ADS7842
ADS
784
2
SBAS103C – SEPTEMBER 2000 – REVISED OCTOBER 2006
12-Bit, 4-Channel Parallel Output Sampling
ANALOG-TO-DIGITAL CONVERTER
DESCRIPTION
FEATURES
● SINGLE SUPPLY: 2.7V to 5V
The ADS7842 is a complete, 4-channel, 12-bit Analog-toDigital Converter (ADC). It contains a 12-bit, capacitorbased, Successive Approximation Register (SAR) ADC with
a sample-and-hold amplifier, interface for microprocessor
use, and parallel, 3-state output drivers. The ADS7842 is
specified at a 200kHz sampling rate while dissipating only
2mW of power. The reference voltage can be varied from
100mV to VCC with a corresponding LSB resolution from
24µV to 1.22mV. The ADS7842 is tested down to 2.7V
operation.
● 4-CHANNEL INPUT MULTIPLEXER
● UP TO 200kHz SAMPLING RATE
●
●
●
●
●
●
FULL 12-BIT PARALLEL INTERFACE
±1LSB INL AND DNL
NO MISSING CODES
72dB SINAD
LOW POWER: 2mW
SSOP-28 PACKAGE
Low power, high speed, and an onboard multiplexer make
the ADS7842 ideal for battery-operated systems such as
portable, multi-channel dataloggers and measurement equipment. The ADS7842 is available in an SSOP-28 package
and is tested over the –40°C to +85°C temperature range.
APPLICATIONS
●
●
●
●
●
DATA ACQUISITION
TEST AND MEASUREMENT
INDUSTRIAL PROCESS CONTROL
MEDICAL INSTRUMENTS
LABORATORY EQUIPMENT
A0
SAR
A1
AIN0
AIN1
ADS7842
3-State
Parallel
Data Bus
4-Channel
MUX
AIN2
CDAC
AIN3
Comparator
Output
Latches
and
3-State
Drivers
CLK
BUSY
WR
CS
VREF
RD
Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of
Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
All trademarks are the property of their respective owners.
Copyright © 2000-2006, Texas Instruments Incorporated
PRODUCTION DATA information is current as of publication date.
Products conform to specifications per the terms of Texas Instruments
standard warranty. Production processing does not necessarily include
testing of all parameters.
www.ti.com
PACKAGE/ORDERING INFORMATION
PRODUCT
MINIMUM
RELATIVE
ACCURACY
(LSB)
SINAD
(dB)
ADS7842E
±2
"
"
ADS7842EB
"
PACKAGE-LEAD
PACKAGE
DESIGNATOR(1)
SPECIFIED
TEMPERATURE
RANGE
PACKAGE
MARKING
ORDERING
NUMBER
TRANSPORT
MEDIA, QUANTITY
68
SSOP-28
DB
–40°C to +85°C
ADS7842E
"
"
"
"
"
ADS7842E
ADS7842E/1K
Rails, 48
Tape and Reel, 1000
±1
70
SSOP-28
DB
–40°C to +85°C
ADS7842EB
"
"
"
"
"
"
ADS7842EB
ADS7842EB/1K
Rails, 48
Tape and Reel, 1000
NOTE: (1) For the most current specifications and package information, refer to our web site at www.ti.com.
ABSOLUTE MAXIMUM RATINGS(1)
PIN DESCRIPTIONS
+VCC to GND ........................................................................ –0.3V to +6V
Analog Inputs to GND ............................................ –0.3V to +VCC + 0.3V
Digital Inputs to GND ........................................................... –0.3V to +6V
Power Dissipation .......................................................................... 250mW
Maximum Junction Temperature ................................................... +150°C
Operating Temperature Range ........................................ –40°C to +85°C
Storage Temperature Range ......................................... –65°C to +150°C
Lead Temperature (soldering, 10s) ............................................... +300°C
PIN
NAME
1
AIN0
Analog Input Channel 0
2
AIN1
Analog Input Channel 1
3
AIN2
Analog Input Channel 2
4
AIN3
Analog Input Channel 3
5
VREF
Voltage Reference Input. See Electrical Characteristics Tables for ranges.
6
AGND
Analog Ground
7
DB11
Data Bit 11 (MSB)
8
DB10
Data Bit 10
9
DB9
Data Bit 9
10
DB8
Data Bit 8
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.
11
DB7
Data Bit 7
12
DB6
Data Bit 6
13
DB5
14
DGND
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.
