Start conversion control, active high
No connection - solder the pin for mechanical support, float or connect to DGND
10
CS
Digital input
Serial interface chip select, active low
11
SCLK
Digital Input
Serial interface shift clock
12
DIN
Digital Input
Serial interface data input
13
DRDY
Digital output
Data ready indicator, active low
14
DOUT/DRDY
Digital output
Dual function serial interface data output and active-low data ready indicator
15
BYPASS
Analog output
Internal subregulator bypass; connect a 1-µF capacitor to DGND
16
DGND
Digital
Digital ground
Digital power supply
17
DVDD
Digital
18
CLKIN
Digital input
NC
—
19-24
1) Internal oscillator: connect to DGND, 2) External clock: connect clock input
No connection - solder the pin for mechanical support, float or connect to DGND
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
3
ADS1235
SBAS824 – OCTOBER 2018
www.ti.com
Pin Functions (continued)
PIN
NO.
TYPE
NAME
DESCRIPTION
25
AIN5
Analog input
Analog input 5
26
AIN4
Analog input
Analog input 4
27
AIN3
Analog input/output
Analog input 3, GPIO3, ACX2
28
AIN2
Analog input/output
Analog input 2, GPIO2, ACX1
29
AIN1
Analog input/output
Analog input 1, GPIO1, ACX2, Reference input 1 negative
30
AIN0
Analog input/output
Analog input 0, GPIO0, ACX1, Reference input 1 positive
31
REFN0
Analog input/output
Reference input 0 negative
32
REFP0
Analog input/output
Reference input 0 positive
—
Thermal Pad
—
Exposed thermal pad - solder the pad for mechanical support; connect to AVSS.
6 Specifications
6.1 Absolute Maximum Ratings (1)
MIN
MAX
–0.3
7
AVSS to DGND
–3
0.3
DVDD to DGND
–0.3
7
AVDD + 0.3
AVDD to AVSS
Power supply voltage
Analog input voltage
AINx, REFP0, REFN0
AVSS – 0.3
Digital input voltage
CS, SCLK, DIN, DOUT/DRDY, DRDY, START, RESET, PWDN, CLKIN
DGND – 0.3 DVDD + 0.3
Input Current
Continuous, all pins except power-supply pins (2)
Temperature
(1)
(2)
–10
Junction, TJ
Storage, Tstg
–60
UNIT
V
V
V
10
mA
150
°C
150
°C
Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. These are stress ratings
only, which do not imply functional operation of the device at these or any other conditions beyond those indicated under Recommended
Operating Conditions. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability.
Input and output pins are diode-clamped to the internal power supplies. Limit the input current to 10 mA in the event the analog input
voltage exceeds AVDD + 0.3 V or AVSS – 0.3 V, or if the digital input voltage exceeds DVDD + 0.3 V or DGND – 0.3 V.
6.2 ESD Ratings
VALUE
V(ESD)
(1)
(2)
4
Electrostatic discharge
Human-body model (HBM), per ANSI/ESDA/JEDEC JS-001 (1)
±2000
Charged-device model (CDM), per JEDEC specification JESD22-C101 (2)
±500
UNIT
V
JEDEC document JEP155 states that 500-V HBM allows safe manufacturing with a standard ESD control process.
JEDEC document JEP157 states that 250-V CDM allows safe manufacturing with a standard ESD control process.
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
ADS1235
www.ti.com
SBAS824 – OCTOBER 2018
6.3 Recommended Operating Conditions
MIN
NOM
MAX
AVDD to AVSS
4.75
5
5.25
AVSS to DGND
–2.6
0
DVDD to DGND
2.7
5.25
UNIT
POWER SUPPLY
Analog power supply
Digital power supply
V
V
ANALOG INPUTS
V(AINx)
Absolute input voltage
VIN
Differential input voltage
PGA mode
See Equation 3
PGA bypassed
AVSS – 0.1
VIN = VAINP – VAINN
V
AVDD + 0.1
±VREF / Gain
See
(1)
V
VOLTAGE REFERENCE INPUTS
VREF
Differential reference voltage
0.9
AVDD – AVSS
V
V(REFNx)
Negative reference voltage
VREF = V(REFPx) – V(REFNx)
AVSS – 0.05
V(REFPx) – 0.9
V
V(REFPx)
Positive reference voltage
V(REFNx) + 0.9
AVDD + 0.05
V
EXTERNAL CLOCK
fCLK
Frequency
1
7.3728
8
MHz
Duty cycle
40%
60%
AVSS
AVDD
V
DGND
DVDD
V
–40
125
°C
GENERAL-PURPOSE INPUTS/OUTPUTS (GPIOs)
Input voltage
DIGITAL INPUTS (other than GPIOs)
Input voltage
TEMPERATURE
TA
(1)
Operating ambient temperature
In PGA mode, the maximum differential input voltage is ±(AVDD – AVSS – 0.6 V) / Gain, when operating with
VREF ≥ AVDD – AVSS – 0.6 V
6.4 Thermal Information
ADS1235
THERMAL METRIC (1)
VQFN (RHB)
UNIT
32 PINS
RθJA
Junction-to-ambient thermal resistance
28.6
°C/W
RθJC(top)
Junction-to-case (top) thermal resistance
17.0
°C/W
RθJB
Junction-to-board thermal resistance
9.7
°C/W
ψJT
Junction-to-top characterization parameter
0.2
°C/W
ψJB
Junction-to-board characterization parameter
9.7
°C/W
RθJC(bot)
Junction-to-case (bottom) thermal resistance
0.7
°C/W
(1)
For more information about traditional and new thermal metrics, see the Semiconductor and IC Package Thermal Metrics application
report.
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
5
ADS1235
SBAS824 – OCTOBER 2018
www.ti.com
6.5 Electrical Characteristics
Minimum and maximum specifications apply from TA = –40°C to +125°C. Typical specifications are at TA = 25°C.
All specifications at AVDD = 5 V, AVSS = 0 V, DVDD = 3.3 V, VREF = 5 V, fCLK = 7.3728 MHz, PGA mode, gain = 1, and data
rate = 20 SPS (unless otherwise noted).
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
4
6
UNIT
ANALOG INPUTS
Absolute input current
PGA mode, V(AINx) = 2.5 V
PGA bypass
200
Absolute input current drift
0.01
PGA mode, VIN = 39 mV
Differential input current
nA/°C
±0.1
PGA mode, VIN = 2.5 V
–3
PGA and Chop modes, VIN = 2.5 V (1)
±1
3
±5
PGA bypass, VIN = 2.5 V
nA
±40
Differential input current drift
Differential input impedance
nA
0.05
PGA mode
PGA bypass
Crosstalk
nA/°C
1
GΩ
50
MΩ
0.1
µV/V
PGA
Gain settings
Antialias filter frequency
Output voltage monitor
CCAPP,
CAPN = 4.7 nF
1, 64, 128
V/V
60
kHz
Low threshold
AVSS + 0.2
High threshold
AVDD – 0.2
V
PERFORMANCE
DR
Resolution
No missing codes
Equivalent input noise density
Gain = 64 and 128
24
Data rate
Integral non-linearity
VOS
Offset voltage
Gain = 1, 64 and 128
TA = 25°C
Chop mode
–10
±2
10
–350 / Gain – 5
±50 / Gain
350 / Gain + 5
–0.5 / Gain –
0.05
±0.2 / Gain
0.5 / Gain +
0.05
Gain = 64 and 128
Chop mode, gain = 1, 64 and 128
Gain error
TA = 25°C
After calibration
–0.5%
Normal-mode rejection ratio (2)
CMRR
Common-mode rejection ratio (3)
PSRR
Power-supply rejection ratio (4)
ppmFSR
µV
100
350
10
50
1
5
±0.05%
0.5%
nV/°C
on the level of noise
Gain drift
NMRR
SPS
on the level of noise
Gain = 1
GE
7200
See Table 1
After calibration
Offset voltage drift
nV/√Hz
2.5
Noise performance
INL
Bits
8
0.5
4
ppm/°C
See Table 5
Data rate = 20 SPS
Data rate = 400 SPS
AVDD and AVSS
DVDD
130
105
115
90
100
100
120
dB
dB
INTERNAL OSCILLATOR
fCLK
Frequency
7.3728
Accuracy
(1)
(2)
(3)
(4)
6
–2%
±0.5%
MHz
2%
Chop-mode input current scales with data rate. See Figure 27 for Chop mode input current at 20 SPS and 1200 SPS.
Normal-mode rejection ratio performance depends on the digital filter configuration.
Common-mode rejection ratio is specified at fIN = 60 Hz.
Power-supply rejection ratio specified at dc.
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
ADS1235
www.ti.com
SBAS824 – OCTOBER 2018
Electrical Characteristics (continued)
Minimum and maximum specifications apply from TA = –40°C to +125°C. Typical specifications are at TA = 25°C.
All specifications at AVDD = 5 V, AVSS = 0 V, DVDD = 3.3 V, VREF = 5 V, fCLK = 7.3728 MHz, PGA mode, gain = 1, and data
rate = 20 SPS (unless otherwise noted).
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNIT
VOLTAGE REFERENCE INPUTS
Reference input current
500
nA
Input current vs voltage
100
nA/V
0.1
nA//V/°C
Input current drift
Input impedance
Differential
Low voltage monitor
Threshold low
5
0.4
MΩ
0.6
V
TEMPERATURE SENSOR
Sensor voltage
TA = 25°C
122.4
Temperature coefficient
mV
420
µV/°C
GENERAL-PURPOSE INPUTS/OUTPUTS (GPIOs) (5)
VOL
Low-level output voltage
IOL = –1 mA
VOH
High-level output voltage
IOH = 1 mA
VIL
Low-level input voltage
VIH
High-level input voltage
0.2 · AVDD
0.8 · AVDD
V
0.3 · AVDD
0.7 · AVDD
Input hysteresis
V
V
V
0.5
V
DIGITAL INPUTS/OUTPUTS (Other than GPIOs)
VOL
Low-level output voltage
VOH
High-level output voltage
VIL
Low-level input voltage
VIH
High-level input voltage
IOL = –1 mA
0.2 · DVDD
IOL = –8 mA
IOH = 1 mA
0.2 · DVDD
0.8 · DVDD
IOH = 8 mA
Input leakage
V
0.75 · DVDD
0.3 · DVDD
0.7 · DVDD
Input hysteresis
V
V
0.1
VIH or VIL
V
–10
V
10
µA
POWER SUPPLY
IAVDD
IAVSS
Analog supply current
PGA bypass
2.7
4.5
PGA mode, gain = 64 and 128
4.3
6.5
Power-down mode
IDVDD
PD
(5)
(6)
Digital supply current
Power dissipation
mA
2
8
µA
0.4
0.65
mA
Power-down mode (6)
30
50
µA
PGA mode
23
35
Power-down mode
0.1
0.2
mW
GPIO voltage with respect to AVSS.
CLKIN input stopped.
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
7
ADS1235
SBAS824 – OCTOBER 2018
www.ti.com
6.6 Timing Requirements
over operating ambient temperature range, DVDD = 2.7 V to 5.25 V, and DOUT/DRDY load: 20 pF || 100 kΩ to DGND
(unless otherwise noted)
MIN
MAX
UNIT
SERIAL INTERFACE
td(CSSC)
Delay time, first SCLK rising edge after CS falling edge (1)
50
ns
tsu(DI)
Setup time, DIN valid before SCLK falling edge
25
ns
th(DI)
Hold time, DIN valid after SCLK falling edge
25
tc(SC)
SCLK period (2)
97
tw(SCH),
tw(SCL)
Pulse duration, SCLK high or low
40
ns
td(SCCS)
Delay time, last SCLK falling edge before CS rising edge
50
ns
tw(CSH)
Pulse duration, CS high to reset interface
25
td(SCIR)
Delay time, SCLK high or low to force interface auto-reset
ns
106
ns
ns
65540
1/fCLK
RESET
tw(RSTL)
Pulse duration, RESET low
4
1/fCLK
CONVERSION CONTROL
tw(STH)
Pulse duration, START high
4
1/fCLK
tw(STL)
Pulse duration, START low
4
1/fCLK
tsu(DRST)
Setup time, START low or STOP command after DRDY low to stop next
conversion (Continuous-conversion mode)
th(DRSP)
Hold time, START low or STOP command after DRDY low to continue next
conversion (Continuous-conversion mode)
(1)
(2)
8
100
150
1/fCLK
1/fCLK
CS can be tied low.
Serial interface time-out mode: minimum SCLK frequency = 1 kHz. Otherwise, no minimum SCLK frequency.
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
ADS1235
www.ti.com
SBAS824 – OCTOBER 2018
6.7 Switching Characteristics
over operating ambient temperature range, DVDD = 2.7 V to 5.25 V, and DOUT/DRDY load: 20 pF || 100 kΩ to DGND
(unless otherwise noted)
PARAMETER
MIN
TYP
MAX
UNIT
tw(DRH)
Pulse duration, DRDY high
16
tp(CSDO)
Propagation delay time, CS falling edge to DOUT/DRDY
driven
tp(SCDO1)
Propagation delay time, SCLK rising edge to valid
DOUT/DRDY
th(SCDO1)
Hold time, SCLK rising edge to invalid data on
DOUT/DRDY
th(SCDO2)
Hold time, last SCLK falling edge of operation to invalid
data on DOUT/DRDY
tp(SCDO2)
Propagation delay time, last SCLK falling edge to valid
data ready function on DOUT/DRDY
110
ns
tp(CSDOZ)
Propagation delay time, CS rising edge to DOUT/DRDY
high impedance
50
ns
SERIAL INTERFACE
1/fCLK
0
50
ns
40
ns
0
ns
15
ns
RESET
tp(RSCN)
Propagation delay time, RESET rising edge or RESET
command to start of conversion
tp(PRCM)
Propagation delay time, power-on threshold voltage to
ADC communication
tp(CMCN)
Propagation delay time, ADC communication to
conversion start
512
1/fCLK
216
1/fCLK
512
1/fCLK
AC EXCITATION
td(ACX)
Delay time, phase-to-phase blanking period
tc(ACX)
ACX period
8
1/fCLK
2
tSTDR
CONVERSION CONTROL
tp(STDR)
Propagation delay time, START high or START command
to DRDY high
2
1/fCLK
tw(CSH)
CS
td(CSSC)
tc(SC)
td(SCCS)
tw(SCH)
SCLK
th(DI)
tsu(DI)
tw(SCL)
DIN
Figure 1. Serial Interface Timing Requirements
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
9
ADS1235
SBAS824 – OCTOBER 2018
www.ti.com
tw(DRH)
DRDY
CS
SCLK
tp(SCDO2)
tp(SCDO1)
tp(CSDO)
DRDY
DOUT/DRDY
(1)
tp(CSDOZ)
(1)
(1)
DATA
DRDY
th(SCDO1)
th(SCDO2)
Before the first SCLK rising edge and after the last SCLK falling edge of a command, the function of DOUT/DRDY is data ready.
