ADS8345
ADS
834
5
ADS
834
5
®
®
SBAS177C – FEBRUARY 2001 – REVISED APRIL 2003
16-Bit, 8-Channel Serial Output Sampling
ANALOG-TO-DIGITAL CONVERTER
FEATURES
DESCRIPTION
● BIPOLAR INPUT RANGE
The ADS8345 is an 8-channel, 16-bit, sampling
Analog-to-Digital (A/D) converter with a synchronous serial
interface. Typical power dissipation is 8mW at a 100kHz
throughput rate and a +5V supply. The reference voltage
(VREF) can be varied between 500mV and VCC/2, providing a
corresponding input voltage range of ±VREF. The device
includes a shutdown mode which reduces power dissipation
to under 15µW. The ADS8345 is ensured down to 2.7V
operation.
● PIN-FOR-PIN COMPATIBLE WITH THE
ADS7844 AND ADS8344
● SINGLE SUPPLY: 2.7V to 5V
● 8-CHANNEL SINGLE-ENDED OR
4-CHANNEL DIFFERENTIAL INPUT
● UP TO 100kHz CONVERSION RATE
● 85dB SINAD
● SERIAL INTERFACE
● QSOP-20 AND SSOP-20 PACKAGES
Low-power, high-speed, and an onboard multiplexer make
the ADS8345 ideal for battery-operated systems such as
personal digital assistants, portable multi-channel data loggers, and measurement equipment. The serial interface also
provides low-cost isolation for remote data acquisition. The
ADS8345 is available in a QSOP-20 or SSOP-20 package
and is ensured over the –40°C to +85°C temperature range.
APPLICATIONS
●
●
●
●
●
DATA ACQUISITION
TEST AND MEASUREMENT EQUIPMENT
INDUSTRIAL PROCESS CONTROL
PERSONAL DIGITAL ASSISTANTS
BATTERY-POWERED SYSTEMS
CH0
SAR
CH1
DCLK
CH2
CH3
CH4
8-Channel
Multiplexer
CH5
CS
Comparator
CDAC
CH6
CH7
COM
Serial
Interface
and
Control
SHDN
DIN
DOUT
BUSY
VREF
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.
Copyright © 2001-2003, 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.
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ABSOLUTE MAXIMUM RATINGS(1)
ELECTROSTATIC
DISCHARGE SENSITIVITY
+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
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.
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.
PACKAGE/ORDERING INFORMATION
PRODUCT
MAXIMUM
INTEGRAL
LINEARITY
ERROR (LSB)
MAXIMUM
GAIN
ERROR (%)
ADS8345E
8
"
8
"
ADS8345N
"
ADS8345EB
"
ADS8345NB
"
PACKAGE-LEAD
PACKAGE
DESIGNATOR(1)
SPECIFIED
TEMPERATURE
RANGE
±0.05
QSOP-20
DBQ
–40°C to +85°C
ADS8345E
Rails, 100
"
"
"
"
ADS8345E/2K5
Tape and Reel, 2500
±0.05
SSOP-20
DB
–40°C to +85°C
ADS8345N
Rails, 100
Tape and Reel, 1000
ORDERING
NUMBER
TRANSPORT
MEDIA, QUANTITY
"
"
"
"
"
ADS8345N/1K
6
±0.024
QSOP-20
DBQ
–40°C to +85°C
ADS8345EB
Rails, 100
"
"
"
"
"
ADS8345EB/2K5
Tape and Reel, 2500
6
±0.024
SSOP-20
DB
–40°C to +85°C
ADS8345NB
Rails, 100
"
"
"
"
"
ADS8345NB/1K
Tape and Reel, 1000
NOTE: (1) For the most current specifications and package information, refer to our web site at www.ti.com.