15
DB4
Data Bit 4
16
DB3
Data Bit 3
17
DB2
Data Bit 2
18
DB1
Data Bit 1
19
DB0
Data Bit 0 (LSB)
20
RD
Read Input. Active LOW. Reads the data outputs in
combination with CS.
21
CS
Chip Select Input. Active LOW. The combination of
CS taken LOW and WR taken LOW initiates a new
conversion and places the outputs in the tri-state
mode.
22
WR
Write Input. Active LOW. Starts a new conversion
and selects an analog channel via address inputs A0
and A1, in combination with CS.
23
BUSY
24
CLK
25, 26
A0, A1
NOTE: (1) Stresses above those listed under “Absolute Maximum Ratings”
may cause permanent damage to the device. Exposure to absolute maximum
conditions for extended periods may affect device reliability.
ELECTROSTATIC
DISCHARGE SENSITIVITY
PIN CONFIGURATION
Top View
SSOP
Data Bit 5
Digital Ground
AIN0
1
28
VANA
AIN1
2
27
VDIG
AIN2
3
26
A1
AIN3
4
25
A0
VREF
5
24
CLK
AGND
6
23
BUSY
DB11
7
22
WR
DB10
8
21
CS
DB9
8
20
RD
DB8 10
19
DB0
A1
A0
Channel Selected
DB7 11
18
DB1
0
0
AIN0
DB2
0
1
AIN1
1
0
AIN2
1
1
AIN3
DB6 12
DB5 13
DGND 14
2
DESCRIPTION
ADS7842E
17
16
15
DB3
DB4
BUSY goes LOW and stays LOW during a
conversion. BUSY rises when a conversion is
complete and enables the parallel outputs.
External Clock Input. The clock speed determines the
conversion rate by the equation fCLK = 16 • fSAMPLE.
Address Inputs. Selects one of four analog input
channels in combination with CS and WR. The
address inputs are latched on the rising edge of
either RD or WR.
27
VDIG
Digital Supply Input. Nominally +5V.
28
VANA
Analog Supply Input. Nominally +5V.
ADS7842
www.ti.com
SBAS103C
ELECTRICAL CHARACTERISTICS: +5V
At TA = –40°C to +85°C, +VCC = +5V, VREF = +5V, fSAMPLE = 200kHz, and fCLK = 16 • fSAMPLE = 3.2MHz, unless otherwise noted.
ADS7842E
PARAMETER
CONDITIONS
MIN
TYP
RESOLUTION
0
VREF
0.15
0.1
30
70
✻
✻
✻
✻
10kHz
10kHz
10kHz
50kHz
–78
71
79
120
68
72
0.1
5
40
2.5
0.001
–72
70
76
3.0
–0.3
3.5
–80
72
81
✻
✻
✻
✻
✻
✻
100
3
±1
±1
✻
✻
±3
✻
5.5
+0.8
✻
✻
✻
4.75
550
300
–40
dB
dB
dB
dB
✻
V
GΩ
µA
µA
µA
✻
✻
✻
V
V
V
V
✻
MHz
✻
✻
✻
✻
V
µA
µA
µA
mW
✻
°C
✻
3.2
✻
5.25
900
✻
✻
✻
3
4.5
Power Dissipation
Clk Cycles
Clk Cycles
kHz
ns
ns
ps
–76
✻
✻
0.4
Straight Binary
0.2
Bits
LSB(1)
LSB
LSB
LSB
LSB
LSB
µVrms
dB
✻
CMOS
Specified Performance
V
pF
µA
✻
✻
✻
+VCC
DCLK Static
| IIH | ≤ +5µA
| IIL | ≤ +5µA
IOH = –250µA
IOL = 250µA
✻
✻
200
at
at
at
at
Bits
✻
500
30
100
5Vp-p
5Vp-p
5Vp-p
5Vp-p
✻
✻
fSAMPLE = 12.5kHz
Power-Down Mode(3), CS = +VCC
TEMPERATURE RANGE
Specified Performance
±0.5
±3
1.0
±4
1.0
12
fSAMPLE = 12.5kHz
DCLK Static
POWER-SUPPLY REQUIREMENTS
+VCC
Quiescent Current
±2
3
=
=
=
=
UNITS
✻
±0.8
VIN
VIN
VIN
VIN
MAX
✻
✻
12
SAMPLING DYNAMICS
Conversion Time
Acquisition Time
Throughput Rate
Multiplexer Settling Time
Aperture Delay
Aperture Jitter
DIGITAL INPUT/OUTPUT
Logic Family
Logic Levels
VIH
VIL
VOH
VOL
Data Format
External Clock
TYP
✻
25
±1
SYSTEM PERFORMANCE
No Missing Codes
Integral Linearity Error
Differential Linearity Error
Offset Error
Offset Error Match
Gain Error
Gain Error Match
Noise
Power-Supply Rejection
REFERENCE INPUT
Range
Resistance
Input Current
MIN
12
ANALOG INPUT
Full-Scale Input Span
Capacitance
Leakage Current
DYNAMIC CHARACTERISTICS
Total Harmonic Distortion(2)
Signal-to-(Noise + Distortion)
Spurious-Free Dynamic Range
Channel-to-Channel Isolation
ADS7842EB
MAX
+85
✻
✻ Same specifications as ADS7842E.