Figure 2. Serial Interface Switching Characteristics
Serial Interface
Auto-Reset
td(SCIR)
Next byte transaction
b7
SCLK
b6
b5
b4
b3
b2
b1
b0
Figure 3. Serial Interface Auto-Reset Characteristics
tw(STH)
START
tw(STL)
Serial
Command
START
STOP
tp(STDR)
STOP
tsu(DRST)
DRDY
th(DRSP)
Figure 4. Conversion Control Timing Requirements
10
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
ADS1235
www.ti.com
SBAS824 – OCTOBER 2018
DVDD
1 V (typ)
VBYPASS
1 V (typ)
AVDD - AVSS
3.5 V (typ)
All supplies reach thresholds
DRDY
DOUT/DRDY
Begin ADC Communication
tp(PRCM)
tp(CMCN)
Conversion
Status
Start of 1st Conversion
Figure 5. Power-Up Characteristics
tw(RSTL)
RESET
Reset
Command
tp(RSCN)
Conversion
Status
Reset
Start
Figure 6. RESET pin and RESET Command Timing Requirements
tc(ACX)
td(ACX)
td(ACX)
ACX1
ACX1
ACX2
ACX2
Figure 7. AC-Excitation Switching Characteristics
DVDD
½ DVDD
50%
DGND
td, th, tp, tw,tc
Figure 8. Timing Voltage-Level Reference
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
11
ADS1235
SBAS824 – OCTOBER 2018
www.ti.com
6.8 Typical Characteristics
at TA = 25°C, AVDD = 5 V, AVSS = 0 V, DVDD = 3.3 V, VREF = 5 V, data rate = 20 SPS, fCLK = 7.3728 MHz and gain = 128
(unless otherwise noted)
10
0.5
PGA Bypass
Gain = 1
Gain = 64 and 128
8
0.3
Offset Voltage (PV)
4
2
0
-2
-4
-6
0.2
0.1
0
-0.1
-0.2
-0.3
-8
-0.4
-10
-40
-20
0
20
40
60
Temperature (qC)
80
100
-0.5
-40
120
-20
After calibration, shorted input
20
40
60
Temperature (qC)
80
100
120
D010
Chop mode, after calibration, shorted input
Figure 9. Offset Voltage vs Temperature
Figure 10. Offset Voltage vs Temperature
100
80
70
70
50
3.6
3
2.4
0.3
0
240
220
200
180
160
140
120
80
0
100
10
0
60
20
10
40
20
20
30
2.1
40
30
1.8
40
1.5
50
60
1.2
60
0.9
Population (%)
80
0
Gain = 1
Gain = 64 and 128
90
3.3
Gain = 1
Gain = 64 and 128
90
2.7
100
Population (%)
0
D009
0.6
Offset Voltage (PV)
6
PGA Bypass
Gain = 1
Gain = 64 and 128
0.4
D012
D011
Input-Referred Offset Drift (nV/qC)
Input-Referred Offset Drift (nV/qC)
Shorted input, 30 units
Chop mode, shorted input, 30 units
Figure 11. Offset Voltage Drift Distribution
Figure 12. Offset Voltage Drift Distribution
200
40
35
Gain = 1
Gain = 64 and 128
150
30
25
Gain Error (ppm)
Offset Voltage (PV)
PGA Bypass
Gain = 1
Gain = 64
Gain = 128
20
15
10
5
0
100
50
0
-50
-5
-10
0.5
1
1.5
2
2.5
3
V REF (V)
3.5
4
4.5
5
-100
-40
D001
After calibration
0
20
40
60
Temperature (qC)
80
100
120
D014
After calibration
Figure 13. Offset Voltage vs Reference Voltage
12
-20
Submit Documentation Feedback
Figure 14. Gain Error vs Temperature
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
ADS1235
www.ti.com
SBAS824 – OCTOBER 2018
Typical Characteristics (continued)
at TA = 25°C, AVDD = 5 V, AVSS = 0 V, DVDD = 3.3 V, VREF = 5 V, data rate = 20 SPS, fCLK = 7.3728 MHz and gain = 128
(unless otherwise noted)
50
60
Gain = 1
Gain = 64
Gain = 128
40
Gain Error (ppm)
Population (%)
40
Gain = 1, 400 SPS
Gain = 1, 7200 SPS
Gain = 128, 400 SPS
Gain = 128, 7200 SPS
50
30
20
10
30
20
10
0
-10
-20
-30
Gain Drift (ppm/qC)
-40
0.5
2.4
2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0
0.2
0
1
1.5
2
D013
Gain = 1
Gain = 64
Gain = 128
5
D002
Gain = 1
Gain = 64
Gain = 128
8
Input-Referred Noise (PVRMS)
Input-Referred Noise (PVRMS )
4.5
9
0.35
0.25
0.2
0.15
0.1
0.05
7
6
5
4
3
2
1
-20
0
20
40
60
Temperature (qC)
80
100
0
-40
120
-20
D003
Exce
0
20
40
60
Temperature (qC)
80
100
120
D004
7200 SPS, Sinc1 mode
20 SPS, Sinc4 mode
Figure 18. Noise vs Temperature
Figure 17. Noise vs Temperature
90
100
Gain = 1, 7200 SPS
Gain = 128, 7200 SPS
80
Population (Data Points)
Gain = 1, 400 SPS
Gain = 128, 400 SPS
Input-Referred Noise (PVRMS )
4
Figure 16. Gain vs Reference Voltage
Figure 15. Gain Drift Distribution
0
-40
3.5
After calibration
30 units
0.3
2.5
3
VREF (V)
10
1
0.1
70
60
50
40
30
20
10
0
1
1.5
2
2.5
3
V REF (V)
3.5
4
4.5
5
D006
-0.25
-0.225
-0.2
-0.175
-0.15
-0.125
-0.1
-0.075
-0.05
-0.025
0
0.025
0.05
0.075
0.1
0.125
0.15
0.175
0.2
0.225
0.25
0.01
0.5
D029
Conversion Data (PV)
20 SPS, Gain = 128, 256 data points, shorted inputs
Figure 19. Noise vs Reference Voltage
Figure 20. Conversion Data Histogram
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
13
ADS1235
SBAS824 – OCTOBER 2018
www.ti.com
Typical Characteristics (continued)
2000
10
1800
8
1600
6
Linearity Error (ppmFSR)
Population (Data Points)
at TA = 25°C, AVDD = 5 V, AVSS = 0 V, DVDD = 3.3 V, VREF = 5 V, data rate = 20 SPS, fCLK = 7.3728 MHz and gain = 128
(unless otherwise noted)
1400
1200
1000
800
600
4
2
0
-2
-4
400
-6
200
-8
0
Gain = 1
Gain = 64
Gain = 128
-3
-2.75
-2.5
-2.25
-2
-1.75
-1.5
-1.25
-1
-0.75
-0.5
-0.25
0
0.25
0.5
0.75
1
1.25
1.5
1.75
2
2.25
2.5
2.75
3
-10
-100 -80
-60
D031
-40 -20
0
20
40
Full Scale Range (%VIN)
60
80
100
D033
Conversion Data (PV)
7200 SPS, Gain = 128, 8192 data points, shorted inputs
Figure 22. Integral Non-Linearity vs VIN
Figure 21. Conversion Data Histogram
60
Gain = 1
Gain = 64
Gain = 128
8
Gain = 1
Gain = 64
Gain = 128
50
Population (%)
6
4
2
40
30
20
10
6
5
5.5
4.5
4
3
120
3.5
100
2.5
80
2
20
40
60
Temperature (qC)
1.5
0
1
0
-20
0
0
-40
0.5
Integral Non-Linearity (ppm FSR)
10
Integral Non-Linearity (ppmFSR)
D007
D015
30 Units
Figure 24. Integral Non-Linearity Distribution
Figure 23. Integral Non-Linearity vs Temperature
2
Gain = 1
Gain = 64
Gain = 128
8
TA = -40qC
TA = 25qC
TA = 85qC
TA = 125qC
1.5
Differential Input Current (nA)
Integral Non-Linearity (ppm FSR)
10
6
4
2
1
0.5
0
-0.5
-1
-1.5
0
2
2.5
3
3.5
VREF (V)
4
4.5
5
-2
-20
D008
-15
-10
-5
0
5
10
Differential Input Voltage (mV)
15
20
D110
Gain = 128
Figure 25. Integral Non-Linearity vs Reference Voltage
14
Submit Documentation Feedback
Figure 26. Differential Input Current
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
ADS1235
www.ti.com
SBAS824 – OCTOBER 2018
Typical Characteristics (continued)
at TA = 25°C, AVDD = 5 V, AVSS = 0 V, DVDD = 3.3 V, VREF = 5 V, data rate = 20 SPS, fCLK = 7.3728 MHz and gain = 128
(unless otherwise noted)
80
1.5
TA = -40qC
TA = 25qC
TA = 85qC
TA = 125qC
1
Differential Input Current (nA)
0.5
0
-0.5
TA = -40qC, 1200 SPS
TA = 25qC, 1200 SPS
TA = 85qC, 1200 SPS
TA = 125qC, 1200 SPS
-1
-1.5
-20
TA
TA
TA
TA
= -40qC, 20 SPS
= 25qC, 20 SPS
= 85qC, 20 SPS
= 125qC, 20 SPS
40
20
0
-20
-40
-60
-80
-15
-10
-5
0
5
10
Differential Input Voltage (mV)
15
-5
20
-4
-3
D112
4
5
D111
Figure 28. Differential Input Current
Figure 27. Differential Input Current
18
15
TA = -40qC
TA = 25qC
TA = 85qC
TA = 125qC
10
7.5
TA = -40qC
TA = 25qC
TA = 85qC
TA = 125qC
15
Absolute Input Current (nA)
12.5
5
2.5
0
-2.5
-5
-7.5
12
9
6
3
0
-3
-6
-9
-12
-10
-15
-12.5
0
0.5
1
1.5
2
2.5
3
3.5
Absolute Input Voltage (V)
4
4.5
0
5
0.5
1
D113
5
D114
40
550
400
4.5
Figure 30. Absolute Input Current
Figure 29. Absolute Input Current
450
4
Chop Mode, Gain = 128
Gain = 128
500
1.5
2
2.5
3
3.5
Absolute Input Voltage (V)
TA = -40qC
TA = 25qC
TA = 85qC
TA = 125qC
30
300
Population (%)
350
REFP
250
200
150
20
10
100
50
REFN
0
4.5
5
D024
124.2
4
123.8
3.5
123.4
2.5
3
VREF (V)
123
2
122.6
1.5
121.4
0
1
121
-50
0.5
122.2
Absolute Input Current (nA)
3
PGA Bypass (gain = 1)
Chop Mode, Gain = 128
Reference Input Current (nA)
-2
-1
0
1
2
Differential Input Voltage (V)
121.8
Differential Input Current (nA)
60
D017
Temperature Sensor Voltage (mV)
VREFN = AVSS
30 units
Figure 31. Reference Input Current vs Reference Voltage
Figure 32. Temperature Sensor Voltage Histogram
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
15
ADS1235
SBAS824 – OCTOBER 2018
www.ti.com
Typical Characteristics (continued)
at TA = 25°C, AVDD = 5 V, AVSS = 0 V, DVDD = 3.3 V, VREF = 5 V, data rate = 20 SPS, fCLK = 7.3728 MHz and gain = 128
(unless otherwise noted)
1
Reference Low Monitor Threshold (V)
Internal Oscillator Error (%)
1
0.5
0
-0.5
-1
-1.5
-40
-20
0
20
40
60
Temperature (qC)
80
100
0.8
0.6
0.4
0.2
0
-40
120
-20
0
D016
30 units
PGA High Monitor Threshold (V)
PGA Low Monitor Threshold (V)
0.2
0.15
0.1
0.05
-20
0
20
40
60
Temperature (qC)
80
100
4.9
4.85
4.8
4.75
4.7
-40
120
-20
0
D026
Figure 35. PGA Low Monitor Threshold vs Temperature
80
100
120
D025
Figure 36. PGA High Monitor Threshold vs Temperature
7
125
6
Operating Current (mA)
120
CMRR and PSRR (dB)
20
40
60
Temperature (qC)
30 units
130
115
110
105
100
95
CMRR
PSRR AVDD, AVSS
PSRR DVDD
-20
0
20
40
60
Temperature (qC)
IAVDD , I AVSS (Gain = 1, PGA mode)
IAVDD , I AVSS (Gain = 64, 128)
IDVDD
5
4
3
2
1
80
100
120
0
-40
-20
D034
Figure 37. CMRR and PSRR vs Temperature
16
D027
4.95
30 units
80
-40
120
5
0.25
85
100
Figure 34. Reference Low Monitor Threshold vs
Temperature
0.3
90
80
30 units
Figure 33. Internal Oscillator Frequency vs Temperature
0
-40
20
40
60
Temperature (qC)
0
20
40
60
Temperature (qC)
80
100
120
D018
Figure 38. Operating Current vs Temperature
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
ADS1235
www.ti.com
SBAS824 – OCTOBER 2018
7 Parameter Measurement Information
7.1 Noise Performance
The ADS1235 noise performance depends on the ADC configuration: data rate, PGA gain, digital filter
configuration, and chop mode. The combination of the parameters affect noise performance. Two significant
factors affecting noise performance are data rate and PGA gain. Since the profile of noise is predominantly white
(flat vs frequency), decreasing the data rate proportionally decreases bandwidth and therefore, decreases total
noise. Since the noise of the PGA is lower than that of the modulator, increasing the gain deceases overall
conversion noise when treated as an input-referred quantity. Noise performance also depends on the digital filter
and chop mode. As the order of the digital filter increases, the noise bandwidth correspondingly decreases
resulting in decreased noise. Further, as a result of two-point data averaging performed in chop mode, noise
decreases by √2 compared to normal operation.
Table 1 shows noise performance in units of μVRMS (RMS = root mean square) and in units of effective resolution
(bits) under the conditions listed. The values in parenthesis are peak-to-peak values (µV) and noise free
resolution (bits). Noise-free resolution is the resolution of the ADC with no code flicker. The noise-free resolution
data are calculated based on the peak-to-peak noise data.
The effective resolution data listed in the tables are calculated using Equation 1:
Effective Resolution or Noise-Free Resolution = ln (FSR / en) / ln (2)
where
•
•
FSR = full scale range = 2 · VREF / Gain (See Recommended Operating Conditions for limitations of FSR)
en = Input referred voltage noise (RMS value to calculate effective resolution, p-p value to calculate noise-free
resolution)
(1)
The data shown in the noise performance table represent typical ADC performance at TA = 25°C. The noiseperformance data are the standard deviation and peak-to-peak computations of the ADC data. The noise data
are acquired with inputs shorted, based on consecutive ADC readings for a period of ten seconds or 8192 data
points, whichever occurs first. Because of the statistical nature of noise, repeated noise measurements may yield
higher or lower noise performance results.
Table 1. Noise in µVRMS (µVPP) and Effective Resolution (Noise-Free Resolution)
at TA = 25°C and VREF = 5 V
Noise (µVRMS, (µVPP))
Effective Resolution (bits), (Noise-Free Resolution (bits))
DATA RATE FILTER
GAIN = 1
GAIN = 64
GAIN = 128
GAIN = 1
GAIN = 64
GAIN = 128
2.5 SPS
FIR
0.21 (0.6)
0.008 (0.028)
0.011 (0.042)
24 (23.8)
24 (22.4)
22.7 (20.8)
2.5 SPS
Sinc1
0.12 (0.3)
0.009 (0.037)
0.008 (0.033)
24 (24)
24 (22)
23.2 (21.2)
2.5 SPS
Sinc2
0.15 (0.3)
0.007 (0.023)
0.006 (0.021)
24 (24)
24 (22.7)
23.7 (21.8)
2.5 SPS
Sinc3
0.15 (0.3)
0.007 (0.023)
0.005 (0.014)
24 (24)
24 (22.7)
24 (22.4)
2.5 SPS
Sinc4
0.15 (0.3)
0.005 (0.019)
0.006 (0.019)
24 (24)
24 (23)
23.7 (22)
5 SPS
FIR
0.29 (0.89)
0.013 (0.051)
0.013 (0.051)
24 (23.2)
23.6 (21.5)
22.5 (20.5)
5 SPS
Sinc1
0.15 (0.3)
0.015 (0.051)
0.01 (0.044)
24 (24)
23.4 (21.5)
22.9 (20.8)
5 SPS
Sinc2
0.17 (0.6)
0.012 (0.047)
0.009 (0.035)
24 (23.8)
23.7 (21.7)
23 (21.1)
5 SPS
Sinc3
0.12 (0.6)
0.011 (0.047)
0.008 (0.037)
24 (23.8)
23.7 (21.7)
23.1 (21)
5 SPS
Sinc4
0.088 (0.3)
0.007 (0.028)
0.007 (0.03)
24 (24)
24 (22.4)
23.3 (21.3)
10 SPS
FIR
0.36 (1.5)
0.022 (0.11)
0.02 (0.096)
24 (22.5)
22.8 (20.5)
21.9 (19.6)
10 SPS
Sinc1
0.28 (0.89)
0.015 (0.065)
0.016 (0.082)
24 (23.2)
23.3 (21.2)
22.2 (19.9)
10 SPS
Sinc2
0.26 (0.89)
0.015 (0.061)
0.013 (0.065)
24 (23.2)
23.3 (21.3)
22.5 (20.2)
10 SPS
Sinc3
0.26 (0.6)
0.014 (0.065)
0.011 (0.047)
24 (23.8)
23.4 (21.2)
22.7 (20.7)
10 SPS
Sinc4
0.24 (0.6)
0.013 (0.056)
0.01 (0.042)
24 (23.8)
23.6 (21.4)
22.9 (20.8)
16.6 SPS
Sinc1
0.41 (1.8)
0.025 (0.12)
0.022 (0.12)
24 (22.2)
22.6 (20.3)
21.8 (19.4)
16.6 SPS
Sinc2
0.32 (1.5)
0.018 (0.089)
0.018 (0.096)
24 (22.5)
23 (20.8)
22 (19.6)
16.6 SPS
Sinc3
0.3 (1.2)
0.017 (0.079)
0.018 (0.091)
24 (22.8)
23.1 (20.9)
22.1 (19.7)
16.6 SPS
Sinc4
0.23 (1.2)
0.015 (0.084)
0.014 (0.072)
24 (22.8)
23.3 (20.8)
22.4 (20)
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
17
ADS1235
SBAS824 – OCTOBER 2018
www.ti.com
Noise Performance (continued)
Table 1. Noise in µVRMS (µVPP) and Effective Resolution (Noise-Free Resolution)
at TA = 25°C and VREF = 5 V (continued)
Noise (µVRMS, (µVPP))
Effective Resolution (bits), (Noise-Free Resolution (bits))
DATA RATE FILTER
GAIN = 1
GAIN = 64
GAIN = 128
GAIN = 1
GAIN = 64
GAIN = 128
20 SPS
FIR
0.51 (2.1)
0.032 (0.16)
0.029 (0.16)
24 (22)
22.2 (19.9)
21.3 (18.9)
20 SPS
Sinc1
0.44 (2.1)
0.025 (0.13)
0.026 (0.13)
24 (22)
22.6 (20.2)
21.5 (19.2)
20 SPS
Sinc2
0.36 (1.2)
0.02 (0.12)
0.02 (0.1)
24 (22.8)
22.9 (20.4)
21.9 (19.5)
20 SPS
Sinc3
0.32 (1.5)
0.017 (0.089)
0.018 (0.096)
24 (22.5)
23.1 (20.8)
22 (19.6)
20 SPS
Sinc4
0.3 (1.2)
0.017 (0.084)
0.018 (0.1)
24 (22.8)
23.1 (20.8)
22.1 (19.6)
50 SPS
Sinc1
0.63 (3.6)
0.04 (0.25)
0.038 (0.23)
23.7 (21.2)
21.9 (19.2)
21 (18.4)
50 SPS
Sinc2
0.57 (3)
0.033 (0.21)
0.032 (0.18)
23.9 (21.5)
22.2 (19.5)
21.2 (18.7)
50 SPS
Sinc3
0.53 (2.4)
0.03 (0.19)
0.03 (0.17)
24 (21.8)
22.3 (19.7)
21.3 (18.8)
50 SPS
Sinc4
0.49 (2.4)
0.028 (0.15)
0.026 (0.16)
24 (21.8)
22.4 (20)
21.5 (18.9)
60 SPS
Sinc1
0.71 (3.9)
0.043 (0.27)
0.042 (0.26)
23.6 (21.1)
21.8 (19.1)
20.8 (18.2)
60 SPS
Sinc2
0.6 (3.3)
0.036 (0.24)
0.034 (0.21)
23.8 (21.4)
22.1 (19.3)
21.1 (18.5)
60 SPS
Sinc3
0.56 (3)
0.032 (0.19)
0.03 (0.17)
23.9 (21.5)
22.2 (19.6)
21.3 (18.8)
60 SPS
Sinc4
0.53 (2.7)
0.031 (0.19)
0.03 (0.18)
24 (21.6)
22.3 (19.7)
21.3 (18.7)
100 SPS
Sinc1
0.8 (4.8)
0.056 (0.34)
0.054 (0.35)
23.4 (20.8)
21.4 (18.8)
20.5 (17.8)
100 SPS
Sinc2
0.68 (4.2)
0.047 (0.29)
0.043 (0.3)
23.6 (21)
21.7 (19)
20.8 (18)
100 SPS
Sinc3
0.67 (4.2)
0.042 (0.28)
0.041 (0.27)
23.6 (21)
21.8 (19.1)
20.9 (18.1)
100 SPS
Sinc4
0.62 (3.6)
0.039 (0.24)
0.039 (0.27)
23.8 (21.2)
21.9 (19.3)
20.9 (18.2)
400 SPS
Sinc1
1.4 (11)
0.11 (0.81)
0.11 (0.75)
22.6 (19.6)
20.4 (17.5)
19.5 (16.7)
400 SPS
Sinc2
1.2 (8.3)
0.09 (0.64)
0.086 (0.6)
22.8 (20)
20.7 (17.9)
19.8 (17)
400 SPS
Sinc3
1.1 (7.7)
0.082 (0.61)
0.078 (0.56)
22.9 (20.1)
20.9 (18)
19.9 (17.1)
400 SPS
Sinc4
1 (7.7)
0.076 (0.59)
0.072 (0.53)
23 (20.1)
21 (18)
20 (17.2)
1200 SPS
Sinc1
2.3 (17)
0.18 (1.3)
0.18 (1.4)
21.9 (19)
19.7 (16.9)
18.8 (15.7)
1200 SPS
Sinc2
2 (14)
0.15 (1.2)
0.15 (1.1)
22.1 (19.3)
20 (17)
19 (16.1)
1200 SPS
Sinc3
1.8 (13)
0.14 (1)
0.13 (1)
22.2 (19.4)
20.1 (17.2)
19.2 (16.2)
1200 SPS
Sinc4
1.7 (13)
0.13 (1)
0.13 (0.94)
22.3 (19.4)
20.2 (17.2)
19.2 (16.3)
2400 SPS
Sinc1
3.2 (26)
0.25 (2)
0.24 (1.8)
21.4 (18.4)
19.2 (16.2)
18.3 (15.4)
2400 SPS
Sinc2
2.7 (20)
0.22 (1.7)
0.21 (1.5)
21.6 (18.7)
19.5 (16.5)
18.5 (15.6)
2400 SPS
Sinc3
2.5 (18)
0.2 (1.4)
0.19 (1.4)
21.7 (18.9)
19.6 (16.7)
18.6 (15.8)
2400 SPS
Sinc4
2.3 (18)
0.18 (1.5)
0.18 (1.4)
21.8 (18.9)
19.7 (16.7)
18.8 (15.8)
4800 SPS
Sinc1
4.4 (35)
0.34 (2.5)
0.32 (2.4)
20.9 (17.9)
18.8 (15.9)
17.9 (15)
4800 SPS
Sinc2
3.9 (30)
0.3 (2.3)
0.29 (2.4)
21.1 (18.1)
19 (16.1)
18 (15)
4800 SPS
Sinc3
3.6 (27)
0.28 (2)
0.26 (2)
21.2 (18.3)
19.1 (16.3)
18.2 (15.2)
4800 SPS
Sinc4
3.4 (27)
0.26 (1.9)
0.25 (1.9)
21.3 (18.3)
19.2 (16.3)
18.2 (15.3)
7200 SPS
Sinc1
5.2 (42)
0.38 (2.9)
0.37 (3)
20.7 (17.7)
18.6 (15.7)
17.7 (14.7)
7200 SPS
Sinc2
4.7 (37)
0.36 (2.8)
0.34 (2.5)
20.8 (17.9)
18.7 (15.8)
17.8 (14.9)
7200 SPS
Sinc3
4.5 (35)
0.34 (2.5)
0.32 (2.4)
20.9 (18)
18.8 (15.9)
17.9 (15)
7200 SPS
Sinc4
4.2 (36)
0.32 (2.5)
0.31 (2.3)
21 (17.9)
18.9 (16)
18 (15.1)
18
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
ADS1235
www.ti.com
SBAS824 – OCTOBER 2018
8 Detailed Description
8.1 Overview
The ADS1235 is a three differential-input, precision 24-bit, ΔΣ ADC with a low-noise PGA and programmable
digital filter. The low-noise, low-drift architecture of the PGA makes the ADC suitable for precision measurement
of low signal level sensors, such as strain-gauge bridges and resistive pressure transducers. The ADC provides
optional chop and ac-bridge excitation modes to eliminate offset drift error.