PIN DESCRIPTIONS
PIN CONFIGURATION
Top View
SSOP
CH0
1
20
+VCC
CH1
2
19
DCLK
CH2
3
18
CS
CH3
4
17
DIN
CH4
5
16
BUSY
PIN
NAME
1
2
3
4
5
6
7
8
9
CH0
CH1
CH2
CH3
CH4
CH5
CH6
CH7
COM
10
SHDN
11
VREF
+VCC
GND
GND
DOUT
ADS8345
2
CH5
6
15
DOUT
CH6
7
14
GND
CH7
8
13
GND
12
13
14
15
COM
9
12
+VCC
16
BUSY
SHDN 10
11
VREF
17
DIN
18
CS
19
DCLK
20
+VCC
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DESCRIPTION
Analog Input Channel 0
Analog Input Channel 1
Analog Input Channel 2
Analog Input Channel 3
Analog Input Channel 4
Analog Input Channel 5
Analog Input Channel 6
Analog Input Channel 7
Common reference for analog inputs. This pin is typically
connected to VREF.
Shutdown. When LOW, the device enters a very
low-power shutdown mode.
Voltage Reference Input. See the Electrical Characteristics Table for ranges.
Power Supply, 2.7V to 5.25V
Ground
Ground
Serial Data Output. Data is shifted on the falling edge of
DCLK. This output is high impedance when CS is HIGH.
Busy Output. Busy goes LOW when the DIN control bits
are being read and also when the device is converting.
The Output is high impedance when CS is HIGH.
Serial Data Input. If CS is LOW, data is latched on rising
edge of DCLK.
Chip Select Input; Active LOW. Data will not be clocked
into DIN unless CS is LOW. When CS is HIGH, DOUT is
high impedance.
External Clock Input. The clock speed determines the
conversion rate by the equation fDCLK = 24 • fSAMPLE.
Power Supply
ADS8345
SBAS177C
ELECTRICAL CHARACTERISTICS: +5V
At TA = –40°C to +85°C, +VCC = +5V, VREF = +2.5V, fSAMPLE = 100kHz, and fCLK = 24 • fSAMPLE = 2.4MHz, unless otherwise noted.
ADS8345E, N
PARAMETER
CONDITIONS
MIN
RESOLUTION
ANALOG INPUT
Full-Scale Input Span
Absolute Input Range
Positive Input-Negative Input
+IN
–IN
1.0
20
3
POWER-SUPPLY REQUIREMENTS
+VCC
Quiescent Current
✻
✻
✻
✻
✻
✻
✻
✻
✻
100
VIN
VIN
VIN
VIN
=
=
=
=
5Vp-p
5Vp-p
5Vp-p
5Vp-p
at
at
at
at
✻
✻
✻
✻
500
30
100
2.4
0.024
0
10kHz
10kHz
10kHz
10kHz
2.4
2.4
✻
✻
0.5
+VCC/2
5
40
0.001
✻
100
3
3.0
–0.3
3.5
5.5
+0.8
4.75
1.5
1.2
Power Dissipation
7.5
–40
Bits
LSB
mV
LSB(1)
%
LSB
µVrms
LSB(1)
CLK Cycles
CLK Cycles
kHz
ns
ns
ps
MHz
MHz
MHz
✻
✻
V
GΩ
µA
µA
✻
✻
✻
✻
5.25
2.0
V
V
V
V
✻
✻
✻
✻
3
10
+85
✻
✻
✻
0.4
Binary Two’s Complement
Specified Performance
V
V
V
pF
µA
dB
dB
dB
dB
✻
✻
✻
✻
CMOS
| IIH | ≤ +5µA
| IIL | ≤ +5µA
IOH = –250µA
IOL = 250µA
✻
✻
✻
✻
✻
✻
–96
85
98
105
DCLK Static
±6
±1
✻
±0.024
✻
✻
16
fSAMPLE = 10kHz
Power-Down Mode(3), CS = +VCC
TEMPERATURE RANGE
Specified Performance
±8
±2
8
±0.05
4
4.5
DCLK Static
DIGITAL INPUT/OUTPUT
Logic Family
Logic Levels
VIH
VIL
VOH
VOL
Data Format
✻
✻
✻
UNITS
Bits
15
4
SHDN = VDD
MAX
✻
✻
14
+4.75V < VCC < 5.25V
TYP
✻
+VREF
+VCC + 0.2
+VCC + 0.2
–VREF
–0.2
–0.2
Data Transfer Only
REFERENCE INPUT
Range
Resistance
Input Current
MIN
25
±1
SAMPLING DYNAMICS
Conversion Time
Acquisition Time
Throughput Rate
Multiplexer Settling Time
Aperture Delay
Aperture Jitter
Internal Clock Frequency
External Clock Frequency
DYNAMIC CHARACTERISTICS
Total Harmonic Distortion(2)
Signal-to-(Noise + Distortion)
Spurious-Free Dynamic Range
Channel-to-Channel Isolation
MAX
16
Capacitance
Leakage Current
SYSTEM PERFORMANCE
No Missing Codes
Integral Linearity Error
Bipolar Error
Bipolar Error Match
Gain Error
Gain Error Match
Noise
Power-Supply Rejection
TYP
ADS8345EB, NB
✻
✻
✻
✻
✻
✻
V
mA
mA
µA
mW
✻
°C
✻ Same specifications as ADS8345E, N.