NOTES: (1) LSB means Least Significant Bit. With VREF equal to +5.0V, one LSB is 1.22mV.
(2) First five harmonics of the test frequency.
(3) Power-down mode at end of conversion when WR, CS, and BUSY conditions have all been met. Refer to Table III of this data sheet.
ADS7842
SBAS103C
www.ti.com
3
ELECTRICAL CHARACTERISTICS: +2.7V
At TA = –40°C to +85°C, +VCC = +2.7V, VREF = +2.5V, fSAMPLE = 125kHz, and fCLK = 16 • fSAMPLE = 2MHz, unless otherwise noted.
ADS7842E
PARAMETER
CONDITIONS
MIN
TYP
RESOLUTION
0
Channel-to-Channel Isolation
REFERENCE INPUT
Range
Resistance
Input Current
POWER-SUPPLY REQUIREMENTS
+VCC
Quiescent Current
±0.8
0.15
0.1
30
70
±2
±0.5
±5
1.0
±4
1.0
✻
✻
✻
✻
✻
V
pF
µA
Bits
LSB(1)
LSB
LSB
LSB
LSB
LSB
µVrms
dB
±1
±1
✻
✻
±3
✻
Clk Cycles
Clk Cycles
kHz
ns
ns
ps
✻
✻
✻
–77
–70
–79
–74
dB
–77
–70
✻
✻
dB
68
71
70
72
dB
68
71
✻
✻
dB
72
78
76
80
dB
72
78
✻
✻
dB
✻
dB
100
0.1
+VCC
DCLK Static
5
13
2.5
0.001
✻
+VCC • 0.7
–0.3
+VCC • 0.8
✻
✻
✻
✻
✻
40
3
5.5
+0.8
✻
✻
✻
2.7
280
220
–40
✻
✻
V
V
V
V
✻
MHz
✻
✻
✻
✻
V
µA
µA
µA
mW
✻
°C
✻
2
✻
3.6
650
✻
✻
✻
3
1.8
Power Dissipation
✻
✻
✻
0.4
Straight Binary
0.2
V
GΩ
µA
µA
µA
✻
CMOS
Specified Performance
Bits
✻
125
| IIH | ≤ +5µA
| IIL | ≤ +5µA
IOH = –250µA
IOL = 250µA
✻
✻
500
30
100
3.6V ≥ VCC ≥ 3.0V, VIN = 2.5Vp-p
at 10kHz
3.0V > VCC ≥ 2.7V, VIN = 2.5Vp-p
at 10kHz
3.6V ≥ VCC ≥ 3.0V, VIN = 2.5Vp-p
at 10kHz
3.0V > VCC ≥ 2.7V, VIN = 2.5Vp-p
at 10kHz
3.6V ≥ VCC ≥ 3.0V, VIN = 2.5Vp-p
at 10kHz
3.0V > VCC ≥ 2.7V, VIN = 2.5Vp-p
at 10kHz
VIN = 2.5Vp-p at 50kHz
UNITS
✻
12
3
fSAMPLE = 12.5kHz
Power-Down Mode(3), CS = +VCC
TEMPERATURE RANGE
Specified Performance
✻
MAX
✻
fSAMPLE = 12.5kHz
DCLK Static
DIGITAL INPUT/OUTPUT
Logic Family
Logic Levels
VIH
VIL
VOH
VOL
Data Format
External Clock
TYP
✻
✻
12
SAMPLING DYNAMICS
Conversion Time
Acquisition Time
Throughput Rate
Multiplexer Settling Time
Aperture Delay
Aperture Jitter
Spurious-Free Dynamic Range
VREF
25
±1
SYSTEM PERFORMANCE
No Missing Codes
Integral Linearity Error
Differential Linearity Error
Offset Error
Offset Error Match
Gain Error
Gain Error Match
Noise
Power-Supply Rejection
Signal-to-(Noise + Distortion)
MIN
12
ANALOG INPUT
Full-Scale Input Span
Capacitance
Leakage Current
DYNAMIC CHARACTERISTICS
Total Harmonic Distortion(2)
ADS7842EB
MAX
+85
✻
✻ Same specifications as ADS7842E.