Key features of the ADC are:
• 1-GΩ input impedance, low-noise PGA
• High-resolution 24-bit ΔΣ ADC
• Four GPIO with ac-bridge excitation control output
• Internal oscillator
• Voltage reference monitor
• Signal overrange monitor
• Temperature sensor
• CRC communication error detection
The analog inputs (AINx) connect to the input multiplexer (MUX). The ADC supports three differential or five
single-ended input measurement configurations. A second voltage reference input and AC-bridge excitation drive
outputs (GPIO) are multiplexed with the analog input pins.
The programmable gain amplifier (PGA) follows the input multiplexer. The gain is programmable to 1, 64 or 128.
The PGA bypass option connects the analog inputs directly to the precharge buffered modulator, extending the
input voltage range to the voltage of the power supplies. The PGA output connects to pins CAPP and CAPN.
The ADC antialias filter is provided at the PGA output with an external capacitor. A monitor is used for detection
of PGA overrange conditions.
The delta-sigma modulator measures the differential input voltage relative to the reference voltage to produce the
24-bit conversion result. The differential input range of the ADC is ±VREF / Gain.
The digital filter averages and decimates the modulator output data to yield the final, down-sampled conversion
result. The sinc filter is programmable (sinc1 through sinc4) allowing optimization of conversion time, conversion
noise and line-cycle rejection. The finite impulse response (FIR) filter mode provides single-cycle settled data
with simultaneous rejection of 50-Hz and 60-Hz at data rates of 20 SPS or less.
Two reference voltage input pairs are provided. The primary reference input pair (REFP0/REFN0) is available as
standalone input pins. A second reference input pair (REFP1/REFN1) is multiplexed with analog inputs AIN0 and
AIN1. A monitor is used for detection of low or missing reference voltage.
The ADC provides four GPIO control lines. The GPIOs are used for input and output of general-purpose logic
signals, as well as providing output drive signals for ac-excited bridges. The GPIOs and ac-bridge excitation drive
outputs are multiplexed to the analog inputs.
The internal temperature sensor voltage is read by the ADC through the analog input multiplexer.
The SPI-compatible serial interface is used to read the conversion data and also to configure and control the
ADC. Data communication errors are detected by CRC. The serial interface consists of four signals: CS, SCLK,
DIN and DOUT/DRDY. The dual function DOUT/DRDY provides data output and also the data ready signal. The
ADC serial interface can be implemented with as little as three pins by tying CS low.
The ADC clock is either internal or external. The ADC detects the mode of clock operation automatically. The
clock frequency is 7.3728 MHz.
Data conversions are controlled by the START pin or by the START command. The ADC is programmable for
continuous or one-shot conversions. The DRDY or DOUT/DRDY pin provides the conversion-data ready signal.
When taken low, the RESET pin resets the ADC. The ADC is powered down by the PWDN pin or is powered
down in software mode.
The ADC operates in either bipolar analog supply configuration (±2.5 V), or in single 5-V supply configuration.
The digital power supply range is 2.7 V to 5 V. The BYPASS pin is the internal subregulator output used for the
ADC digital core.
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
19
ADS1235
SBAS824 – OCTOBER 2018
www.ti.com
8.2 Functional Block Diagram
AVDD
2-V Core
Voltage
REFP0
REFN0
DVDD
BYPASS
Ref
Mux
LDO
I/O Voltage
START
AIN0
AIN1
Input
Mux
AIN2
Ref
Monitor
Temp
Sensor
Control
DRDY
Buf
AIN3
CS
AIN4
AIN5
24-Bit
û ADC
PGA
Digital
Filter
Serial
Interface
GPIO
AC-EXC
DIN
SCLK
DOUT/DRDY
PGA
Monitor
AVSS
20
RESET
PWDN
CAPP CAPN
Submit Documentation Feedback
Internal
Oscillator
Clock
Mux
CLKIN
DGND
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
ADS1235
www.ti.com
SBAS824 – OCTOBER 2018
8.3 Feature Description
The following sections describe the functional blocks of the ADC.
8.3.1 Analog Inputs
Figure 39 shows the analog input circuit consisting of ESD-protection diodes, the input multiplexer and the PGA.
The ADS1235 has six analog inputs to support three differential-input measurement channels. In addition, there
are two internal (system) measurements, and an option to disconnect all inputs.
ESD Diodes
Positive Input Multiplexer
MUXP[3:0] bits 7:4 of INPMUX
(register address = 11h)
AVDD
- and Reserved
Reserved
Reserved
AIN0
AIN0
AIN1
AIN2
AIN3
AIN4
AIN5
0001
0010
0100
AIN2
0101
AIN3
0110
AIN4
0111
AIN5
1000
Reserved
1001
1010
VCOM: (AVDD + AVSS) / 2
Negative Input Multiplexer
MUXN[3:0] bits 3:0 of INPMUX
(register address = 11h)
0011
AIN1
Reserved
Temperature Sensor
Reserved
Reserved
All Open
AVSS
0000
+
PGA
-
ADC
1011
1100
1101
1110
1111
ESD Diodes
Figure 39. Analog Input Block Diagram
8.3.1.1 ESD Diodes
ESD diodes are incorporated to protect the ADC inputs from possible ESD events occurring during the
manufacturing process and during PCB assembly when manufactured in an ESD-controlled environment. For
system-level ESD protection, consider the use of external ESD protection devices for pins that are exposed to
ESD, including the analog inputs.
If either input is driven below AVSS – 0.3 V, or above AVDD + 0.3 V, the internal protection diodes may conduct.
If these conditions are possible, use external clamp diodes, series resistors, or both to limit the input current to
the specified maximum value.
8.3.1.2 Input Multiplexer
The input multiplexer selects the signal for measurement. The multiplexer consists of independently
programmable positive and negative sections. See Figure 39 for multiplexer register settings. The multiplexers
select any input as positive and any input as negative for connection to the PGA. For example, to select AIN5
and AIN4 as a differential input with (+) and (-) polarity, program the INPMUX register to the value of 87h.
When the multiplexer is changed, a break-before-make sequence is performed in order to reduce charge
injection into the next measurement channel. Be aware that over-driving unused channels beyond the power
supplies can effect conversions taking place on active channels. See the Input Overload section for more
information.
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
21
ADS1235
SBAS824 – OCTOBER 2018
www.ti.com
Feature Description (continued)
8.3.1.3 Temperature Sensor
The ADC has an internal temperature sensor. The temperature sensor is comprised of two internal diodes with
one diode having 80 times the current density of the other. The difference in current density of the diodes yields
a differential output voltage that is proportional to absolute temperature. The temperature sensor reading is
converted by the ADC. See Figure 39 for register settings to select the temperature sensor for measurement.
Equation 2 shows how to convert the temperature sensor reading to degrees Celsius (˚C):
Temperature (°C) = [(Temperature Reading (µV) – 122,400) / 420 µV/°C] + 25°C
(2)
Measure the temperature sensor with PGA on, gain = 1 and ac-bridge excitation mode disabled. As a result of
the low package-to-PCB thermal resistance, the internal temperature closely tracks the PCB temperature.
8.3.1.4 Inputs Open
This configuration opens the inputs to the PGA. Use this configuration to disconnect the PGA from the sensor.
With all inputs disconnected, the conversion data are invalid due to the floating input condition. See Figure 39 for
the register setting value to open all inputs.
8.3.1.5 Internal VCOM Connection
This configuration connects the PGA inputs to the internal VCOM voltage as defined: (AVDD + AVSS) / 2. Use this
connection to short the inputs to measure the ADC noise performance and offset voltage, or to short the inputs
for offset calibration. See Figure 39 for register settings for the internal VCOM connection.
8.3.1.6 Alternate Functions
The analog input pins have multiplexed alternate functions. The alternate functions are the second reference
input and GPIO to provide the ac-bridge excitation drive signals. The functions are enabled by programming the
associated function registers. The analog inputs retain measurement capability if the alternate functions are
programmed. Table 2 summarizes the alternate functions multiplexed to the analog input pins.
Table 2. Analog Input Alternate Functions
ANALOG INPUTS
REFERENCE INPUTS
GPIO/AC-BRIDGE EXCITATION (2-wire mode)
GPIO/AC-BRIDGE EXCITATION (4-wire mode)
AIN0
REFP1
GPIO0/ACX1
GPIO0/ACX1
AIN1
REFN1
GPIO1/ACX2
GPIO1/ACX2
AIN2
GPIO2/ACX1
AIN3
GPIO3/ACX2
AIN4
AIN5
22
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
ADS1235
www.ti.com
SBAS824 – OCTOBER 2018
8.3.2 PGA
The PGA is a low-noise, CMOS differential-input, differential-output amplifier. The PGA extends the dynamic
range of the ADC, important when used with low-level output sensors. Gain is controlled by the GAIN[2:0]
register bits as shown in Figure 40. In PGA bypass mode, the input voltage range extends to the analog
supplies. The PGA is powered down in bypass mode.
BYPASS bit 7 of PGA
(register address = 10h)
0: PGA active (shown)
1: PGA bypass
280 Ÿ
350 Ÿ
VAINP
8 pF
+
A1
±
GAIN[2:0] bits 2:0 of PGA
(register address = 10h)
CAPP
12 pF
000: 1
001: reserved
010: reserved
011: reserved
100: reserved
101: reserved
110: 64
111:128
Positive
Monitor
12 pF
VAINN
4.7 nF
C0G
ADC
12 pF
Negative
Monitor
±
A2
+
350 Ÿ
CAPN
280 Ÿ
8 pF
Figure 40. PGA Block Diagram
The PGA consists of two chopper-stabilized amplifiers (A1 and A2), and a resistor network that determines the
PGA gain. The resistor network is precision-matched, providing low drift performance. The PGA has internal
noise filters to reduce sensitivity to electromagnetic-interference (EMI). The PGA output is monitored to provide
indication of a possible PGA overload condition.
Pins CAPP and CAPN are the PGA positive and negative outputs, respectively. Connect an external 4.7-nF
capacitor (type C0G) as shown in Figure 40. The capacitor filters the sample pulses caused by the modulator,
and with the internal resistors the antialias filter is provided. Place the capacitor as close as possible to the pins
using short, direct traces. Avoid running clock traces or other digital traces close to these pins.
8.3.2.1 Input Voltage Range
The input voltage range is determined by the magnitude of the reference voltage and ADC gain. As shown in
Figure 19, conversion voltage noise is constant over the specified reference voltage range. Table 3 shows the
differential input voltage range verses gain for VREF = 5 V.
Table 3. Input Voltage Range
GAIN[2:0] BITS
GAIN
FULL-SCALE DIFFERENTIAL INPUT VOLTAGE RANGE (1)
000
1
±5.000 V
110
64
±0.078 V
111
128
±0.039 V
(1)
VREF = 5 V. Input voltage range scales with VREF. For gain = 1 and PGA mode, the input voltage range
is limited by evaluation of Equation 3.
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
23
ADS1235
SBAS824 – OCTOBER 2018
www.ti.com
As with many amplifiers, the PGA has an input voltage range specification that must not be exceeded in order to
maintain linear operation. The input range is specified as an absolute voltage (signal plus common mode
voltage) at both positive and negative inputs. As specified in Equation 3, the maximum and minimum absolute
input voltage depends on gain, the expected maximum differential voltage, and the minimum value of the analog
power supply voltage.
AVSS + 0.3 V + VIN · (Gain – 1) / 2 · < VAINP and VAINN < AVDD – 0.3 V – VIN · (Gain – 1) / 2
where
•
•
•
•
•
VAINP, VAINN = absolute input voltage
VIN = VAINP – VAINN, maximum differential input voltage
Gain (for gains = 64 and 128, use 32 for calculation)
AVDD = minimum AVDD voltage
AVSS = maximum AVSS voltage
(3)
The relationship of the PGA input to the PGA output is shown graphically in Figure 41. The PGA output voltages
(VOUTP, VOUTN) depend on the respective absolute input voltage, the differential input voltage, and the PGA gain.
To maintain the PGA within the linear operating range, the PGA output voltages must be restricted within
AVDD – 0.3 V and AVSS + 0.3 V. The diagram depicts a positive differential input voltage that results in a
positive differential output voltage.
PGA Input
PGA Output
AVDD
AVDD ± 0.3 V
VOUTP = VAINP + VIN Â (Gain ± 1) / 2
VAINP
VIN = VAINP Â 9AINN
VAINN
VOUTN = VAINN ± VIN Â (Gain ± 1) / 2
AVSS + 0.3 V
AVSS
Figure 41. PGA Input/Output Range
8.3.2.2 PGA Bypass Mode
Bypass the PGA to extend the input voltage range to the analog power supply voltages. In bypass mode, the
PGA is bypassed and the analog inputs are connected directly to the precharge buffers of the modulator, thereby
extending the input voltage range. Be aware of the increased input current in bypass mode. See the Electrical
Characteristics for the input current specification.
8.3.3 PGA Voltage Monitor
The PGA has internal monitors to alarm of possible overrange conditions. Overrange conditions are possible if
the signal voltage is over-driven, the common-mode voltage is out of range or if too much gain is used for the
normal range of input signal. When overranged, the PGA output nodes are in saturation resulting in invalid
conversion data. The high alarm bit asserts high (PGAH_ALM) If either the positive or negative PGA output is
greater than AVDD – 0.2 V. Similarly, the low alarm bit asserts high (PGAL_ALM) if either positive or negative
PGA output is less than AVSS + 0.2 V. The status of the alarm bits are read in the STATUS byte. The alarm bits
are read-only and automatically reset at the start of the next conversion cycle after the overrange condition is
cleared. The PGA voltage monitor diagram and threshold values are shown in Figure 42 and Figure 43.
24
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
ADS1235
www.ti.com
SBAS824 – OCTOBER 2018
AVDD ± 0.2 V
PGAH_ALM Condition
±
+
PGAH_ALM
±
P
PGA
N
STATUS Byte
PGA Output
P or N
P or N
+
6
7
5
4
3
2
1
0
±
+
AVDD
AVDD ± 0.2 V
AVSS + 0.2 V
AVSS
PGAL_ALM
±
AVSS + 0.2 V
+
PGAL_ALM Condition
Figure 42. PGA Voltage Monitor Diagram
Figure 43. PGA Monitor Thresholds
The PGA voltage monitors are fast-responding voltage comparators. Comparator operation is disabled during
multiplexer changes to minimize triggering of false alarms. However, it is possible the alarms can trigger on other
transient overload conditions that may occur after gain changes, sensor connection changes, and so on.
8.3.4 Reference Voltage
The ADC requires a reference voltage for operation. The ADC allows two external inputs and the internal analog
power supply as reference options. The reference voltage is selected by independent positive and negative
multiplexers. The default reference is the 5-V analog power supply (AVDD – AVSS). Figure 44 shows the block
diagram of the reference multiplexer.
Reserved
AVDD
REFP0
AIN0 (REFP1)
00
01
10
11
RMUXP[1:0] bits 3:2 of REF
(register address = 06h)
VREFP
Ref Monitor
Reserved
AVSS
REFN0
AIN1 (REFN1)
00
01
VREFN
BUF
ADC
10
11
RMUXN[1:0] bits 1:0 of REF
(register address = 06h)
Figure 44. Reference Input Block Diagram
Program the RMUXP[1:0] and RMUXN[1:0] bits of the REF register to select the positive and negative reference
voltages, respectively. The positive reference selections are AVDD, REFP0 and AIN0 (REFP1). The negative
reference input selections are internal AVSS, REFN0, AIN1 (REFN1). The reference low-voltage monitor is
located after the reference multiplexer. See the Reference Monitor section for more information.
8.3.4.1 External Reference
Use the external reference by applying the reference voltage to the designated reference input pins. The
reference input pins are differential with positive and negative inputs. Program the reference multiplexer bits
RMUXP[1:0] and RMUXN[1:0] to select the respective reference voltage for operation. For example, to select
REFP0 and REFN0 as the reference voltage, program the REF register to the value of 0Ah. Follow the specified
absolute and differential reference voltage operating conditions, as specified in the Recommended Operating
Conditions.
Be aware of the reference input current when reference impedances are present, such as by the use of a resistor
divider. Consider the effect of the resistance to system accuracy. Connect a capacitor across the reference input
pins to filter noise. When R-C filters are used, match the time constants of the input signal filter to the reference
voltage filter to maintain constant conversion noise over the signal operating range.
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
25
ADS1235
SBAS824 – OCTOBER 2018
www.ti.com
8.3.4.2 AVDD – AVSS Reference (Default)
A third reference option is the 5-V analog power supply (AVDD – AVSS). Select this reference option by
programming the REF register to 05h. For 6-wire strain-gauge bridge applications that use excitation-sense
connections, or for ac-bridge excitation operation, connect the excitation sense lines to the reference input pins
and program the ADC for external reference operation.
8.3.4.3 Reference Monitor
The ADC incorporates an internal low-voltage monitor of the reference voltage. As shown in Figure 45 and
Figure 46, the REFL_ALM bit of the STATUS byte asserts if the reference voltage (VREF = VREFP – VREFN) falls
below 0.4 V. The alarm is read-only and resets at the next conversion after the low reference condition is no
longer present.
Use the reference monitor to detect a missing or failed reference voltage. To implement detection of a missing
reference, use a 100-kΩ resistor across the reference inputs. If either reference input is disconnected, the
resistor biases the differential reference input toward 0 V so that the reference monitor detects the disconnected
reference.
Differential
Reference Voltage
STATUS Byte
x
6
7
+
External
Reference
_
5
3
4
2
1
0.4 V, Typical
0
REFP0
100 k
REFN0
Ref
Mux
REFL_ALM
+
_
x
_
0.4 V
Reference Low Condition
+
Figure 45. Reference Monitor
Figure 46. Reference Monitor Threshold
8.3.5 General-Purpose Input/Outputs (GPIOs)
The ADC includes four GPIO pins, GPIO0 through GPIO3. The GPIOs are digital inputs/outputs that are
referenced to analog AVDD and AVSS. The GPIOs are read and written by the GPIO_DAT bits of the MODE3
register. The GPIOs are multiplexed with analog inputs AIN0 to AIN3. As shown in Figure 47, the GPIOs are
configured through a series of programming registers. Bits GPIO_CON[3:0] connect the GPIOs to the associated
pin (1 = connect). Bits GPIO_DIR program the direction of the GPIOs; (0 = output, 1 = input). The input voltage
threshold is the voltage value between AVDD and AVSS. Bits GPIO_DAT[3:0] are the data values for the GPIOs.