NOTES: (1) LSB means Least Significant Bit. With VREF equal to +2.5V, one LSB is 610mV.
(2) First five harmonics of the test frequency.
(3) Power-down mode at end of conversion when WR, CS, and BUSY conditions have all been met. Refer to Table III of this data sheet.
4
ADS7842
www.ti.com
SBAS103C
TYPICAL CHARACTERISTICS: +5V
At TA = +25°C, +VCC = +5V, VREF = +5V, fSAMPLE = 200kHz, and fCLK = 16 • fSAMPLE = 3.2MHz, unless otherwise noted.
FREQUENCY SPECTRUM
(4096 Point FFT; fIN = 10.3kHz, –0.2dB)
0
0
–20
–20
–40
–40
Amplitude (dB)
–60
–80
–60
–80
–100
–100
–120
–120
0
25
50
75
100
0
25
50
Frequency (kHz)
SIGNAL-TO-NOISE RATIO AND SIGNAL-TO(NOISE + DISTORTION) vs INPUT FREQUENCY
100
SPURIOUS-FREE DYNAMIC RANGE AND TOTAL
HARMONIC DISTORTION vs INPUT FREQUENCY
74
–85
85
SNR
SFDR
73
–80
80
72
SFDR (dB)
SNR and SINAD (dB)
75
Frequency (kHz)
SINAD
71
THD
75
–75
70
–70
70
69
1
10
1
100
10
Input Frequency (kHz)
Input Frequency (kHz)
EFFECTIVE NUMBER OF BITS
vs INPUT FREQUENCY
CHANGE IN SIGNAL-TO-(NOISE + DISTORTION)
vs TEMPERATURE
0.6
12.0
0.4
11.8
Delta from +25°C (dB)
Effective Number of Bits
–65
100
65
68
11.6
11.4
11.2
0.2
0.0
–0.2
–0.4
fIN = 10kHz, –0.2dB
–0.6
11.0
1
10
–40
100
ADS7842
SBAS103C
–20
0
20
40
60
80
100
Temperature (°C)
Input Frequency (kHz)
www.ti.com
5
THD (dB)
Amplitude (dB)
FREQUENCY SPECTRUM
(4096 Point FFT; fIN = 1,123Hz, –0.2dB)
TYPICAL CHARACTERISTICS: +2.7V
At TA = +25°C, +VCC = +2.7V, VREF = +2.5V, fSAMPLE = 125kHz, and fCLK = 16 • fSAMPLE = 2MHz, unless otherwise noted.
FREQUENCY SPECTRUM
(4096 Point FFT; fIN = 10.6kHz, –0.2dB)
0
0
–20
–20
–40
–40
Amplitude (dB)
–60
–80
–60
–80
–100
–100
–120
–120
0
15.6
31.3
46.9
0
62.5
62.5
SPURIOUS-FREE DYNAMIC RANGE AND TOTAL
HARMONIC DISTORTION vs INPUT FREQUENCY
90
–90
85
74
–85
SFDR
70
SFDR (dB)
SNR and SINAD (dB)
46.9
SIGNAL-TO-NOISE RATIO AND SIGNAL-TO(NOISE + DISTORTION) vs INPUT FREQUENCY
SNR
66
SINAD
62
58
54
80
–80
75
–75
70
–70
THD
65
–65
60
–60
55
–55
50
1
10
Input Frequency (kHz)
–50
1
100
10
100
Input Frequency (kHz)
EFFECTIVE NUMBER OF BITS
vs INPUT FREQUENCY
CHANGE IN SIGNAL-TO-(NOISE + DISTORTION)
vs TEMPERATURE
12.0
0.4
11.5
0.2
fIN = 10kHz, –0.2dB
Delta from +25°C (dB)
Effective Number of Bits
31.3
Frequency (kHz)
78
11.0
10.5
10.0
9.5
0.0
–0.2
–0.4
–0.6
9.0
–0.8
1
10
100
–40
Input Frequency (kHz)
6
15.6
Frequency (kHz)
–20
0
20
40
60
80
100
Temperature (°C)
ADS7842
www.ti.com
SBAS103C
THD (dB)
Amplitude (dB)
FREQUENCY SPECTRUM
(4096 Point FFT; fIN = 1,129Hz, –0.2dB)
TYPICAL CHARACTERISTICS: +2.7V (Continued)
At TA = +25°C, +VCC = +2.7V, VREF = +2.5V, fSAMPLE = 125kHz, and fCLK = 16 • fSAMPLE = 2MHz, unless otherwise noted.