Observe that if a GPIO pin is programmed as an output, the value read is the value previously written to the
register data, not the actual voltage at the pin.
The GPIOs also provide the ac-bridge excitation drive signals. AC-bridge excitation mode overrides the GPIO
register data values. See the AC-Bridge Excitation Mode section for details.
AVDD
Write
CHOP[1:0] bits 6:5 of MODE1
(register address = 03h)
GPIO_CON[3:0] bits 7:4 of MODE2
(register address = 04h)
Write
0: GPIO not connected (default)
1: GPIO connected
AIN0
AIN1
AIN2
AIN3
AC-Bridge Excitation Mode
GPIO_DAT[3:0] bits 3:0 of MODE3
(register address = 05h)
0: VGPIO low (default)
1: VGPIO high
0
GPIO0
00: Normal mode (default)
01: Chop mode
10: 2-wire AC-bridge excitation mode
11: 4-wire AC-bridge excitation mode
Read
1
GPIO1
GPIO2
+
GPIO3
±
AVDD + AVSS
2
MUX
Read Select
GPIO_DIR[3:0] bits 3:0 of MODE2
(register address = 04h)
0: GPIO is output (default)
1: GPIO is input
AVSS
Figure 47. GPIO Block Diagram
26
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
ADS1235
www.ti.com
8.3.6
SBAS824 – OCTOBER 2018
Modulator
The modulator is an inherently stable, fourth-order, 2 + 2 pipelined ΔΣ modulator. The modulator samples the
analog input voltage at a high sample rate (fMOD = fCLK / 8) and converts the analog input to a ones-density bitstream with the density given by the ratio of the input signal to the reference voltage. The modulator shapes the
noise of the converter to high frequency, where the noise is removed by the digital filter.
8.3.7 Digital Filter
The ADC operates on the principle of oversampling. Oversampling is defined as the ratio of the sample rate of
the modulator to that of the ADC output data rate. Oversampling improves ADC noise by digital bandwidth
limiting (low-pass filtering) of the data.
The digital filter receives the modulator output data and produces a high-resolution conversion result. The digital
filter low-pass filters and decimates the modulator data (data rate reduction), yielding the final data output. By
adjusting the type of filtering, tradeoffs are made between resolution, data throughput and line-cycle rejection.
The digital filter has two selectable modes: sin (x) / x (sinc) mode and finite impulse response (FIR) mode (see
Figure 48). The sinc mode provides data rates of 2.5 SPS through 7200 SPS with variable sinc orders of 1
through 4. The FIR filter provides simultaneous rejection of 50-Hz and 60-Hz frequencies with data rates of 2.5
SPS through 20 SPS while providing single-cycle settled conversions.
Sinc Filter Section
fCLK / 8
Modulator
Sinc5 Filter
7200 SPS to 2.5 SPS
SincN Filter
FIR Filter Section
20 SPS
FIR
DR[3:0] bits 7:3 of MODE0
(register address = 02h)
0000: 2.5 SPS
0001: 5 SPS
0010: 10 SPS
0011: 16.6 SPS
0100: 20 SPS
0101: 50 SPS
0110: 60 SPS
0111: 100 SPS
1000: 400 SPS
1001: 1200 SPS
1010: 2400 SPS
1011: 4800 SPS
1100: 7200 SPS
FILTER[2:0] bits 2:0 of MODE0
(register address = 02h)
Filter
Mux
To Offset/Gain
Calibration
Averager
10 SPS
5 SPS
2.5 SPS
(
= Data rate reduction)
000: sinc1
001: sinc2
010: sinc3
011: sinc4
100: FIR mode
Figure 48. Digital Filter Block Diagram
8.3.7.1 Sinc Filter
The sinc filter is comprised of two stages: a fixed-decimation sinc5 filter, followed by a variable-decimation,
variable-order sinc filter. The first stage filters and down-samples the input data from the modulator to produce an
intermediate data rate of 14400 SPS. The second stage receives the intermediate data to provide final output
data rates of 7200 SPS through 2.5 SPS. The second stage has programmable orders of sinc.
The data rate is programmed by the DR[3:0] bits of the MODE0 register. The filter mode is programmed by the
FILTER[2:0] bits of the MODE0 register (see Figure 48).
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
27
ADS1235
SBAS824 – OCTOBER 2018
www.ti.com
8.3.7.1.1 Sinc Filter Frequency Response
The overall frequency response of the sinc filter is low pass. The filter reduces signal and noise beginning at the
-3-dB bandwidth. Changing the data rate and filter order changes the filter bandwidth together with the rate of
frequency roll-off. See the Filter Bandwidth section for the bandwidth of the filter settings.
Figure 49 shows the frequency response of the sinc filter at 2400 SPS for various orders of the sinc filter. The
peaks and nulls are characteristic of the sinc filter response. The frequency response nulls occur at f (Hz) = N ·
fDATA, where N = 1, 2, 3 and so on. At the null frequencies, the filter has zero gain. The response nulls are
superimposed with the larger nulls beginning at 14400 Hz. The larger nulls are produced by the first stage. The
frequency response is similar to that of data rates 2.5 SPS through 7200 SPS. Figure 50 shows the frequency
response nulls for 10 SPS.
0
0
sinc1
sinc2
sinc3
sinc4
-20
-40
Amplitude (dB)
Amplitude (dB)
-40
sinc1
sinc2
sinc3
sinc4
-20
-60
-80
-100
-60
-80
-100
-120
-120
-140
-140
-160
-160
0
5
10
15
20
25
30
Frequency (kHz)
35
40
45
0
10
20
30
40
D003
Figure 49. Sinc Frequency Response (2400 SPS)
50 60 70 80
Frequency (Hz)
90 100 110 120
D004
Figure 50. Sinc Frequency Response (10 SPS)
Figure 51 and Figure 52 show the frequency response of data rates 50 SPS and 60 SPS, respectively. Increase
the attenuation at 50 Hz or 60 Hz and harmonics by increasing the order of the sinc filter, as shown in the
figures.
0
0
sinc1
sinc2
sinc3
sinc4
-20
-60
-80
-100
-60
-80
-100
-120
-120
-140
-140
-160
-160
0
50 100 150 200 250 300 350 400 450 500 550 600
Frequency (Hz)
D005
Figure 51. Sinc Frequency Response (50 SPS)
28
-40
Amplitude (dB)
Amplitude (dB)
-40
sinc1
sinc2
sinc3
sinc4
-20
0
60
120
180
240 300 360
Frequency (Hz)
420
480
540
600
D006
Figure 52. Sinc Frequency Response (60 SPS)
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
ADS1235
www.ti.com
SBAS824 – OCTOBER 2018
Figure 53 and Figure 54 show the detailed frequency response at 50 SPS and 60 SPS, respectively.
0
0
sinc1
sinc2
sinc3
sinc4
-20
-40
Ampliude (dB)
Amplitude (dB)
-40
-60
-80
-100
-60
-80
-100
-120
-120
-140
-140
-160
45
46
47
48
49
50
51
Frequency (Hz)
52
53
54
sinc1
sinc2
sinc3
sinc4
-20
-160
55
55
56
57
58
D009
Figure 53. Detail Sinc Frequency Response (50 SPS)
59
60
61
Frequency (Hz)
62
63
64
65
D010
Figure 54. Detail Sinc Frequency Response (60 SPS)
8.3.7.2 FIR Filter
The finite impulse response (FIR) filter is a coefficient based filter that provides an overall low-pass filter
response. The filter provides simultaneous attenuation of 50 Hz and 60 Hz and harmonics at data rates of 2.5
SPS through 20 SPS. The conversion latency time of the FIR filter data rates is single-cycle. As shown in
Figure 48, the FIR filter receives pre-filtered data from the sinc filter. The FIR filter decimates the data to yield the
output data rates of 20 SPS. A variable averager (sinc1) provides data rates of 10 SPS, 5 SPS, and 2.5 SPS.
Table 4 lists the bandwidth of the data rates in FIR filter mode.
8.3.7.2.1 FIR Filter Frequency Response
0
0
-20
-20
-40
-40
Amplitude (dB)
Amplitude (dB)
Figure 55 and Figure 56 show the FIR filter attenuation at 50 Hz and 60 Hz provided by a series of response
nulls placed close to these frequencies. The response nulls are repeated at harmonics of 50 Hz and 60 Hz.
-60
-80
-100
-60
-80
-100
-120
-120
-140
-140
-160
0
30
60
90
120 150 180
Frequency (Hz)
210
240
270
300
-160
40
D011
Figure 55. FIR Frequency Response (20 SPS)
45
50
55
60
Frequency (Hz)
65
70
D012
Figure 56. FIR Frequency Response Detail (20 SPS)
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
29
ADS1235
SBAS824 – OCTOBER 2018
www.ti.com
Figure 57 is the FIR filter response at 10 SPS. As a result of the variable averager used to produce rates of 10
SPS and lower, new frequency nulls are superimposed to the response. The first null appears at the data rate. At
10 SPS, additional nulls occur at frequencies folded around multiples of 20 Hz.
0
-20
Amplitude (dB)
-40
-60
-80
-100
-120
-140
-160
0
30
60
90
120 150 180
Frequency (Hz)
210
240
270
300
D013
Figure 57. FIR Frequency Response (10 SPS)
8.3.7.3 Filter Bandwidth
The bandwidth of the digital filter depends on the data rate, filter type and order. Be aware that the bandwidth of
the entire system is the combined response of the digital filter, the antialias filter and the use of external analog
filters. Table 4 lists the bandwidth of the digital filter versus data rate and filter mode.
Table 4. Filter Bandwidth
-3-dB BANDWIDTH (Hz)
DATA RATE (SPS)
FIR
SINC1
SINC2
SINC3
SINC4
2.5
1.2
1.10
0.80
0.65
0.58
5
2.4
2.23
1.60
1.33
1.15
10
4.7
4.43
3.20
2.62
2.28
16.6
—
7.38
5.33
4.37
3.80
20
13
8.85
6.38
5.25
4.63
50
—
22.1
16.0
13.1
11.4
30
60
—
26.6
19.1
15.7
13.7
100
—
44.3
31.9
26.2
22.8
400
—
177
128
105
91.0
1200
—
525
381
314
273
2400
—
1015
751
623
544
4800
—
1798
1421
1214
1077
7200
—
2310
1972
1750
1590
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
ADS1235
www.ti.com
SBAS824 – OCTOBER 2018
8.3.7.4 50-Hz and 60-Hz Normal Mode Rejection
To reduce 50-Hz and 60-Hz noise interference, configure the data rate and filter to reject noise at 50 Hz and 60
Hz. Table 5 summarizes the 50-Hz and 60-Hz noise rejection versus data rate and filter mode. The table values
are based on 2% and 6% tolerance of signal frequency to ADC clock frequency. For the sinc filter, increase noise
rejection by increasing the order of the filter. Common-mode noise is also rejected at these frequencies.
Table 5. 50-Hz and 60-Hz Normal Mode Rejection
DIGITAL FILTER RESPONSE (dB)
DATA RATE (SPS)
FILTER TYPE
50 Hz ±2%
60 Hz ±2%
50 Hz ±6%
60 Hz ±6%
2.5
FIR
–113
–99
–88
–80
2.5
Sinc1
–36
–37
–40
–37
2.5
Sinc2
–72
–74
–80
–74
2.5
Sinc3
–108
–111
–120
–111
2.5
Sinc4
–144
–148
–160
–148
5
FIR
–111
–95
–77
–76
5
Sinc1
–34
–34
–30
–30
5
Sinc2
–68
–68
–60
–60
5
Sinc3
–102
–102
–90
–90
5
Sinc4
–136
–136
–120
–120
10
FIR
–111
–94
–73
–68
10
Sinc1
–34
–34
–25
–25
10
Sinc2
–68
–68
–50
–50
10
Sinc3
–102
–102
–75
–75
10
Sinc4
–136
–136
–100
–100
16.6
Sinc1
–34
–21
–24
–21
16.6
Sinc2
–68
–42
–48
–42
16.6
Sinc3
–102
–63
–72
–63
16.6
Sinc4
–136
–84
–96
–84
20
FIR
–95
–94
–66
–66
20
Sinc1
–18
–34
–18
–24
20
Sinc2
–36
–68
–36
–48
20
Sinc3
–54
–102
–54
–72
20
Sinc4
–72
–136
–72
–96
50
Sinc1
–34
–15
–24
–15
50
Sinc2
–68
–30
–48
–30
50
Sinc3
–102
–45
–72
–45
50
Sinc4
–136
–60
–96
–60
60
Sinc1
–13
–34
–12
–24
60
Sinc2
–27
–68
–24
–48
60
Sinc3
–40
–102
–36
–72
60
Sinc4
–53
–136
–48
–96
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
31
ADS1235
SBAS824 – OCTOBER 2018
www.ti.com
8.4 Device Functional Modes
8.4.1 Conversion Control
Conversions are controlled by either the START pin or by the START command. If using commands to control
conversions, keep the START pin low to avoid contentions between pin and commands. Commands take affect
on the 16th falling SCLK edge (CRC mode disabled) or on the 32nd falling SCLK edge (CRC mode enabled).
See Figure 4 for conversion-control timing details.
The ADC provides two conversion modes: continuous and pulse. The continuous-conversion mode performs
conversions indefinitely until stopped by the user. Pulse-conversion mode performs one conversion and then
stops. The conversion mode is programmed by the CONVRT bit (bit 4 of register MODE0).
8.4.1.1 Continuous-Conversion Mode
This conversion mode performs continuous conversions until stopped by the user. To start conversions, take the
START pin high or send the START command. DRDY is driven high at the time the conversion is initiated. DRDY
is driven low when the conversion data are ready. Conversion data are available to read at that time.
Conversions are stopped by taking the START pin low or by sending the STOP command. When conversions
are stopped, the conversion in progress runs to completion. To restart a conversion that is in progress, toggle the
START pin low-then-high or send a new START command.
8.4.1.2 Pulse-Conversion Mode
In pulse-conversion mode, the ADC performs one conversion when START is taken high or when the START
command is sent. When the conversion completes, further conversions stop. The DRDY output is driven high to
indicate the conversion is in progress, and is driven low when the conversion data are ready. Conversion data
are available to read at that time. To restart a conversion in progress, toggle the START pin low-then-high or
send a new START command. Driving START low or sending the STOP command does not interrupt the current
conversion.
8.4.1.3 Conversion Latency
The digital filter averages data from the modulator in order to produce the conversion result. The stages of the
digital filter must have settled data in order to provide fully-settled output data. The order and the decimation ratio
of the digital filter determine the amount of data averaged, and in turn, affect the latency of the conversion data.
The FIR and sinc1 filter modes are zero latency because the ADC provides the conversion result in one
conversion cycle. Latency time is an important consideration for the data throughput rate in multiplexed
applications.
Table 6 lists the conversion latency values of the ADC. Conversion latency is defined as the time from the start
of the first conversion, by taking the START pin high or sending the START command, to the time when fully
settled conversion data are ready. If the input signal is settled, then the ADC provides fully settled data. The
conversion latency values listed in the table are with the start-conversion delay parameter = 50 µs, and include
the overhead time needed to process the data. After the first conversion completes (in continuous conversion
mode), the period of the following conversions are equal to 1/fDATA. The first conversion latency in chop and acexcitation modes are twice the values listed in the table. Also when operating in these modes, the period of
following conversions are equal to the values listed in the table.
32
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
ADS1235
www.ti.com
SBAS824 – OCTOBER 2018
Device Functional Modes (continued)
Table 6. Conversion Latency
(1)
CONVERSION LATENCY - t(STDR) (1) (ms)
DATA RATE
(SPS)
FIR
SINC1
SINC2
SINC3
SINC4
2.5
402.2
400.4
800.4
1,200
1,600
5
202.2
200.4
400.4
600.4
800.4
10
102.2
100.4
200.4
300.4
400.4
16.6
—
60.43
120.4
180.4
240.4
20
52.23
50.43
100.4
150.4
200.4
50
—
20.43
40.43
60.43
80.43
60
—
17.09
33.76
50.43
67.09
100
—
10.43
20.43
30.43
40.43
400
—
2.925
5.425
7.925
10.43
1200
—
1.258
2.091
2.925
3.758
2400
—
0.841
1.258
1.675
2.091
4800
—
0.633
0.841
1.050
1.258
7200
—
0.564
0.702
0.841
0.980
Chop mode off, conversion-start delay = 50 µs (DELAY[3:0] = 0001)
If the input signal changes while free-running conversions, the conversion data are a mix of old and new data, as
shown in Figure 58. After an input change, the number of conversion periods required for fully settled data are
determined by dividing the conversion latency by the period of the data rate, plus add one conversion period to
the result. In chop and ac-bridge excitation modes, use twice the latency values listed in the table.
VIN = VAINP - VAINN
Old VIN
New VIN
Old data
Mix of old data
and new data
Fully settled
new data
DRDY pin
Figure 58. Input Change During Conversions
8.4.1.4 Start-Conversion Delay
Some applications require a delay at the start of a conversion in order to allow settling time for the PGA antialias
filter or to allow time after input and configuration changes. The ADC provides a user programmable delay time
that delays the start of a new conversion. The default value is 50 μs. 50 μs allows for settling of the antialiasing
filter placed at the PGA output. Use additional delay time as needed to provide settling time for external
components. The delay time increases the conversion latency values listed in Table 6. As an alternative to the
programmable start-conversion delay, manually delay the start of conversion after input and configuration
changes.
Start-conversion delay is an important consideration for operation in ac-bridge excitation mode. In this mode, the
reference inputs to the bridge, and therefore, the bridge output signals are reversed for each conversion. As a
result, time delay is required to allow for settling of external filter components after the bridge voltage is reversed.
As a general guideline, set the start-conversion delay parameter to a minimum of 15 times the R-C time constant
of the signal input and reference input filters.
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
33
ADS1235
SBAS824 – OCTOBER 2018
www.ti.com
8.4.2 Chop Mode
The PGA and modulator are chopper-stabilized at high frequency in order to reduce offset voltage, offset voltage
drift and 1/f noise. The offset and noise artifacts are modulated to a high frequency by the chop operation, which
are removed by the digital filter. Although chopper stabilization is designed to remove all offset, a small offset
voltage may remain. The optional global chop mode removes the remaining offset errors, providing near zero
offset voltage drift performance.
Chop mode alternates the signal polarity between consecutive conversions in order to remove offset. The ADC
subtracts consecutive, alternate-polarity conversions to yield the final conversion data. The result of subtraction
removes the offset.
CHOP[1:0] bits 6:5 of MODE1
(register address = 03h)
00: Normal mode
01: Chop mode
10: 2-wire AC-bridge excitation mode
11: 4-wire AC-bridge excitation mode
Chop Switch
VOFS
VAINP
Input
MUX
+ -
VAINN
PGA
ADC
AIN0
ADC
Digital
Filter
Chop
Control
Offset
Cal
Full-Scale
Cal
Conversion
Output
AIN5
Figure 59. ADC Chop Mode
As shown in Figure 59, the internal chop switch reverses the signal after the input multiplexer. VOFS models the
internal offset voltage. The operational sequence of chop mode is as follows:
Conversion C1: VAINP – VAINN – VOFS → First conversion withheld after start
Conversion C2: VAINN – VAINP – VOFS → Output 1 = (C1 – C2) / 2 = VAINP – VAINN
Conversion C3: VAINP – VAINN – VOFS → Output 2 =-(C3 – C2) / 2 = VAINP – VAINN
The sequence repeats for all conversions. Because of the required settling time to alternate the internal polarity,
the effective data rate in chop mode operation is reduced. The chop mode data rate is proportional to the order
of the sinc filter. Referring to Table 6, the new data rate is equal to 1 / latency values; and be aware the chop
mode first conversion latency is 2 × latency values. As a consequence of the internal data subtraction, two data
points are effectively averaged together. Averaging of data reduces noise by √2. Divide the noise data values
shown in Table 1 by √2 to derive the chop mode noise performance data. The null frequencies of the digital filter
are not changed in chop-mode operation. However, new null frequencies appear at multiples of fDATA / 2 as a
result of averaging.