POWER-DOWN SUPPLY CURRENT
vs TEMPERATURE
400
140
350
120
Supply Current (nA)
Supply Current (µA)
SUPPLY CURRENT vs TEMPERATURE
300
250
200
150
100
80
60
40
100
20
–40
–20
0
20
40
60
80
100
–40
–20
0
40
60
80
100
DIFFERENTIAL LINEARITY ERROR vs CODE
1.00
1.00
0.75
0.75
0.50
0.50
DLE (LSB)
ILE (LSB)
INTEGRAL LINEARITY ERROR vs CODE
0.25
0.00
–0.25
0.25
0.00
–0.25
–0.50
–0.50
–0.75
–0.75
–1.00
000H
–1.00
000H
FFFH
800H
FFFH
800H
Output Code
Output Code
CHANGE IN GAIN vs TEMPERATURE
CHANGE IN OFFSET vs TEMPERATURE
0.15
0.6
0.10
0.4
Delta from +25°C (LSB)
Delta from +25°C (LSB)
20
Temperature (°C)
Temperature (°C)
0.05
0.00
–0.05
0.2
0.0
–0.2
–0.4
–0.10
–0.6
–0.15
–40
–20
0
20
40
60
80
–40
100
0
20
40
60
80
100
Temperature (°C)
Temperature (°C)
ADS7842
SBAS103C
–20
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7
TYPICAL CHARACTERISTICS: +2.7V (Continued)
At TA = +25°C, +VCC = +2.7V, VREF = +2.5V, fSAMPLE = 125kHz, and fCLK = 16 • fSAMPLE = 2MHz, unless otherwise noted.
REFERENCE CURRENT vs TEMPERATURE
18
12
16
Reference Current (µA)
Reference Current (µA)
REFERENCE CURRENT vs SAMPLE RATE
14
10
8
6
4
14
12
10
8
2
6
0
0
25
50
75
100
–40
125
–20
0
20
40
60
80
100
Temperature (°C)
Sample Rate (kHz)
SUPPLY CURRENT vs +VCC
MAXIMUM SAMPLE RATE vs +VCC
1M
320
300
Sample Rate (Hz)
Supply Current (µA)
fSAMPLE = 12.5kHz
280
VREF = +VCC
260
240
220
100k
10k
VREF = +VCC
200
1k
180
2
2.5
3
3.5
4
4.5
2
5
8
2.5
3
3.5
4
4.5
5
+VCC (V)
+VCC (V)
ADS7842
www.ti.com
SBAS103C
THEORY OF OPERATION
some operating modes. While the converter is in the hold
mode, or after the sampling capacitor has been fully charged,
the input impedance of the analog input is greater than 1GΩ.
The ADS7842 is a classic SAR ADC. The architecture is
based on capacitive redistribution which inherently includes
a sample-and-hold function. The converter is fabricated on a
0.6µm CMOS process.
EXTERNAL CLOCK
The ADS7842 requires an external clock to run the conversion process. This clock can vary between 200kHz (12.5kHz
throughput) and 3.2MHz (200kHz throughput). The duty
cycle of the clock is unimportant as long as the minimum
HIGH and LOW times are at least 150ns and the clock period
is at least 300ns. The minimum clock frequency is set by the
leakage on the capacitors internal to the ADS7842.
The basic operation of the ADS7842 is shown in Figure 1.
The device requires an external reference and an external
clock. It operates from a single supply of 2.7V to 5.25V. The
external reference can be any voltage between 100mV and
+VCC. The value of the reference voltage directly sets the
input range of the converter. The average reference input
current depends on the conversion rate of the ADS7842.
BASIC OPERATION
ANALOG INPUTS
The ADS7842 features four, single-ended inputs. The input
current into each analog input depends on input voltage and
sampling rate. Essentially, the current into the device must
charge the internal hold capacitor during the sample period.