8.4.3 AC-Bridge Excitation Mode
Resistive bridge sensors are excited by dc or ac voltages; or by dc or ac currents. DC voltage excitation is the
most common type of excitation. AC excitation reverses the polarity of the excitation voltage by the use of
external switching components. Similar in concept to chop mode, the result of the voltage reversal removes
offset voltage in the connections leading from the bridge to the ADC inputs. This also includes the offset voltage
of the ADC itself. The ADC provides the signals necessary to drive the external switching components in order to
reverse the bridge voltage.
The timing of the drive signals is synchronized to the ADC conversion phase. During one conversion phase, the
voltage polarity is normal. For the alternate conversion phase, the voltage polarity is reversed. The ADC
compensates the reversed polarity conversion by internal reversing the reference voltage. The ADC subtracts the
data corresponding to the normal and reverse phases in order to remove offset voltage from the input.
The ADC output drive signals are non-overlapping in order to avoid bridge cross-conduction that can otherwise
occur during excitation voltage reversal. The switch rate of the ac-excitation drive signals are performed at the
data rate to avoid unnecessary fast switching. See Figure 7 for output drive timing.
34
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
ADS1235
www.ti.com
SBAS824 – OCTOBER 2018
Table 7 shows the ac-bridge excitation drive signals and the associated GPIO pins. Program the ac-bridge
excitation mode using the CHOP[1:0] bits in register MODE1. AC-bridge excitation can be programmed for twowire or four-wire drive mode. For two-wire operation, two drive signals are provided on the GPIOs. If needed, use
two external inverters to derive four signals to drive discrete transistors. The GPIO drive levels are referred to the
5-V analog supply. Be aware that the ac-bridge excitation mode changes the nominal data rate, depending on
the order of the sinc filter. See the Chop Mode section for details of the effective data rate.
Table 7. AC-Bridge Excitation Drive Pins
DEVICE PIN
GPIO
2-WIRE MODE (CHOP[1:0] = 10)
4-WIRE MODE (CHOP[1:0] = 11)
AIN0
GPIO0
ACX1
ACX1
AIN1
GPIO1
ACX2
ACX2
AIN2
GPIO2
—
ACX1
AIN3
GPIO3
—
ACX2
8.4.4 ADC Clock Mode
Operate the ADC with an external clock or with the internal oscillator. The clock frequency is 7.3728 MHz. For
external clock operation, apply the clock signal to CLKIN. For internal-clock operation, connect CLKIN to DGND.
The internal oscillator begins operation immediately at power-up. The ADC automatically selects the clock mode
of operation. Read the clock mode bit in the STATUS register to determine the clock mode.
8.4.5 Power-Down Mode
The ADC has two power-down modes: hardware and software. In both power-down modes, the digital outputs
remain driven. The digital inputs must be maintained at VIH or VIL levels (do not float the digital inputs). The
internal low-dropout regulator remains on, drawing 25 µA (typical) from DVDD.
8.4.5.1 Hardware Power-Down
Take the PWDN pin low to engage hardware power-down mode. Except for the internal LDO, all ADC functions
are disabled. To exit hardware power-down mode (wake-up) take the PWDN pin high. The register values are
not reset at wake-up.
8.4.5.2 Software Power-Down
Set the PWDN bit (bit 7 of register MODE3) to engage software power-down mode. Similar to the operation of
hardware power-down mode, software mode powers down the internal functions except in this case the serial
interface. Exit the software power-down mode by clearing the PWDN bit. The register values are not reset.
8.4.6 Reset
The ADC is reset in three ways: at power-on, by the RESET pin, and by the RESET command. When reset, the
serial interface, conversion-control logic, digital filter, and register values are reset. The RESET bit of the
STATUS byte is set to indicate a device reset has occurred by any of the three reset methods. Clear the bit to
detect the next device reset. If the START pin is high after reset, the ADC begins conversions.
8.4.6.1 Power-on Reset
At power-on, after the supply voltages cross the reset-voltage thresholds, the ADC is reset and 216 fCLK cycles
later the ADC is ready for communication. Until this time, DRDY is held low. DRDY is driven high to indicate
when the ADC is ready for communication. If the START pin is high, the conversion cycle starts 512 / fCLK cycles
after DRDY asserts high. Figure 5 shows the power-on reset behavior.
8.4.6.2 Reset by Pin
Reset the ADC by taking the RESET pin low and then returning the pin high. After reset, the conversion starts
512 / fCLK cycles later. See Figure 6 for RESET timing.
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
35
ADS1235
SBAS824 – OCTOBER 2018
www.ti.com
8.4.6.3 Reset by Command
Reset the ADC by the RESET command. Toggle CS high to make sure the serial interface resets before sending
the command. For applications that tie CS low, see the Serial Interface Auto-Reset section for information on
how to reset the serial interface. After reset, the conversion starts 512 / fCLK cycles later. See Figure 6 for timing
details.
8.4.7 Calibration
The ADC incorporates calibration registers and associated commands to calibrate offset and full-scale errors.
Calibrate by using calibration commands, or calibrate by writing to the calibration registers directly (user
calibration). To calibrate by command, send the offset or full-scale calibration commands. To user calibrate, write
values to the calibration registers based on calculations of the conversion data. Perform offset calibration before
full-scale calibration.
8.4.7.1 Offset and Full-Scale Calibration
Use the offset and full-scale (gain) registers to correct offset or full-scale errors, respectively. As shown in
Figure 60, the offset calibration register is subtracted from the output data before multiplication by the full-scale
register, which is divided by 400000h. After the calibration operation, the final output data are clipped to 24 bits.
VAINP
+
Digital
Filter
ADC
ADC
VAINN
Output Data
Clipped to 24 bits
Final
Output
1/400000h
FSCAL[2:0] registers
(register addresses = 0Ah, 0Bh, 0Ch)
OFCAL[2:0] registers
(register addresses = 07h, 08h, 09h)
Figure 60. Calibration Block Diagram
Equation 4 shows the internal calibration.
Final Output Data = (Filter Output - OFCAL[2:0]) · FSCAL[2:0] / 400000h
(4)
8.4.7.1.1 Offset Calibration Registers
The offset calibration word is 24 bits, consisting of three 8-bit registers, as listed in Table 8. The offset value is
subtracted from the conversion result. The offset value is in two's complement format with a maximum positive
value equal to 7FFFFFh, and a maximum negative value equal to 800000h. A register value equal to 000000h
has no offset correction. Although the offset calibration register provides a wide range of possible offset values,
the input signal after calibration cannot exceed ±106% of the pre-calibrated range; otherwise, the ADC is
overranged. Table 9 lists example values of the offset register.
Table 8. Offset Calibration Registers
REGISTER
BYTE
ORDER
ADDRESS
OFCAL0
LSB
07h
B7
B6
B5
B4
B3
B2
B1
OFCAL1
MID
08h
B15
B14
B13
B12
B11
B10
B9
B8
OFCAL2
MSB
09h
B23 (MSB)
B22
B21
B20
B19
B18
B17
B16
BIT ORDER
B0 (LSB)
Table 9. Offset Calibration Register Values
(1)
36
OFCAL[2:0] REGISTER VALUE
IDEAL OUTPUT VALUE (1)
000001h
FFFFFFh
000000h
000000h
FFFFFFh
000001h
Output value with no offset error
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
ADS1235
www.ti.com
SBAS824 – OCTOBER 2018
8.4.7.1.2 Full-Scale Calibration Registers
The full-scale calibration word is 24 bits consisting of three 8-bit registers, as listed in Table 10. The full-scale
calibration value is in straight-binary format, normalized to a unity-gain factor at a value of 400000h. Table 11
lists register values for selected gain factors. Gain errors greater than unity are corrected by using full-scale
values less than 400000h. Although the full-scale register provides a wide range of possible values, the input
signal after calibration must not exceed ±106% of the precalibrated input range; otherwise, the ADC is
overranged.
Table 10. Full-Scale Calibration Registers
REGISTER
BYTE
ORDER
ADDRESS
FSCAL0
LSB
0Ah
B7
B6
B5
B4
B3
B2
B1
FSCAL1
MID
0Bh
B15
B14
B13
B12
B11
B10
B9
B8
FSCAL2
MSB
0Ch
B23 (MSB)
B22
B21
B20
B19
B18
B17
B16
BIT ORDER
B0 (LSB)
Table 11. Full-Scale Calibration Register Values
FSCAL[2:0] REGISTER VALUE
GAIN FACTOR
433333h
1.05
400000h
1.00
3CCCCCh
0.95
8.4.7.2 Offset Self-Calibration (SFOCAL)
The offset self-calibration command corrects offset errors internal to the ADC. When the offset self-calibration
command is sent, the ADC disconnects the external inputs, shorts the inputs to the PGA, and then averages 16
conversion results to compute the calibration value. Averaging the data reduces conversion noise to improve
calibration accuracy. When calibration is complete, the ADC restores the user input and performs one conversion
using the new calibration value.
8.4.7.3 Offset System-Calibration (SYOCAL)
The offset system-calibration command corrects system offset errors. For this type of calibration, the user shorts
the inputs to either the ADC or to the system. When the command is sent, the ADC averages 16 conversion
results to compute the calibration value. Averaging the data reduces conversion noise to improve calibration
accuracy. When calibration is complete, the ADC performs one conversion using the new calibration value.
8.4.7.4 Full-Scale Calibration (GANCAL)
The full-scale calibration command corrects gain error. To calibrate, apply a positive full-scale calibration voltage
to the ADC, wait for the signal to settle, and then send the calibration command. The ADC averages 16
conversion results to compute the calibration value. Averaging the data reduces conversion noise to improve
calibration accuracy. The ADC computes the full-scale calibration value so that the calibration voltage is scaled
to positive full scale output code. When calibration is complete, the ADC performs one new conversion using the
new calibration value.
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
37
ADS1235
SBAS824 – OCTOBER 2018
www.ti.com
8.4.7.5 Calibration Command Procedure
Use the following procedure to calibrate using commands. The register-lock mode must be UNLOCK for all
calibration commands. After power-on, make sure the reference voltage has stabilized before calibrating.
Perform offset calibration before full-scale calibration.
1. Configure the ADC as required.
2. Apply the appropriate calibration signal (zero or full-scale)
3. Take the START pin high or send the START command to start conversions. DRDY is driven high.
4. Before the conversion cycle completes, send the calibration command. Keep CS low otherwise the command
is cancelled. Send no other commands during the calibration period.
5. Calibration time depends on the data rate and digital filter mode. See Table 12. DRDY asserts low when
calibration is complete. The offset or full-scale calibration registers are updated with new values. At
calibration completion, new conversion data are ready using the new calibration value.
Table 12. Calibration Time (ms)
(1)
FILTER MODE
(1)
DATA RATE
(SPS)
FIR
SINC1
SINC2
SINC3
SINC4
2.5
6805
6801
7601
8401
9201
5
3405
3401
3801
4201
4601
10
1705
1701
1901
2101
2301
16.6
—
1021
1141
1261
1381
20
854.5
850.9
951.0
1051
1151
50
—
340.9
380.9
420.9
460.9
60
—
284.2
317.5
350.9
384.2
100
—
170.9
190.9
210.9
230.9
400
—
43.36
48.36
53.36
58.36
1200
—
15.02
16.69
18.36
20.02
2400
—
7.938
8.772
9.605
10.44
4800
—
4.397
4.813
5.230
5.647
7200
—
3.216
3.494
3.772
4.050
Nominal clock frequency. Chop and AC-Excitation modes disabled.
8.4.7.6 User Calibration Procedure
To user calibrate, apply the calibration voltage, acquire conversion data, and compute the calibration value. The
computed value is written to the corresponding calibration registers. Before starting calibration, preset the offset
and full-scale registers to 000000h and 400000h, respectively.
To offset calibrate, short the ADC inputs (or inputs to the system) and average n number of the conversion
results. Averaging conversion data reduces noise to improve calibration accuracy. Write the averaged value of
the conversion data to the offset registers.
To gain calibrate using a full scale calibration voltage, temporarily reduce the full scale register 95% to avoid
output clipped codes (set FSCAL[2:0] to 3CCCCCh). Acquire n number of conversions and average the
conversions to reduce noise to improve calibration accuracy. Compute the full-scale calibration value as shown
in Equation 5:
Full-Scale Calibration Value = Expected Code / Actual Code · 400000h
where
•
38
Expected code = 799998h using full scale calibration signal and 95% scale factor
Submit Documentation Feedback
(5)
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
ADS1235
www.ti.com
SBAS824 – OCTOBER 2018
8.5 Programming
8.5.1 Serial Interface
The serial interface is SPI-compatible and is used to read conversion data, configure registers, and control the
ADC. The serial interface consists of four control lines: CS, SCLK, DIN, and DOUT/DRDY. Most microcontroller
SPI peripherals can operate with the ADC. The interface operates in SPI mode 1, where CPOL = 0 and CPHA =
1. In SPI mode 1, SCLK idles low and data are updated or changed on SCLK rising edges; data are latched or
read on SCLK falling edges. Timing details of the SPI protocol are found in Figure 1 and Figure 2.
8.5.1.1 Chip Select (CS)
CS is an active-low input that selects the serial interface for communication. CS must be low during the entire
data transaction. When CS is taken high, the serial interface resets, SCLK input activity is ignored (blocking
commands), and DOUT/DRDY enters the high-impedance state. The operation of DRDY is not effected by CS. If
the ADC is a single device connected to the serial bus, CS can be tied low in order to reduce the serial interface
to three lines.
8.5.1.2 Serial Clock (SCLK)
SCLK is the serial clock input that shifts data into and out of the ADC. Output data are updated on the rising
edge of SCLK and input data are latched on the falling edge of SCLK. Return SCLK low after the data operation
is completed. SCLK is a Schmidt-triggered input designed to improve noise immunity. Even though SCLK is
noise resistant, keep SCLK as noise-free as possible to avoid unintentional SCLK transitions. Avoid ringing and
overshoot on the SCLK input. Place a series termination resistor close to the SCLK drive pin to reduce ringing.
8.5.1.3 Data Input (DIN)
DIN is the serial data input to the ADC. DIN is used to input commands and register data to the ADC. Data are
latched on the falling edge of SCLK.
8.5.1.4 Data Output/Data Ready (DOUT/DRDY)
The DOUT/DRDY pin is a dual-function output. The functions of this pin are data output and data ready. The
functionality changes automatically based on whether a read data operation is in progress. During a read data
operation, the functionality is data output. After the read operation is complete, the functionality changes to data
ready.
In data output mode, data are updated on the SCLK rising edge, therefore the host latches the data on the falling
edge of SCLK. In data-ready mode, the pin functions the same as DRDY (if CS is low) by asserting low when
data are ready. Therefore, monitor either DOUT/DRDY or DRDY to determine when data are ready. When CS is
high, the DOUT/DRDY pin is in the high-impedance mode (tri-state).
8.5.1.5 Serial Interface Auto-Reset
The serial interface is reset by taking CS high. Applications that tie CS low do not have the ability to reset the
serial interface by CS. If a false SCLK occurs (for example, caused by a noise pulse or clocking glitch), the serial
interface may inadvertently advance one or more bit positions, resulting in loss of synchronization to the host. If
loss of synchronization occurs, the ADC interface does not respond correctly until the interface is reset.
For applications that tie CS low, the serial interface auto-reset feature recovers the interface in the event that an
unintentional SCLK glitch occurs. When the first SCLK low-to-high transition occurs (either caused by a glitch or
by normal SCLK activity), seven SCLK transitions must occur within 65536 fCLK cycles (8.9 ms) to complete the
byte transaction, otherwise the serial interface resets. After reset, the interface is ready to begin the next byte
transaction. If the byte transaction is completed within the 65536 fCLK cycles, the serial interface does not reset.
The cycle of SCLK detection re-starts at the next rising edge of SCLK. The serial interface is reset by holding
SCLK low for a minimum 65536 fCLK cycles.
The auto-reset function is enabled by the SPITIM bit (default is off). See Figure 3 for timing details.
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
39
ADS1235
SBAS824 – OCTOBER 2018
www.ti.com
Programming (continued)
8.5.2 Data Ready (DRDY)
DRDY is an output that asserts low when conversion data are ready. After power-up, DRDY also indicates when
the ADC is ready for communication. The operation of DRDY depends on the conversion mode (continuous or
pulse) and whether the conversion data are retrieved or not. Figure 61 shows DRDY operation with and without
data retrieval in the two modes of conversion.
DRDY - with data retrieval
(Continuous-conversion mode)
DRDY ± w/o data retrieval
(Continuous-conversion mode)
DRDY ± w or w/o data retreival
(Pulse-conversion mode)
START
Command
START
STOP
START
STOP
Figure 61. DRDY Operation
8.5.2.1 DRDY in Continuous-Conversion Mode
In continuous-conversion mode, DRDY is driven high when conversions are started and is driven low when
conversion data are ready. During data readback, DRDY returns high at the end of the read operation. If the
conversion data are not read, DRDY pulses high 16 fCLK cycles prior to the next falling edge.
To read conversion data before the next conversion is ready, send the complete read-data command 16 fCLK
cycles before the next DRDY falling edge. If the readback command is sent less than 16 fCLK cycles before the
DRDY falling edge, either old or new conversion data are provided, depending on the timing of when the
command is sent. In the case that old conversion data are provided, DRDY driven low is delayed until after the
read data operation is completed. In this case, the DRDY bit of the STATUS byte is cleared to indicate the same
data have been read. If new conversion data are provided, DRDY transitions low at the normal period of the data
rate. In this case, the DRDY bit of the STATUS byte is set to indicate that new data have been read. To make
sure new data are read back, wait until DRDY asserts low before starting the data read operation.
8.5.2.2 DRDY in Pulse-Conversion Mode
DRDY is driven high at conversion start and is driven low when the conversion data are ready. During the data
read operation DRDY remains low until a new conversion is started.
8.5.2.3 Data Ready by Software Polling
Use software polling of data ready in lieu of hardware polling of DRDY or DOUT/DRDY. To software poll, read
the STATUS register and poll the DRDY bit. In order to not skip conversion data in continuous conversion mode,
poll the bit at least as often as the period of the data rate. If the DRDY bit is set, then conversion data are new
since the previous data read operation. If the bit is cleared, conversion data are not new since the previous data
read operation. In this case, the previous conversion data are returned.
8.5.3 Conversion Data
Conversion data are read by the RDATA command. To read data, take CS low and issue the read data
command. The data field response consists of the optional STATUS byte, three data bytes, and the optional
CRC byte. The CRC is computed over the combination of status byte and conversion data bytes. See the
RDATA Command for details to read conversion data.
40
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
ADS1235
www.ti.com
SBAS824 – OCTOBER 2018
Programming (continued)
8.5.3.1 Status byte (STATUS)
The status byte contains information on the operating state of the ADC. The STATUS byte is included with the
conversion data by enabling bit STATENB of register MODE3. Optionally, read the STATUS register to directly
determine status information without the need to read conversion data. See Figure 67 for details.
8.5.3.2 Conversion Data Format
The conversion data are 24 bits, in two's-complement format to represent positive and negative values. The data
output begins with the most significant bit (sign bit) first. The data are scaled so that VIN = 0 V results in an
uncalibrated code value of 000000h; positive full scale equals 7FFFFFh and negative full scale equals 800000h;
see Table 13 for the uncalibrated code values. The data are clipped to 7FFFFFh (positive full scale) and
800000h (negative full scale) during positive and negative signal overdrive, respectively.
Table 13. ADC Conversion Data Codes
DESCRIPTION
Positive Full Scale
1 LSB
INPUT SIGNAL (V)
23
≥ VREF / Gain · (2
000001h
000000h
23
–VREF / (Gain · 2
)
≤ –VREF / Gain
(1)
7FFFFFh
0
Negative Full Scale
(1)
- 1) / 2
VREF / (Gain · 223 )
Zero scale
-1 LSB
24-BIT CONVERSION DATA
23
FFFFFFh
800000h
Ideal (calibrated) conversion data
8.5.4 CRC
Cyclic redundancy check (CRC) is an error checking code that detects communication errors to and from the
host. CRC is the division remainder of the data payload bytes by a fixed polynomial. The data payload is 1, 2, 3
or 4 bytes depending on the data operation. The CRC mode is optional and is enabled by the CRCENB bit. See
Table 33 to program the CRC mode.
The user computes the CRC corresponding to the two command bytes and appends the CRC to the command
string (3rd byte). A 4th, zero-value byte completes the command field. The ADC repeats the CRC calculation and
compares the calculation to the received CRC. If the user and repeated CRC values match, the command
executes and the ADC responds by transmitting the repeated CRC during the 4th byte of the command. If the
operation is conversion data or register data read, the ADC responds with a 2nd CRC that is computed over the
requested data payload bytes. The response data payload is 1, 3, or 4 bytes depending on the data operation.
If the user and repeated CRC values do not match, the command does not execute and the ADC responds with
an inverted CRC for the actual received command bytes. The inverted CRC is intended to signal the host of the
failed operation. The user terminates transmission of the command bytes to match the action of ADC termination.
The CRCERR bit is set in the STATUS register when a CRC error is detected. The ADC is ready to accept the
next command after a CRC error occurs at the end of the 4th byte.
The CRC data byte is the 8-bit remainder of the bitwise exclusive-OR (XOR) operation of the argument by a
CRC polynomial. The CRC polynomial is based on the CRC-8-ATM (HEC): X8 + X2 + X0 + 1. The nine binary
polynomial coefficients are: 100000111. The CRC calculation is preset with "1" data values.
The CRC mnemonics apply to the following command sections.
• CRC-2: Input CRC of command bytes 1 and 2. Except for WREG command, the value of byte 2 is arbitrary
• Out CRC-1: Output CRC of one register data byte
• Out CRC-2: Output CRC of two command bytes, inverted value if input CRC error detected
• Out CRC-3: Output CRC of three conversion data bytes
• Out CRC-4: Output CRC of three conversion data bytes plus STATUS byte
• Echo Byte 1: Echo of received input byte 1
• Echo Byte 2: Echo of received input byte 2
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
41
ADS1235
SBAS824 – OCTOBER 2018
www.ti.com
8.5.5 Commands
Commands read conversion data, control the ADC, and read and write register data. See Table 14 for the list of
commands. Send only the commands that are listed in Table 14. The ADC executes commands at completion of
the 2nd byte (no CRC verification) or at completion of the 4th byte (with CRC verification). Follow the two byte or
four byte format according to the CRC mode. Except for register write commands, the value of the second
command byte is arbitrary but the value is included in the CRC calculation (total of two-byte CRC). If a CRC error
is detected, the ADC does not execute the command. Taking CS high before the command is completed results
in termination of the command. When CS is taken low, the communication frame is reset to begin a new
command.
Table 14. Command Byte Summary
MNEMONIC
DESCRIPTION
BYTE 1
BYTE 2
BYTE 3
(CRC Mode Only)
BYTE 4
(CRC Mode only)
Control Commands
NOP
No operation
00h
Arbitrary
CRC-2
00h
RESET
Reset
06h
Arbitrary
CRC-2
00h
START
Start conversion
08h
Arbitrary
CRC-2
00h
STOP
Stop conversion
0Ah
Arbitrary
CRC-2
00h
12h
Arbitrary
CRC-2
00h
Read Data Command
RDATA
Read conversion data
Calibration Commands
SYOCAL
System offset calibration
16h
Arbitrary
CRC-2
00h
GANCAL
Gain calibration
17h
Arbitrary
CRC-2
00h
SFOCAL
Self offset calibration
19h
Arbitrary
CRC-2
00h
Register Commands
RREG
Read register data
20h + rrh (1)
Arbitrary
CRC-2
00h
WREG
Write register data
40h + rrh (1)
Register data
CRC-2
00h
Protection Commands
LOCK
Register lock
F2h
Arbitrary
CRC-2
00h
UNLOCK
Register unlock
F5h
Arbitrary
CRC-2
00h
(1)
rrh = 5-bit register address.
8.5.5.1 NOP Command
This command is no operation. Use the NOP command to validate the CRC response byte and error detection
without affecting normal operation. Table 15 shows the NOP command byte sequence.
Table 15. NOP Command
DIRECTION
BYTE 1
BYTE 2
BYTE 3
BYTE 4
No CRC mode
DIN
00h
Arbitrary
DOUT/DRDY
FFh
Echo byte 1
CRC mode
42
DIN
00h
Arbitrary
CRC-2
00h
DOUT/DRDY
FFh
Echo byte 1
Echo byte 2
Out CRC-2
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
ADS1235
www.ti.com
SBAS824 – OCTOBER 2018
8.5.5.2 RESET Command
The RESET command resets ADC operation and resets the registers to default values. See the Reset by
Command section for details. Table 16 shows the RESET command byte sequence.
Table 16. RESET Command
DIRECTION
BYTE 1
BYTE 2
BYTE 3
BYTE 4
DIN
06h
Arbitrary
DOUT/DRDY
FFh
Echo byte 1
DIN
06h
Arbitrary
CRC-2
00H
DOUT/DRDY
FFh
Echo byte 1
Echo byte 2
Out CRC-2
No CRC mode
CRC mode
8.5.5.3 START Command
This command starts conversions. See the Conversion Control section for details. Table 17 shows the START
command byte sequence.
Table 17. START Command
DIRECTION
BYTE 1
BYTE 2
BYTE 3
BYTE 4
No CRC mode
DIN
08h
Arbitrary
DOUT/DRDY
FFh
Echo byte 1
DIN
08h
Arbitrary
CRC-2
00h
DOUT/DRDY
FFh
Echo byte 1
Echo byte 2
Out CRC-2
CRC mode
8.5.5.4 STOP Command
This command stops conversions. See the Conversion Control section for details. Table 18 shows the STOP
command byte sequence.
Table 18. STOP Command
DIRECTION
BYTE 1
BYTE 2
BYTE 3
BYTE 4
DIN
0Ah
Arbitrary
DOUT/DRDY
FFh
Echo byte 1
DIN
0Ah
Arbitrary
CRC-2
00h
DOUT/DRDY
FFh
Echo byte 1
Echo byte 2
Out CRC-2
No CRC mode
CRC mode
8.5.5.5 RDATA Command
This command reads conversion data. Because the data are buffered, the data can be read at any time during
the conversion phase. If data are read near the completion of the next conversion, old or new conversion data
are returned. See the Data Ready (DRDY) section for details.
The response of conversion data varies in length from 3 to 5 bytes depending if the STATUS byte and CRC
bytes are included. See the Conversion Data Format section for the numeric data format. See Table 19,
Figure 62 (minimum configuration) and Figure 63 (maximum configuration) for operation of the RDATA
command.
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
43
ADS1235
SBAS824 – OCTOBER 2018
www.ti.com
Table 19. RDATA Command
DIRECTION
BYTE 1
BYTE 2
BYTE 3
BYTE 4
BYTE 5
BYTE 6
BYTE 7
BYTE 8
BYTE 9
No CRC mode
DIN
12h
Arbitrary
DOUT/DRDY
FFh
Echo byte 1
00h
DIN
12h
Arbitrary
CRC-2
00h
DOUT/DRDY
FFh
Echo byte 1
Echo byte 2
Out CRC-2
STATUS
(1)
00h
00h
00h
MSB data
MID data
LSB data
CRC mode
(1)
00h
STATUS
(1)
00h
00h
00h
00h
MSB data
MID data
LSB data
Out CRC-3 or
Out CRC-4
Optional STATUS byte
CS
(1)
1
9
17
25
33
SCLK
DIN
12h
FFh
DOUT/DRDY
(1)
00h
00h
00h
MSB data
MID data
LSB data
Arbitrary
Echo byte 1
CS can be tied low
Figure 62. Conversion Data Read Operation (STATUS Byte and CRC Mode Disabled)
(1)
CS
1
9
17
25
41
33
49
57
65
SCLK
DIN
12h
DOUT/DRDY
FFh
A.
Arbitrary
Echo byte 1
CRC-2
00h
00h
Echo byte 2
Out CRC-2
STATUS
00h
00h
MID data
MSB data
00h
00h
LSB DATA
Out CRC-4
CS can be tied low
Figure 63. Conversion Data Read Operation (STATUS Byte and CRC Mode Enabled)
8.5.5.6 SYOCAL Command
This command is used for system offset calibration. See the Offset System-Calibration (SYOCAL) section for
details. Table 20 shows the SYOCAL command byte sequence.
Table 20. SYOCAL Command
DIRECTION
BYTE 1
BYTE 2
BYTE 3
BYTE 4
DIN
16h
Arbitrary
DOUT/DRDY
FFh
Echo byte 1
DIN
16h
Arbitrary
CRC-2
00h
DOUT/DRDY
FFh
Echo byte 1
Echo Byte 2
Out CRC-2
No CRC mode
CRC mode
44
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
ADS1235
www.ti.com
SBAS824 – OCTOBER 2018
8.5.5.7 GANCAL Command
This command is for gain calibration. See the Calibration section for details. Full-Scale Calibration (GANCAL)
shows the GANCAL command byte sequence.
Table 21. GANCAL Command
DIRECTION
BYTE 1
BYTE 2
BYTE 3
BYTE 4
DIN
17h
Arbitrary
DOUT/DRDY
FFh
Echo byte 1
DIN
17h
Arbitrary
CRC-2
00h
DOUT/DRDY
FFh
Echo byte 1
Echo Byte 2
Out CRC-2
No CRC mode
CRC mode
8.5.5.8 SFOCAL Command
This command is used for self offset calibration. See the Offset Self-Calibration (SFOCAL) section for details.
Table 22 shows the SFOCAL command byte sequence.
Table 22. SFOCAL Command
DIRECTION
BYTE 1
BYTE 2
BYTE 3
BYTE 4
No CRC mode
DIN
19h
Arbitrary
DOUT/DRDY
FFh
Echo byte 1
DIN
19h
Arbitrary
CRC-2
00h
DOUT/DRDY
FFh
Echo byte 1
Echo Byte 2
Out CRC-2
CRC mode
8.5.5.9 RREG Command
Use the RREG command to read register data. The register data are read one byte at a time by issuing the
RREG command for each operation. Add the register address (rrh) to the base opcode (20h) to construct the
command byte (20h + rrh). Table 23 shows the command byte sequence. The ADC responds with the register
data byte, most significant bit first. The response to registers outside the valid address range is 00h. Figure 64
shows an example of the register read operation. The Out CRC-1 byte is the CRC calculated for the register data
byte.
Table 23. RREG Command
DIRECTION
BYTE 1
BYTE 2
BYTE 3
BYTE 4
BYTE 5
BYTE 6
DIN
20h + rrh (1)
Arbitrary
00h
DOUT/DRDY
FFh
Echo byte 1
Register data
DIN
20h + rrh
Arbitrary
CRC-2
00h
00h
00h
DOUT/DRDY
FFh
Echo byte 1
Echo byte 2
Out CRC-2
Register data
Out CRC-1
No CRC mode
CRC mode
(1)
rrh = 5-bit register address
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
45
ADS1235
SBAS824 – OCTOBER 2018
www.ti.com
(1)
CS
1
9
17
25
33
41
SCLK
DIN
22h
Arbitrary
CRC-2
00h
DOUT/DRDY
FFh
Echo byte 1
Echo byte 2
Out CRC-2
(1)
00h
00h
Reg data
Out CRC-1
CS can be tied low
Figure 64. Register Read Operation (address = 02h, CRC Mode Enabled)
8.5.5.10 WREG Command
Use the WREG command to write register data. The register data are written one byte at a time by issuing the
WREG command for each operation. Add the register address (rrh) to the base opcode (40h) to construct the
command byte (40h + rrh). Table 24 shows the command byte sequence. Figure 65 shows an example of the
WREG operation. Be aware that writing to certain registers results in conversion restart. Table 27 lists the
registers that restart an ongoing conversion when written to. Do not write to registers outside the address range.
Table 24. WREG Command
DIRECTION
BYTE 1
BYTE 2
DIN
40h + rrh (1)
Register data
DOUT/DRDY
FFh
Echo byte 1
BYTE 3
BYTE 4
No CRC mode
CRC mode
(1)
DIN
40h + rrh
Register data
CRC-2
00h
DOUT/DRDY
FFh
Echo byte 1
Echo byte 2
Out CRC-2
rrh = 5-bit register address.
(1)
CS
1
9
17
25
SCLK
DIN
42h
DOUT/DRDY
A.
FFh
Reg Data
CRC-2
00h
Echo byte 1
Echo byte 2
Out CRC-2
CS can be tied low
Figure 65. Register Write Operation (address = 02h, CRC Mode Enabled)
46
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
ADS1235
www.ti.com
SBAS824 – OCTOBER 2018
8.5.5.11 LOCK Command
The LOCK command locks-out write access to the registers including the calibration registers that are changed
by calibration commands. The default mode is UNLOCK. Read access is allowed in LOCK mode. Table 25
shows the LOCK command byte sequence.
Table 25. LOCK Command
DIRECTION
BYTE 1
BYTE 2
BYTE 3
BYTE 4
No CRC mode
DIN
F2h
Arbitrary
DOUT/DRDY
FFh
Echo byte 1
CRC mode
DIN
F2h
Arbitrary
CRC-2
00h
DOUT/DRDY
FFh
Echo byte 1
Echo Byte2
out CRC-2
8.5.5.12 UNLOCK Command
The UNLOCK command allows register write access, including access to the contents of the calibration registers
that can be changed by the calibration commands. Table 26 shows the UNLOCK command byte sequence.
Table 26. UNLOCK Command
DIRECTION
BYTE 1
BYTE 2
BYTE 3
BYTE 4
No CRC mode
DIN
F5h
Arbitrary
DOUT/DRDY
FFh
Echo byte 1
DIN
F5h
Arbitrary
CRC-2
00h
DOUT/DRDY
FFh
Echo byte 1
Echo Byte2
Out CRC-2
CRC mode
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
47
ADS1235
SBAS824 – OCTOBER 2018
www.ti.com
8.6 Register Map
The register map consists of 18, one-byte registers. Collectively, the registers are used to configure the ADC to
the desired operating mode. Access the registers by using the RREG and WREG (read-register and writeregister) commands. Register data are accessed one register byte at a time for each command operation. At
power-on or device reset, the registers are reset to the default values, as shown in the Default column of
Table 27. Writing new data to certain registers causes the ADC conversion in progress to restart. The affected
registers are listed in the Restart column in Table 27.
Register-write access is enabled or disabled by the UNLOCK and LOCK commands, respectively. The default
mode is register UNLOCK. See the LOCK Command section for more details.
Table 27. Register Map Summary
(rrh)
REGISTER
DEFAULT
RESTART
00h
ID
Cxh
BIT 7
BIT 6
BIT 5
BIT 4
BIT 3
01h
STATUS
01h
02h
MODE0
24h
Yes
0
03h
MODE1
01h
Yes
0
04h
MODE2
00h
05h
MODE3
00h
06h
REF
05h
07h
OFCAL0
00h
08h
OFCAL1
00h
OFC[15:8]
09h
OFCAL2
00h
OFC[23:16]
0Ah
FSCAL0
00h
FSC[7:0]
0Bh
FSCAL1
00h
FSC[15:8]
BIT 2
DEV_ID[3:0]
LOCK
CRCERR
PGAL_ALM
PGAH_ALM
REFL_ALM
DRDY
DR[3:0]
BIT 0
CHOP[1:0]
CLOCK
RESET
FILTER[2:0]
CONVRT
DELAY[3:0]
GPIO_CON[3:0]
GPIO_DIR[3:0]
PWDN
STATENB
CRCENB
SPITIM
0
0
0
0
Yes
BIT 1
REV_ID[3:0]
GPIO_DAT[3:0]
RMUXP[1:0]
RMUXN[1:0]
OFC[7:0]
0Ch
FSCAL2
40h
FSC[23:16]
0Dh
RESERVED
FFh
FFh
0Eh
RESERVED
00h
00h
0Fh
RESERVED
00h
10h
PGA
00h
Yes
11h
INPMUX
FFh
Yes
00h
BYPASS
0
0
0
0
MUXP[3:0]
GAIN[2:0]
MUXN[3:0]
8.6.1 Device Identification (ID) Register (address = 00h) [reset = Cxh]
Figure 66. ID Register
7
6
5
4
3
2
DEV_ID[3:0]
1
0
REV_ID[3:0]
NOTE: Reset values are device dependent
LEGEND: R/W = Read/Write; R = Read only; -n = value after reset
Table 28. ID Register Field Descriptions
Bit
Field
Type
Reset
Description
7:4
DEV_ID[3:0]
R
Ch
Device ID
3:0
REV_ID[3:0]
R
xh
1100
Revision ID
Note: Revision ID can change without notification
48
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
ADS1235
www.ti.com
SBAS824 – OCTOBER 2018
8.6.2 Device Status (STATUS) Register (address = 01h) [reset = 01h]
Figure 67. STATUS Register
7
LOCK
R-0h
6
CRCERR
R/W-0h
5
PGAL_ALM
R-0h
4
PGAH_ALM
R-0h
3
REFL_ALM
R-0h
2
DRDY
R-0h
1
CLOCK
R-xh
0
RESET
R/W-1h
LEGEND: R/W = Read/Write; R = Read only; -n = value after reset
Table 29. STATUS Register Field Descriptions
Bit
Field
Type
Reset
Description
7
LOCK
R
0h
Register Lock Status
Indicates register lock status. Register writes are locked by the
LOCK command and unlocked by the UNLOCK command.
0: Register write not locked (default)
1: Register write locked
6
CRCERR
R/W
0h
CRC Error
Indicates that a CRC error is detected by the ADC. The CRC
error bit remains set until cleared by the user.
0: No CRC error
1: CRC error
5
PGAL_ALM
R
0h
PGA Low Alarm
Indicates PGA output voltage is below the low limit. The alarm
resets at the start of conversion cycles.
0: No Alarm
1: Alarm
4
PGAH_ALM
R
0h
PGA High Alarm
Indicates PGA output voltage is above the high limit. The alarm
resets at the start of conversion cycles.
0: No Alarm
1: Alarm
3
REFL_ALM
R
0h
Reference Low Alarm
Indicates reference voltage is below the low limit. The alarm
resets at the start of conversion cycles.
0: No Alarm
1: Alarm
2
DRDY
R
0h
Data Ready
Indicates conversion data ready.
0: Conversion data not new since the previous read operation
1: Conversion data new since the previous read operation
1
CLOCK
R
xh
Clock
Indicates internal or external clock mode. The ADC automatically
selects the clock source.