After this capacitance has fully charged, there is no further
input current. The source of the analog input voltage must be
able to charge the input capacitance to a 12-bit settling level
within the same period, which can be as little as 350ns in
Figure 1 shows the simple circuit required to operate the
ADS7842 with Channel 0 selected. A conversion can be
initiated by bringing the WR pin (pin 22) LOW for a minimum
of 25ns. BUSY (pin 23) will output a LOW during the
conversion process and rises only after the conversion is
complete. The 12 bits of output data will be valid on pins
7-13 and 15-19 following the rising edge of BUSY.
ADS7842
0V to VREF
1
+5V
+
2.2µF
AIN0
VANA 28
+
0.1µF
+
+5V Analog Supply
10µF
2
AIN1
VDIG 27
3
AIN2
A1 26
4
AIN3
A0 25
5
VREF
CLK 24
3.2MHz Clock
6
AGND
BUSY 23
BUSY Output
7
DB11
WR 22
8
DB10
CS 21
9
DB9
RD 20
10 DB8
DB0 19
11 DB7
DB1 18
12 DB6
DB2 17
13 DB5
DB3 16
14 DGND
DB4 15
Write Input
Read Input
FIGURE 1. Basic Operation of the ADS7842.
ADS7842
SBAS103C
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9
STARTING A CONVERSION
SYMBOL
DESCRIPTION
tCONV
tACQ
tCKP
tCKL
tCKH
t1
t2
t3
t4
t5
t6
t7
t8
t9
t10
t11
t12
t13
t14
t15
t16
t17
Conversion Time
Acquisition Time
Clock Period
Clock LOW
Clock HIGH
CS to WR/RD Setup Time
Address to CS Hold Time
CS LOW
CLK to WR Setup Time
CS to BUSY LOW
CLK to WR LOW
CLK to WR HIGH
WR to CLK HIGH
Address Hold Time
Address Setup Time
BUSY to RD Delay
CLK LOW to BUSY HIGH
BUS Access
BUS Relinquish
Address to RD HIGH
Address Hold Time
RD HIGH to CLK LOW
A conversion is initiated on the falling edge of the WR input,
with valid signals on A0, A1, and CS . The ADS7842 will
enter the conversion mode on the first rising edge of the
external clock following the WR pin going LOW. The ADS7842
will start the conversion on the 1st clock cycle. The MSB will
be approximated by the Capacitive Digital-to-Analog Converter (CDAC) on the 1st clock cycle, the 2nd-MSB on the
2nd cycle, and so on until the LSB has been decided on the
12th clock cycle. The BUSY output will go LOW 20ns after
the falling edge of the WR pin. The BUSY output will return
HIGH just after the ADS7842 has finished a conversion and
the data will be valid on pins 7-13, 15-19. The rising edge of
BUSY can be used to latch the data. It is recommended that
the data be read immediately after each conversion. The
switching noise of the asynchronous data transfer can cause
digital feedthrough degrading the converter’s performance.
See Figure 2.
MIN TYP MAX UNITS
6.5
1.5
500
150
150
0
0
25
25
20
5
25
25
5
5
0
10
25
25
2
2
50
µs
µs
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
READING DATA
Data from the ADS7842 will appear at pins 7-13 and
15-19. The MSB will output on pin 7 while the LSB will
output on pin 19. The outputs are coded in Straight Binary
(with 0V = 000H and VREF = FFFH, see Table IV). Following
a conversion, the BUSY pin will go HIGH. After BUSY goes
HIGH, the CS and RD pins may be brought LOW to enable
the 12-bit output bus. CS and RD must be held LOW for at
least 25ns seconds following BUSY HIGH. Data will be valid
25ns seconds after the falling edge of both CS and RD. The
output data will remain valid for 25ns seconds following the
rising edge of both CS and RD. See Figure 4 for the read
cycle timing diagram.
TABLE I. Timing Specifications (+VCC = +2.7V to 3.6V,
TA = –40°C to +85°C, CLOAD = 50pF).
SYMBOL
DESCRIPTION
tCONV
tACQ
tCKP
tCKL
tCKH
t1
t2
t3
t4
t5
t6
t7
t8
t9
t10
t11
t12
t13
t14
t15
t16
t17
Conversion Time
Acquisition Time
Clock Period
Clock LOW
Clock HIGH
CS to WR/RD Setup Time
Address to CS Hold Time
CS LOW
CLK to WR Setup Time
CS to BUSY LOW
CLK to WR LOW
CLK to WR HIGH
WR to CLK HIGH
Address Hold Time
Address Setup Time
BUSY to RD Delay
CLK LOW to BUSY HIGH
BUS Access
BUS Relinquish
Address to RD HIGH
Address Hold Time
RD HIGH to CLK LOW
POWER-DOWN MODE
The ADS7842 incorporates a unique method of placing the
ADC in the power-down mode. Rather than adding an extra
pin to the package, the A0 address pin is used in conjunction
with the RD pin to place the device in power-down mode and
also to ‘wake-up’ the ADC following power-down. In this
shutdown mode, all analog and digital circuitry is turned off.