0: ADC clock is internal
1: ADC clock is external
0
RESET
R/W
1h
Reset
Indicates ADC reset. Clear the bit to detect next device reset.
0: No reset
1: Reset (default)
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
49
ADS1235
SBAS824 – OCTOBER 2018
www.ti.com
8.6.3 Mode 0 (MODE0) Register (address = 02h) [reset = 24h]
Figure 68. MODE0 Register
7
0
R/W-0h
6
5
4
3
2
DR[3:0]
R/W-4h
1
FILTER[2:0]
R/W-4h
0
LEGEND: R/W = Read/Write; R = Read only; -n = value after reset
Table 30. MODE0 Register Field Descriptions
Bit
7
Field
Type
Reset
Description
0
R/W
0h
Reserved
DR[3:0]
R/W
4h
Always write 0
6:3
Data Rate
Select the ADC data rate.
0000: 2.5 SPS
0001: 5 SPS
0010: 10 SPS
0011: 16.6 SPS
0100: 20 SPS (default)
0101: 50 SPS
0110: 60 SPS
0111: 100 SPS
1000: 400 SPS
1001: 1200 SPS
1010: 2400 SPS
1011: 4800 SPS
1100: 7200 SPS
1101 - 1111: Reserved
2:0
FILTER[2:0]
R/W
4h
Digital Filter
Select the digital filter mode.
000: sinc1
001: sinc2
010: sinc3
011: sinc4
100: FIR (default)
101 - 111: Reserved
50
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
ADS1235
www.ti.com
SBAS824 – OCTOBER 2018
8.6.4 Mode 1 (MODE1) Register (address = 03h) [reset = 01h]
Figure 69. MODE1 Register
7
0
R/W-0h
6
5
4
CONVRT
R/W-0h
CHOP[1:0]
R/W-0h
3
2
1
0
DELAY[3:0]
R/W-1h
LEGEND: R/W = Read/Write; R = Read only; -n = value after reset
Table 31. MODE1 Register Field Descriptions
Bit
7
Field
Type
Reset
Description
0
R/W
0h
Reserved
CHOP[1:0]
R/W
0h
Always write 0
6:5
Chop and AC-Bridge Excitation Modes
Select the Chop and ac-bridge excitation modes.
00: Normal mode (default)
01: Chop mode
10: 2-wire ac-bridge excitation mode
11: 4-wire ac-bridge excitation mode
4
CONVRT
R/W
0h
ADC Conversion Mode
Select the ADC conversion mode.
0: Continuous conversions (default)
1: Pulse (one shot) conversion
3:0
DELAY[3:0]
R/W
1h
Conversion Start Delay
Program the time delay at conversion start. Delay values are
with fCLK = 7.3728 MHz.
0000: 0 µs
0001: 50 µs (default)
0010: 59 µs
0011: 67 µs
0100: 85 µs
0101: 119 µs
0110: 189 µs
0111: 328 µs
1000: 605 µs
1001: 1.16 ms
1010: 2.27 ms
1011: 4.49 ms
1100: 8.93 ms
1101: 17.8 ms
1110, 1111: Reserved
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
51
ADS1235
SBAS824 – OCTOBER 2018
www.ti.com
8.6.5 Mode 2 (MODE2) Register (address = 04h) [reset = 00h]
Figure 70. MODE2 Register
7
6
5
4
3
2
GPIO_CON[3:0]
R/W-0h
1
0
GPIO_DIR[3:0]
R/W-0h
LEGEND: R/W = Read/Write; R = Read only; -n = value after reset
Table 32. MODE2 Register Field Descriptions
Bit
7
Field
Type
Reset
Description
GPIO_CON[3]
R/W
0h
GPIO3 Pin Connection
Connect GPIO3 to analog input AIN3.
0: GPIO3 not connected to AIN3 (default)
1: GPIO3 connected to AIN3
6
GPIO_CON[2]
R/W
0h
GPIO2 Pin Connection
Connect GPIO2 to analog input AIN2.
0: GPIO2 not connected to AIN2 (default)
1: GPIO2 connected to AIN2
5
GPIO_CON[1]
R/W
0h
GPIO1 Pin Connection
Connect GPIO1 to analog input AIN1.
0: GPIO1 not connected to AIN1 (default)
1: GPIO1 connected to AIN1
4
GPIO_CON[0]
R/W
0h
GPIO0 Pin Connection
Connect GPIO0 to analog input AIN0
0: GPIO0 not connected to AIN0 (default)
1: GPIO0 connected to AIN0
3
GPIO_DIR[3]
R/W
0h
GPIO3 Pin Direction
Configure GPIO3 as a GPIO input or GPIO output on AIN3.
0: GPIO3 is an output (default)
1: GPIO3 is an input
2
GPIO_DIR[2]
R/W
0h
GPIO2 Pin Direction
Configure GPIO2 as a GPIO input or GPIO output on AIN2.
0: GPIO2 is an output (default)
1: GPIO2 is an input
1
GPIO_DIR[1]
R/W
0h
GPIO1 Pin Direction
Configure GPIO1 as a GPIO input or GPIO output on AIN1.
0: GPIO1 is an output (default)
1: GPIO1 is an input
0
GPIO_DIR[0]
R/W
0h
GPIO0 Pin Direction
Configure GPIO0 as a GPIO input or GPIO output on AIN0.
0: GPIO0 is an output (default)
1: GPIO0 is an input
52
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
ADS1235
www.ti.com
SBAS824 – OCTOBER 2018
8.6.6 Mode 3 (MODE3) Register (address = 05h) [reset = 00h]
Figure 71. MODE3 Register
7
PWDN
R/W-0h
6
STATENB
R/W-0h
5
CRCENB
R/W-0h
4
SPITIM
R/W-0h
3
2
1
0
GPIO_DAT[3:0]
R/W-0h
LEGEND: R/W = Read/Write; R = Read only; -n = value after reset
Table 33. MODE3 Register Field Descriptions
Bit
7
Field
Type
Reset
Description
PWDN
R/W
0h
Software Power-down Mode
Enable the software power-down mode.
0: Normal mode (default)
1: Software power-down mode
6
STATENB
R/W
0h
STATUS Byte
Enable the Status byte in the conversion data read operation.
0: No Status byte (default)
1: Status byte enabled
5
CRCENB
R/W
0h
CRC Data Verification
Enable the CRC data verification.
0: No CRC (default)
1: CRC enabled
4
SPITIM
R/W
0h
SPI Auto-Reset Function
Enable the SPI auto-reset function.
0: SPI auto-reset disabled (default)
1: SPI auto-reset enabled
3
GPIO_DAT[3]
R/W
0h
GPIO3 Data
Read or write the GPIO3 data on AIN3.
0: GPIO3 is low (default)
1: GPIO3 is high
2
GPIO_DAT[2]
R/W
0h
GPIO2 Data
Read or write the GPIO2 data on AIN2.
0: GPIO2 is low (default)
1: GPIO2 is high
1
GPIO_DAT[1]
R/W
0h
GPIO1 Data
Read or write the GPIO1 data on AIN1.
0: GPIO1 is low (default)
1: GPIO1 is high
0
GPIO_DAT[0]
R/W
0h
GPIO0 Data
Read or write the GPIO1 data on AIN0.
0: GPIO0 is low (default)
1: GPIO0 is high
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
53
ADS1235
SBAS824 – OCTOBER 2018
www.ti.com
8.6.7 Reference Configuration (REF) Register (address = 06h) [reset = 05h]
Figure 72. REF Register
7
0
R/W-0h
6
0
R/W-0h
5
0
R/W-0h
4
0
R/W-0h
3
2
RMUXP[1:0]
R/W-1h
1
0
RMUXN[1:0]
R/W-1h
LEGEND: R/W = Read/Write; R = Read only; -n = value after reset
Table 34. REF Register Field Descriptions
Bit
Field
Type
Reset
Description
7:4
0
R/W
0h
Reserved
3:2
RMUXP[1:0]
R/W
1h
Always write 0h
Reference Positive Input
Select the positive reference input.
00: Reserved
01: AVDD (default)
10: REFP0
11: AIN0 (REFP1)
1:0
RMUXN[1:0]
R/W
1h
Reference Negative Input
Select the negative reference input.
00: Reserved
01: AVSS (default)
10: REFN0
11: AIN1 (REFN1)
54
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
ADS1235
www.ti.com
SBAS824 – OCTOBER 2018
8.6.8 Offset Calibration (OFCALx) Registers (address = 07h, 08h, 09h) [reset = 00h, 00h, 00h]
Figure 73. OFCAL0, OFCAL1, OFCAL2 Registers
7
6
5
4
3
2
1
0
11
10
9
8
19
18
17
16
OFC[7:0]
R/W-00h
15
14
13
12
OFC[15:8]
R/W-00h
23
22
21
20
OFC[23:16]
R/W-00h
LEGEND: R/W = Read/Write; R = Read only; -n = value after reset
Table 35. OFCAL0, OFCAL1, OFCAL2 Registers Field Description
Bit
23:0
Field
Type
Reset
Description
OFC[23:0]
R/W
000000h
Offset Calibration
These three registers are the 24-bit offset calibration word. The
offset calibration is two's complement format. The ADC subtracts
the offset value from the conversion result before the full-scale
operation.
8.6.9 Full-Scale Calibration (FSCALx) Registers (address = 0Ah, 0Bh, 0Ch) [reset = 00h, 00h, 40h]
Figure 74. FSCAL0, FSCAL1, FSCAL2 Registers
7
6
5
4
3
2
1
0
11
10
9
8
19
18
17
16
FSC[7:0]
R/W-00h
15
14
13
12
FSC[15:8]
R/W-00h
23
22
21
20
FSC[23:16]
R/W-40h
LEGEND: R/W = Read/Write; R = Read only; -n = value after reset
Table 36. FSCAL0, FSCAL1, FSCAL2 Registers Field Description
Bit
23:0
Field
Type
Reset
Description
FSC[23:0]
R/W
400000h
Full-Scale Calibration
These three registers are the 24-bit full scale calibration word.
The full-scale calibration is straight binary format. The ADC
divides the register value by 400000h then multiplies the result
with the conversion data. The scaling operation occurs after the
offset operation.
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
55
ADS1235
SBAS824 – OCTOBER 2018
www.ti.com
8.6.10 Reserved (RESERVED) Register (address = 0Dh) [reset = FFh]
Figure 75. RESERVED Register
7
1
R-0h
6
1
R-0h
5
1
R-0h
4
1
R-0h
3
1
R-0h
2
1
R-0h
1
1
R-0h
0
1
R-0h
LEGEND: R/W = Read/Write; R = Read only; -n = value after reset
Table 37. RESERVED Register Field Descriptions
Bit
Field
Type
Reset
Description
7:0
0
R
FFh
Reserved
These bits are read only and always return 0
8.6.11 Reserved (RESERVED) Register (address = 0Eh) [reset = 00h]
Figure 76. RESERVED Register
7
0
R-0h
6
0
R-0h
5
0
R-0h
4
0
R-0h
3
0
R-0h
2
0
R-0h
1
0
R-0h
0
0
R-0h
LEGEND: R/W = Read/Write; R = Read only; -n = value after reset
Table 38. RESERVED Register Field Descriptions
Bit
Field
Type
Reset
Description
7:0
0
R
0h
Reserved
These bits are read only and always return 0
8.6.12 Reserved (RESERVED) Register (address = 0Fh) [reset = 00h]
Figure 77. RESERVED Register
7
0
R-0h
6
0
R-0h
5
0
R-0h
4
0
R-0h
3
0
R-0h
2
0
R-0h
1
0
R-0h
0
0
R-0h
LEGEND: R/W = Read/Write; R = Read only; -n = value after reset
Table 39. RESERVED Register Field Descriptions
Bit
Field
Type
Reset
Description
7:0
0
R
0h
Reserved
These bits are read only and always return 0
56
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
ADS1235
www.ti.com
SBAS824 – OCTOBER 2018
8.6.13 PGA Configuration (PGA) Register (address = 10h) [reset = 00h]
Figure 78. PGA Register
7
BYPASS
R/W-0h
6
0
R/W-0h
5
0
R/W-0h
4
0
R/W-0h
3
0
R/W-0h
2
1
GAIN[2:0]
R/W-0h
0
LEGEND: R/W = Read/Write; R = Read only; -n = value after reset
Table 40. PGA Register Field Descriptions
Bit
7
Field
Type
Reset
Description
BYPASS
R/W
0h
PGA Bypass Mode
Select the PGA mode.
0: PGA mode (default)
1: PGA bypass
6:3
0
R/W
0h
2:0
GAIN[2:0]
R/W
0h
Reserved
Always write 0
Gain
Select the gain.
000: 1 (default)
001 - 101: Reserved
110: 64
111: 128
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
57
ADS1235
SBAS824 – OCTOBER 2018
www.ti.com
8.6.14 Input Multiplexer (INPMUX) Register (address = 11h) [reset = FFh]
Figure 79. INPMUX Register
7
6
5
4
3
2
MUXP[3:0]
R/W-Fh
1
0
MUXN[3:0]
R/W-Fh
LEGEND: R/W = Read/Write; R = Read only; -n = value after reset
Table 41. INPMUX Register Field Descriptions
Bit
Field
Type
Reset
Description
7:4
MUXP[3:0]
R/W
Fh
Positive Input Multiplexer
Select the positive multiplexer input.
0000 - 0010: Reserved
0011: AIN0
0100: AIN1
0101: AIN2
0110: AIN3
0111: AIN4
1000: AIN5
1001, 1010: Reserved
1011: Internal temperature sensor positive
1100, 1101: Reserved
1110: PGA P input open
1111: Internal connection to VCOM (default)
3:0
MUXN[3:0]
R/W
Fh
Negative Input Multiplexer
Select the negative multiplexer input.
0000 - 0010: Reserved
0011: AIN0
0100: AIN1
0101: AIN2
0110: AIN3
0111: AIN4
1000: AIN5
1001, 1010: Reserved
1011: Internal temperature sensor negative
1100, 1101: Reserved
1110: PGA N Input open
1111: Internal connection to VCOM (default)
58
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
ADS1235
www.ti.com
SBAS824 – OCTOBER 2018
9 Application and Implementation
NOTE
Information in the following applications sections is not part of the TI component
specification, and TI does not warrant its accuracy or completeness. TI’s customers are
responsible for determining suitability of components for their purposes. Customers should
validate and test their design implementation to confirm system functionality.
9.1 Application Information
9.1.1 Input Range
The input voltage must be maintained within the specified input range for linear ADC operation otherwise the
conversion data is invalid. Use Equation 3 to verify the input voltage of the PGA is within specification. The input
requirement can also be verified by measuring the PGA output voltages (pins CAPP and CAPN) with a voltmeter
under the conditions of maximum expected input signal, ADC gain, and worst case (low) power-supply voltage.
Check that voltages measured on the pins are within the range: AVSS + 0.3 V < V(CAPP) and V(CAPN) < AVDD –
0.3 V.
9.1.2 Input Overload
Observe the input overvoltage precautions as outlined in the ESD Diodes section. If an overvoltage condition
occurs on an unused channel, the overvoltage channel may crosstalk to the measurement channel. One solution
is to externally clamp the inputs with low-forward voltage diodes as shown in Figure 80. The external diodes
divert the overvoltage current around the ADC inputs to the power supply and ground. Be aware of the reverse
leakage current of the Schottky diodes that may lead to measurement errors.
IFAULT
Schottky
Diode
RLIM
5V
AVDD
AINx
ADC
AVSS ± 0.3 V > VCLAMP > AVDD + 0.3 V
Schottky
Diode
AVSS
IFAULT
Figure 80. External Diode Clamps
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
59
ADS1235
SBAS824 – OCTOBER 2018
www.ti.com
Application Information (continued)
9.1.3 Unused Inputs and Outputs
• Analog Inputs
To minimize input leakage of the measurement channel, tie unused inputs to mid-supply voltage (AVDD +
AVSS) / 2 or to AVDD.
• Digital I/O
Not all the digital I/Os may be needed to operate the ADC. Be sure not to float both used and unused digital
inputs, including during power-down mode. The following is a summary of the optional digital I/Os connection:
• CS: Tie CS low to permanently enable the serial interface.
• CLKIN: Tie CLKIN to DGND to permanently operate the ADC with the internal oscillator.
• START: Tie START to DGND to control conversions by command. Tie START to DVDD to permanently
free-run conversions (Continuous-conversion mode only)
• RESET: Tie RESET to DVDD if not using hardware reset. The ADC is reset at power-on. The ADC is also
reset by the RESET command.
• PWDN: Tie PWDN to DVDD if not using the hardware power-down mode. The ADC can be powered down
by software.
• DRDY: The functionality of the DRDY output is also provided by the dual-mode DOUT/DRDY pin. The
DOUT/DRDY output is active when CS is low. Data ready is also determined by software polling. Because
the conversion data are buffered, data can be read at any time without the need to synchronize to data
ready.
9.1.4 Multiplexed 2-Bridge Input Example
Figure 81 shows an example of a multiplexed, two-bridge system. The figure is a simplified diagram and
excludes input filter components. The bridges are connected with independent excitation sense lines leading
from each bridge to the ADC for accurate reference voltage tracking. Adjust the time delay parameter of the ADC
as necessary to provide delay for settling time that is required after changing the ADC input multiplexer for each
bridge measurement.
5V
AVDD
ADS1235
5V
SEN1 +
REFP0
(REFP0)
SIG1 +
AIN4
(AINP)
SIG1 -
AIN5
(AINN)
SEN1 -
REFN0
(REFN0)
5V
SEN2 +
AIN0
SIG2 +
AIN2
SIG2 -
AIN3
SEN2 -
AIN1
(REFP1)
(AINP)
(AINN)
(REFN1)
AVSS
Figure 81. Multiplexed 2-Bridge Application
60
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
ADS1235
www.ti.com
SBAS824 – OCTOBER 2018
Application Information (continued)
9.1.5 AC-Bridge Excitation Example
Figure 82 shows an ac-excited bridge measurement system in the 4-wire, GPIO-control mode. Signal and
reference-input filter components are omitted for clarity. The transistors switch the polarity of the excitation
voltage provided to the bridge by the drive signals from the ADC GPIO drivers via the spare analog input pins.
The timing of the drive signals are synchronized to the ADC conversions. The drive signals are non-overlapping
in order to avoid commutation errors that can occur during the switching phase. The resistors located at the
gates of each transistor maintain the transistors off at power-on, while the ADC drive signals are initialized by the
host after system power up. See Figure 7 for timing of the drive signals.
5V
AVDD
5V
ADS1235
100 NŸ
AIN0
AIN3
(ACX1)
(ACX2)
100 NŸ
REFP0
SEN +
SIG +
AIN4
SIG -
AIN5
SEN -
REFN0
(AINP)
(AINN)
5V
100 NŸ
AIN1
AIN2
(ACX2)
(ACX1)
100 NŸ
AVSS
Figure 82. AC-Bridge Excitation Application
The recommended configuration sequence for ac-bridge excitation mode follows:
1. Stop conversions by taking the START pin low, or by control of conversions in software mode; send the
STOP command
2. Program the signal and reference input multiplexers, gain, data rata, filter mode and other configurations as
needed
3. Program the 2-wire or 4-wire ac-bridge excitation mode. 2-wire mode requires complementary output
switching devices
4. Program the GPIO internal connection to the analog input pins
5. Program the GPIO as outputs to enable drive signals at the analog input pins. The bridge output drive
signals appear on the GPIO pins.
Start the conversions. Adjust the time delay parameter as necessary to provide sufficient bridge switch delay.
The delay is based on the time constant of the input and reference filters.
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
61
ADS1235
SBAS824 – OCTOBER 2018
www.ti.com
Application Information (continued)
9.1.6 Serial Interface and Digital Connections
Figure 83 shows an example of the digital connections between the host µC and ADC. Not all I/O connections
are necessary for basic ADC operation; see the Unused Inputs and Outputs section. Impedance-matching
resistors in series with the I/O PCB traces help reduce overshoot and ringing, and are particularly helpful over
long trace runs.
Optional
Clock
Source
ADC
47
CLKIN
Host µC
47
PWDN
47
RESET
47
START
Port Pin
Port Pin
Port Pin
Port Pin
(CS)
47
CS
47
SCLK
47
DIN
SCLK
MOSI
47
MISO
47
,54;
DOUT
/DRDY
DRDY
Figure 83. Serial Interface and Digital I/O Connections
62
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
ADS1235
www.ti.com
SBAS824 – OCTOBER 2018
9.2 Typical Application
Figure 84 shows an application of the ADS1235 with a bridge circuit. The excitation voltage provided to the
bridge is the ADC power supply voltage (5 V). Due to the low input-referred noise of the ADS1235, in many
applications there is no need for an additional gain amplifier. The excitation voltage sense lines are connected to
the reference inputs of the ADC with a noise filter. This configuration provides ratiometric operation that cancels
noise and drift of the excitation voltage.
The input signal and reference voltage paths are filtered with equal-value components to remove high frequency
noise from affecting the measurement.
5V
Exc +
1µF
10 µ F
0.1 µF
1 nF
1µF
AVDD
100
REFP0
REFN0
10 nF
100 k
BYPASS
DVDD
ADS1235
Ref
Mux
100
START
Sen +
1 nF
AIN0
AIN1
5V
1k
AIN2
AIN3
AIN4
Sen -
Input
Mux
Ref
Monitor
Temp
Sensor
Sig +
100
Sig -
100
RESET
PWDN
DRDY
Buf
CS
AIN5
1 nF
Control
24-Bit
û ADC
PGA
Digital
Filter
Serial
Interface
DIN
SCLK
DOUT/DRDY
PGA
Monitor
10 nF
CAPP
1 nF
Exc -
CAPN
Internal
Oscillator
AVSS
Clock
Mux
CLKIN
DGND
10 nF
(C0G)
Figure 84. Bridge Input Application
9.2.1 Design Requirements
The ADC can be configured to provide tradeoffs between conversion noise, sample rate and conversion settling
time. Table 42 summarizes the design performance goals. Table 43 summarizes the design parameters.
1 kΩ fixed-value precision resistors simulate the bridge circuit. One of the four resistor values is unbalanced
(1.008 kΩ) in order to generate a 10 mV output signal to simulate a full scale output with 2 mV/V bridge gauge
factor when used with 5 V excitation.
Table 42. Design Goals
DESIGN GOAL
VALUE
Noise free resolution (counts)
> 100,000 counts
Sample rate
10 SPS
Settling time
200 ms
Table 43. Design Parameters
DESIGN PARAMETER
DESIGN VALUE
Bridge resistance
1 kΩ
Bridge excitation voltage
5V
Bridge gauge factor
2 mV/V
Bridge full scale signal
10 mV
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
63
ADS1235
SBAS824 – OCTOBER 2018
www.ti.com
9.2.2 Detailed Design Procedure
A key consideration in the design of a bridge transducer for weigh applications is noise-free resolution. Noisefree resolution is defined by the ratio of full scale signal to the conversion noise of the ADC. Other considerations
are data throughput rate and input signal settling time.
Table 1 shows the ADC conversion noise expressed as an input-referred quantity. The table shows various
tradeoffs among gain, sample rate and sinc filter in order to optimize noise for a given design. For this example,
the configuration of the ADC that yields the lowest noise while achieving the sample rate and settling time
requirement is gain = 128, 10 SPS, filter order = sinc 1 and by using the chop mode. Use of the chop mode has
the additional advantage of eliminating offset drift from the ADC.
Configuring the ADC for 10 SPS, the sinc4 filter order and disabling chop mode yields approximately the same
noise performance compared to the target configuration (shown above) but do not satisfy the settling time
requirement of 200 ms. The sinc4 filter order settles in four conversion periods, or 400 ms.
Noise-free counts are improved by increasing the signal output from the bridge. Increasing the signal output is
possible by the use of a bridge with a higher gauge-factor, or by increasing the excitation voltage. Operation with
an excitation voltage above 5 V requires voltage division of the bridge sense voltage before it is input to the ADC
reference pins.
External filter components filter the signal and reference inputs of the ADC. The filters remove both differential
and common-mode high-frequency noise. Component value mismatch in the common-mode filter converts
common-mode noise into differential noise. To minimize the effect of the mismatch, the differential filter capacitor
values (10 nF) are 10x higher value than the common-mode capacitors (1 nF). Increase the capacitor values to
provides additional noise filtering. Maintain the resistors at low values to minimize thermal noise. For consistent
noise performance, match the corner frequencies of the input and reference filters. More information is found in
the RTD Ratiometric Measurements and Filtering Using the ADS1148 and ADS1248 Family of Devices
Application Report.
9.2.3 Application Curves
Figure 85 and Figure 86 show the conversion data that was acquired over a 60 second interval. 60 seconds of
data provides evaluation of noise performance under actual conditions. The acquired conversion data is taken
with a full-scale signal (10 mV) to show the noise cancelling effects of the ADC provided by ratiometric operation.
Figure 85 and Figure 86 show conversion data in normal mode and in chop mode operation, respectively. In
normal mode, the approximate noise free resolution is 100,000 counts. In chop mode, approximate noise free
resolution is 120,000 counts. Chop mode provides the additional advantage of zero offset drift, but requires an
additional conversion period for fully settled data after an input step change occurs (200 ms total).
10007.75
10009.65
Chop mode
V N = 0.014 PV RMS
10009.6 V N = 0.084 PV P-P
Conversion Data (PV)
Conversion Data (PV)
Normal Mode
V N = 0.019 PV RMS
10007.7 V N = 0.107 PV P-P
10007.65
10007.6
10007.55
10007.5
10009.55
10009.5
10009.45
10009.4
10007.45
10009.35
0
10
20
30
Time (s)
40
50
60
0
D107
Figure 85. Conversion Data (Normal Mode)
64
Submit Documentation Feedback
10
20
30
Time (s)
40
50
60
D108
Figure 86. Conversion Data (Chop Mode)
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
ADS1235
www.ti.com
SBAS824 – OCTOBER 2018
9.3 Initialization Setup
Figure 87 shows a general configuration and measurement procedure.
Apply Power
Set RESET and PWDN High
/* These pins must be high for operation
Y
External clock?
Apply clock to XTAL1
/* ADC automatically detects external clock
(external clock can be applied at power-on)
N
16
/* The ADC is internally held in reset for 216 clocks after power-on
Wait 2 clock cycles
Y
/* If START pin is high, conversions are free-running
If START pin is low, conversions are stopped
START High?
DRDY pulses at 20 Hz
N
Set START low
/* For simplicity, stop conversions before register configuration
DRDY not pulsing
Issue Write Register
command to configure the
ADC
/* Readings are suspended until Write Register command completes
Issue Read Register
command to verify registers
/* Readings are suspended until Read Register command completes
Set START pin
or START command
/* Start or re-start new ADC conversion
N
Hardware DRDY?
Read STATUS byte
Y
N
DRDY bit = 1 ?
N
DRDY low ?
/* Wait for new data
Y
Y
Read Data
N
Change ADC
Settings ?
Y
Figure 87. ADC Configuration and Measurement Procedure
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
65
ADS1235
SBAS824 – OCTOBER 2018
www.ti.com
10 Power Supply Recommendations
The ADC requires an analog power supply (AVDD, AVSS) and digital power supply (DVDD). The analog power
supply can be bipolar (AVDD = +2.5 V and AVSS = –2.5 V) or unipolar (AVDD = 5 V and AVSS = DGND). The
digital supply range is 2.7 V to 5.25 V. DVDD powers the ADC core by use of an internal regulator. DVDD also
sets the digital I/O voltage. Keep in mind that the GPIO I/O voltages are AVDD and AVSS. Voltage ripple
produced by switch-mode power supplies may interfere with the ADC conversions. Use low-dropout regulators
(LDOs) to reduce voltage ripple caused by switch-mode power supplies.
10.1 Power-Supply Decoupling
Good power-supply decoupling is important in order to achieve rated performance. Power supplies must be
decoupled close to the power supply pins using short, direct connections to ground. For the analog supply, place
0.1-µF and 10-µF capacitors between AVDD and AVSS. Connect a 1-µF capacitor from DVDD to the ground
plane. Connect a 1-µF capacitor from BYPASS to the ground plane.
10.2 Analog Power-Supply Clamp
It is important to evaluate circumstances when an input signal is present with the ADC, both powered and
unpowered. When the input signal exceeds the power-supply voltage, it is possible to backdrive the analog
power-supply voltage with the input signal through a conduction path of the internal ESD diodes. Backdriving the
ADC power supply can also occur when the power-supply is on. The backdriven current path is illustrated in
Figure 88. Depending on how the power supply responds during a backdriven condition, it is possible to exceed
the maximum rated ADC supply voltage. The ADC voltage must not be exceeded at all times. One solution is to
clamp the analog supply to safe voltage using an external zener diode.
ADC supply On or Off
IFAULT
+V
+5 V Reg
AVDD
RLIMIT
AINx
Input Voltage
+
±
ESD Diode
ADC
Optional
6-V Zener Diode
AINx
ESD Diode
IFAULT
IFAULT
AVSS
Figure 88. Analog Power-Supply Clamp
10.3 Power-Supply Sequencing
The power supplies can be sequenced in any order, but do not allow the analog or digital inputs to exceed the
respective analog or digital power-supply voltage without external limits of the possible input fault currents.
66
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
ADS1235
www.ti.com
SBAS824 – OCTOBER 2018
11 Layout
Good layout practices are crucial to realize the full-performance of the ADC. Poor grounding can quickly degrade
the noise performance. The following layout guidelines help provide the best results.
11.1 Layout Guidelines
For best performance, dedicate an entire PCB layer to a ground plane and do not route any other signal traces
on this layer. However, depending on restrictions imposed by specific end equipment, a dedicated ground plane
may not be practical. If ground plane separation is necessary, make a direct connection of the planes at the
ADC. Do not connect individual ground planes at multiple locations because this configuration creates ground
loops.
Route digital traces away from the CAPP and CAPN pins and away from all analog inputs and associated
components in order to minimize interference.
Avoid long traces on DOUT/DRDY, because high capacitance on this pin can lead to increased ADC noise
levels. Use a series resistor or a buffer if long traces are used.
C0G capacitors are preferred for the analog input filters. Evaluate other types of capacitors carefully for input
filtering use. Use a C0G-type capacitor for the CAPP to CAPN capacitor. Use X7R-type capacitors for the power
supply decoupling capacitors. High-K type capacitors (Y5V) are not recommended. Place the capacitors as close
as possible to the device pins using short, direct traces. For optimum performance, use low-impedance
connections on the ground-side connections of the bypass capacitors.
When applying an external clock, be sure the clock is free of overshoot and glitches. A source-termination
resistor placed at the clock buffer helps control reflections and overshoot. Glitches present on the clock signal
can lead to increased noise and possible mis-operation.
11.2 Layout Example
Figure 89 is an example layout of the ADS1235, requiring a minimum of three PCB layers. The example circuit is
shown with single supply operation (AVSS = DGND). In this example, the inner layer is dedicated to the ground
plane and the outer layers are used for signal and power traces. If a four-layer PCB is used, dedicate the
additional inner layer as the power plane. In this example, the ADC is oriented in such a way to minimize
crossover of the analog and digital signal traces.
DVDD
Supply
ADC Clock Options:
Option 1: To enable INTERNAL
oscillator, tie CLKIN to
GND
1 µF
DVDD
17
NC
NC
NC
20
CLKIN
NC
21
18
NC
22
19
NC
23
10 nF
24
Option 2: Connect EXTERNAL
clock source to CLKIN
1 µF
25
16
AIN4
26
15
BYPASS
AIN3
27
14
DOUT/ DRDY
AIN5
DGND
(Differential Input Pair)
ADS1235
(Differential Input Pair)
AIN2
28
13
DRDY
AIN1
29
12
DIN
10 nF
(Differential Input Pair)
Connect thermal pad to
AVSS
AIN0
30
11
SCLK
REFN0
31
10
CS
32
9
START
47
47
7
8
47
RESET
4.7 nF
PWDN
6
NC
5
AVSS
4
AVDD
3
CAPN
1
NC
10 nF
CAPP
REFP0
2
(Reference Input)
Additional
analog
compenents
To
MCU
47
1 µF
C0G
AVDD
Supply
10 nF
10 µF
(0805 shown)
For Bipolar Supply,
AVSS = -2.5 V
(0603 shown)
(9-mil traces shown)
Figure 89. ADS1235 Layout Example
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
67
ADS1235
SBAS824 – OCTOBER 2018
www.ti.com
12 Device and Documentation Support
12.1 Documentation Support
12.1.1 Related Documentation
For related documentation see the following:
ADS1235 Evaluation Module User's Guide
12.2 Receiving Notification of Documentation Updates
To receive notification of documentation updates, navigate to the device product folder on ti.com. In the upper
right corner, click on Alert me to register and receive a weekly digest of any product information that has
changed. For change details, review the revision history included in any revised document.
12.3 Community Resources
The following links connect to TI community resources. Linked contents are provided "AS IS" by the respective
contributors. They do not constitute TI specifications and do not necessarily reflect TI's views; see TI's Terms of
Use.
TI E2E™ Online Community TI's Engineer-to-Engineer (E2E) Community. Created to foster collaboration
among engineers. At e2e.ti.com, you can ask questions, share knowledge, explore ideas and help
solve problems with fellow engineers.
Design Support TI's Design Support Quickly find helpful E2E forums along with design support tools and
contact information for technical support.
12.4 Trademarks
E2E is a trademark of Texas Instruments.
All other trademarks are the property of their respective owners.
12.5 Electrostatic Discharge Caution
This integrated circuit can be damaged by ESD. Texas Instruments recommends that all integrated circuits be handled with
appropriate precautions. Failure to observe proper handling and installation procedures can cause damage.
ESD damage can range from subtle performance degradation to complete device failure. Precision integrated circuits may be more
susceptible to damage because very small parametric changes could cause the device not to meet its published specifications.
12.6 Glossary
SLYZ022 — TI Glossary.
This glossary lists and explains terms, acronyms, and definitions.
13 Mechanical, Packaging, and Orderable Information
The following pages include mechanical packaging and orderable information. This information is the most
current data available for the designated devices. This data is subject to change without notice and revision of
this document. For browser-based versions of this data sheet, refer to the left-hand navigation.
68
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
ADS1235
www.ti.com
SBAS824 – OCTOBER 2018
PACKAGE OUTLINE
RHB0032E
VQFN - 1 mm max height
SCALE 3.000
PLASTIC QUAD FLATPACK - NO LEAD
5.1
4.9
A
B
PIN 1 INDEX AREA
5.1
4.9
C
1 MAX
SEATING PLANE
0.05
0.00
0.08 C
2X 3.5
(0.2) TYP
3.45 0.1
9
EXPOSED
THERMAL PAD
16
28X 0.5
8
17
2X
3.5
SYMM
33
32X
24
1
PIN 1 ID
(OPTIONAL)
32
0.3
0.2
0.1
0.05
C A B
C
25
SYMM
0.5
32X
0.3
4223442/A 11/2016
NOTES:
1. All linear dimensions are in millimeters. Any dimensions in parenthesis are for reference only. Dimensioning and tolerancing
per ASME Y14.5M.
2. This drawing is subject to change without notice.
3. The package thermal pad must be soldered to the printed circuit board for thermal and mechanical performance.
www.ti.com
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
69
ADS1235
SBAS824 – OCTOBER 2018
www.ti.com
EXAMPLE BOARD LAYOUT
RHB0032E
VQFN - 1 mm max height
PLASTIC QUAD FLATPACK - NO LEAD
( 3.45)
SYMM
32
25
32X (0.6)
1
24
32X (0.25)
(1.475)
28X (0.5)
33
SYMM
(4.8)
( 0.2) TYP
VIA
8
17
(R0.05)
TYP
9
(1.475)
16
(4.8)
LAND PATTERN EXAMPLE
SCALE:18X
0.07 MIN
ALL AROUND
0.07 MAX
ALL AROUND
SOLDER MASK
OPENING
METAL
SOLDER MASK
OPENING
METAL UNDER
SOLDER MASK
NON SOLDER MASK
DEFINED
(PREFERRED)
SOLDER MASK
DEFINED
SOLDER MASK DETAILS
4223442/A 11/2016
NOTES: (continued)
4. This package is designed to be soldered to a thermal pad on the board. For more information, see Texas Instruments literature
number SLUA271 (www.ti.com/lit/slua271).
5. Vias are optional depending on application, refer to device data sheet. If any vias are implemented, refer to their locations shown
on this view. It is recommended that vias under paste be filled, plugged or tented.
www.ti.com
70
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
ADS1235
www.ti.com
SBAS824 – OCTOBER 2018
EXAMPLE STENCIL DESIGN
RHB0032E
VQFN - 1 mm max height
PLASTIC QUAD FLATPACK - NO LEAD
4X ( 1.49)
(0.845)
(R0.05) TYP
32
25
32X (0.6)
1
24
32X (0.25)
28X (0.5)
(0.845)
SYMM
33
(4.8)
17
8
METAL
TYP
16
9
SYMM
(4.8)
SOLDER PASTE EXAMPLE
BASED ON 0.125 mm THICK STENCIL
EXPOSED PAD 33:
75% PRINTED SOLDER COVERAGE BY AREA UNDER PACKAGE
SCALE:20X
4223442/A 11/2016
NOTES: (continued)
6. Laser cutting apertures with trapezoidal walls and rounded corners may offer better paste release. IPC-7525 may have alternate
design recommendations.
www.ti.com
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Product Folder Links: ADS1235
71
PACKAGE OPTION ADDENDUM
www.ti.com
10-Dec-2020
PACKAGING INFORMATION
Orderable Device
Status
(1)
Package Type Package Pins Package
Drawing
Qty
Eco Plan
(2)
Lead finish/
Ball material
MSL Peak Temp
Op Temp (°C)
Device Marking
(3)
(4/5)
(6)
ADS1235IRHBR
ACTIVE
VQFN
RHB
32
3000
RoHS & Green
NIPDAU
Level-3-260C-168 HR
-40 to 125
ADS
1235
ADS1235IRHBT
ACTIVE
VQFN
RHB
32
250
RoHS & Green
NIPDAU
Level-3-260C-168 HR
-40 to 125
ADS
1235
(1)
The marketing status values are defined as follows:
ACTIVE: Product device recommended for new designs.
LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect.
NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design.
PREVIEW: Device has been announced but is not in production. Samples may or may not be available.
OBSOLETE: TI has discontinued the production of the device.
(2)
RoHS: TI defines "RoHS" to mean semiconductor products that are compliant with the current EU RoHS requirements for all 10 RoHS substances, including the requirement that RoHS substance
do not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, "RoHS" products are suitable for use in specified lead-free processes. TI may
reference these types of products as "Pb-Free".
RoHS Exempt: TI defines "RoHS Exempt" to mean products that contain lead but are compliant with EU RoHS pursuant to a specific EU RoHS exemption.
Green: TI defines "Green" to mean the content of Chlorine (Cl) and Bromine (Br) based flame retardants meet JS709B low halogen requirements of
工商网监
湘ICP备2023018690号