The simplest way to place the ADS7842 in power-down
mode is immediately following a conversion. After a conversion has been completed and the BUSY output has returned
HIGH, CS and RD must be brought LOW for a minimum of
25ns. While keeping CS LOW, RD is brought HIGH and the
ADS7842 enters the power-down mode, provided the A0 pin
is HIGH (see Figure 5 and Table III). In order to ‘wake-up’ the
device following power-down, A0 must be LOW when RD
switches from LOW to HIGH a second time (see Figure 6).
MIN TYP MAX UNITS
3.5
1.5
300
150
150
0
0
25
25
20
5
25
25
5
5
0
10
25
25
2
2
50
µs
µs
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
TABLE II. Timing Specifications (+VCC = +4.75V to +5.25V,
TA = –40°C to +85°C, CLOAD = 50pF).
The typical supply current of the ADS7842 with a 5V supply
and 200kHz sampling rate is 550µA. In the power-down
mode the current is typically reduced to 3µA.
10
ADS7842
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SBAS103C
CS
WR
BUSY
A0
A1
COMMENTS
0
RD
X
1
1
X
Power-Down Mode
0
X
1
0
X
Wake-Up Mode
DIGITAL OUTPUT
STRAIGHT BINARY
DESCRIPTION
ANALOG INPUT
Least Significant Bit (LSB)
Full-Scale
Midscale
Midscale –1LSB
Zero Full-Scale
means rising edge triggered. X = Don't care.
TABLE III. Truth Table for Power-Down and Wake-Up Modes.
1.2207mV
4.99878V
2.5V
2.49878V
0V
BINARY CODE
1111
1000
0111
0000
1111
0000
1111
0000
HEX CODE
1111
0000
1111
0000
FFF
800
7FF
000
TABLE IV. Ideal Input Voltages and Output Codes (VREF = 5V).
CS
Latching in Address for Next Channel
WR
Sample
Conversion
CLK
1
2
3
4
5
6
7
9
8
10
11
12
13
14
15
16
BUSY
RD
A0
A1
DATA VALID
DB0-DB11
FIGURE 2. Normal Operation, 16 Clocks per Conversion.
CS
t1
t2
t3
WR
t6
t7
t8
t4
CLK
tCKL
t5
BUSY
t10
t9
N + 1(1)
A0, A1
NOTE: (1) Addresses for next conversion (N + 1) latched in with rising edge of current WR (N).
FIGURE 3. Initiating a Conversion.
ADS7842
SBAS103C
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11
CS
t1
t3
RD
CLK
t12
t11
BUSY
n–1
Conversion n
To prevent PWD
A0 must be 0
A0
t13
DB0-DB11
t14
n-1 DATA VALID
NOTE: Internal register of current conversion updated 1/2 clock cycle prior to BUSY going HIGH.
FIGURE 4. Read Timing Following a Conversion.
CS
t2
t1
t3
RD
CLK
t12
t11
BUSY
t15
t16
A0
NOTE: Rising edge of RD while A0 = 1 initiates power down immediately.
FIGURE 5. Entering Power-Down Using RD and A0.
CS
t1
t2
t3
RD
t15
A0
t16
NOTE: Rising edge of 2nd RD while A0 = 0 places the ADS7842 in sample mode.
FIGURE 6. Initiating Wake-Up Using RD and A0.
12
ADS7842
www.ti.com
SBAS103C
LAYOUT
REFERENCE INPUT
The external reference sets the analog input range. The
ADS7842 will operate with a reference in the range of 100mV
to +VCC.
There are several critical items concerning the reference
input and its wide voltage range. As the reference voltage is
reduced, the analog voltage weight of each digital output
code is also reduced. This is often referred to as the LSB size
and is equal to the reference voltage divided by 4096. Any
offset or gain error inherent in the ADC will appear to
increase, in terms of LSB size, as the reference voltage is
reduced. For example, if the offset of a given converter is
2LSBs with a 2.5V reference, then it will typically be 10LSBs
with a 0.5V reference. In each case, the actual offset of the
device is the same, 1.22mV.
Likewise, the noise or uncertainty of the digitized output will
increase with lower LSB size. With a reference voltage of
100mV, the LSB size is 24µV. This level is below the internal
noise of the device. As a result, the digital output code will not
be stable and vary around a mean value by a number of
LSBs. The distribution of output codes will be gaussian and
the noise can be reduced by simply averaging consecutive
conversion results or applying a digital filter.
With a lower reference voltage, care should be taken to
provide a clean layout including adequate bypassing, a clean
(low-noise, low-ripple) power supply, a low-noise reference,
and a low-noise input signal. Because the LSB size is lower,
the converter will also be more sensitive to nearby digital
signals and electromagnetic interference.
The voltage into the VREF input is not buffered and directly
drives the CDAC portion of the ADS7842. Typically, the input
current is 13µA with a 2.5V reference. This value will vary by
microamps depending on the result of the conversion. The
reference current diminishes directly with both conversion
rate and reference voltage. As the current from the reference
is drawn on each bit decision, clocking the converter more
quickly during a given conversion period will not reduce
overall current drain from the reference.
Data Format
The ADS7842 output data is in Straight Offset Binary format,
see Table IV. This table shows the ideal output code for the
given input voltage and does not include the effects of offset,
gain, or noise.
For optimum performance, care should be taken with the
physical layout of the ADS7842 circuitry. This is particularly
true if the reference voltage is low and/or the conversion rate
is high.
The basic SAR architecture is sensitive to glitches or sudden
changes on the power supply, reference, ground connections, and digital inputs that occur just prior to latching the
output of the analog comparator. Thus, during any single
conversion for an n-bit SAR converter, there are n “windows”
in which large external transient voltages can easily affect the
conversion result. Such glitches might originate from switching power supplies, nearby digital logic, and high-power
devices. The degree of error in the digital output depends on
the reference voltage, layout, and the exact timing of the
external event. The error can change if the external event
changes in time with respect to the DCLK input.
With this in mind, power to the ADS7842 should be clean and
well bypassed. A 0.1µF ceramic bypass capacitor should be
placed as close to the device as possible. In addition, a 1µF
to 10µF capacitor and a 5Ω or 10Ω series resistor may be
used to low-pass filter a noisy supply.
The reference should be similarly bypassed with a 0.1µF
capacitor. Again, a series resistor and large capacitor can be
used to low-pass filter the reference voltage. If the reference
voltage originates from an op amp, make sure that it can
drive the bypass capacitor without oscillation (the series
resistor can help in this case). The ADS7842 draws very little
current from the reference on average, but it does place
larger demands on the reference circuitry over short periods
of time (on each rising edge of CLK during a conversion).
The ADS7842 architecture offers no inherent rejection of
noise or voltage variation in regards to the reference input.
This is of particular concern when the reference input is tied
to the power supply. Any noise and ripple from the supply will
appear directly in the digital results. While high frequency
noise can be filtered out as discussed in the previous
paragraph, voltage variation due to line frequency (50Hz or
60Hz) can be difficult to remove.
The GND pin should be connected to a clean ground point. In
many cases, this will be the “analog” ground. Avoid connections which are too near the grounding point of a microcontroller
or digital signal processor. If needed, run a ground trace
directly from the converter to the power-supply entry point. The
ideal layout will include an analog ground plane dedicated to
the converter and associated analog circuitry.
ADS7842
SBAS103C
www.ti.com
13
Revision History
DATE
REVISION
PAGE
SECTION
DESCRIPTION
10/06
C
4
Electrical Characteristics
Dynamic Characteristics—total harmonic distortion: added new conditions.
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
14
ADS7842
www.ti.com
SBAS103C
PACKAGE OPTION ADDENDUM
www.ti.com
7-Oct-2021
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)
ADS7842E
ACTIVE
SSOP
DB
28
50
RoHS & Green
Call TI
Level-2-260C-1 YEAR
-40 to 85
ADS7842E/1K
ACTIVE
SSOP
DB
28
1000
RoHS & Green
Call TI
Level-2-260C-1 YEAR
ADS7842E
ADS7842EB
ACTIVE
SSOP
DB
28
50
RoHS & Green
Call TI
Level-2-260C-1 YEAR
ADS7842E
B
ADS7842EB/1K
ACTIVE
SSOP
DB
28
1000
RoHS & Green
Call TI
Level-2-260C-1 YEAR
ADS7842E
B
ADS7842EG4
ACTIVE
SSOP
DB
28
50
RoHS & Green
Call TI
Level-2-260C-1 YEAR
-40 to 85
ADS7842E
ADS7842E
(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
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