NOTES: (1) LSB means Least Significant Bit. With VREF equal to +2.5V, one LSB is 76µV. (2) First nine harmonics of the test frequency. (3) Auto power-down mode
(PD1 = PD0 = 0) active or SHDN = GND.
ADS8345
SBAS177C
www.ti.com
3
ELECTRICAL CHARACTERISTICS: +2.7V
At TA = –40°C to +85°C, +VCC = +2.7V, VREF = +1.25V, fSAMPLE = 100kHz, and fCLK = 24 • fSAMPLE = 2.4MHz, unless otherwise noted.
ADS8345E, N
PARAMETER
CONDITIONS
MIN
RESOLUTION
ANALOG INPUT
Full-Scale Input Span
Absolute Input Range
SAMPLING DYNAMICS
Conversion Time
Acquisition Time
Throughput Rate
Multiplexer Settling Time
Aperture Delay
Aperture Jitter
Internal Clock Frequency
External Clock Frequency
Positive Input-Negative Input
+IN
–IN
REFERENCE INPUT
Range
Resistance
Input Current
1
20
3
POWER-SUPPLY REQUIREMENTS
+VCC
Quiescent Current
✻
✻
✻
✻
✻
✻
✻
✻
✻
100
✻
✻
✻
✻
500
30
100
2.4
0.024
0.024
0
VIN = 2.5Vp-p at 1kHz
VIN = 2.5Vp-p at 1kHz
VIN = 2.5Vp-p at 1kHz
VIN = 2.5Vp-p at 10kHz
2.4
2.0
2.4
✻
✻
✻
0.5
+VCC/2
5
13
0.001
✻
40
3
+VCC • 0.7
–0.3
+VCC • 0.8
5.5
+0.8
2.7
1.2
950
Power Dissipation
3.2
–40
Bits
LSB
mV
LSB
% of FSR
LSB
µVrms
LSB(1)
CLK Cycles
CLK Cycles
kHz
ns
ns
ps
MHz
MHz
MHz
MHz
✻
✻
V
GΩ
µA
µA
✻
✻
✻
✻
3.6
1.85
V
V
V
V
✻
✻
✻
✻
3
5
+85
✻
✻
✻
0.4
Binary Two’s Complement
Specified Performance
V
V
V
pF
µA
dB
dB
dB
dB
✻
✻
✻
✻
CMOS
| IIH | ≤ +5µA
| IIL | ≤ +5µA
IOH = –250µA
IOL = 250µA
✻
✻
✻
✻
✻
✻
✻
–95
81
95
108
DCLK Static
±6
±0.5
✻
±0.024
✻
✻
16
4.5
fSAMPLE = 10kHz
Power-Down Mode(3), CS = +VCC
TEMPERATURE RANGE
Specified Performance
±8
±1.0
4
±0.05
4
UNITS
Bits
15
2
SHDN = VDD
MAX
✻
✻
14
+2.7 < VCC < +3.3V
TYP
✻
✻
✻
25
±1
DCLK Static
DIGITAL INPUT/OUTPUT
Logic Family
Logic Levels
VIH
VIL
VOH
VOL
Data Format
MIN
✻
+VREF
+VCC + 0.2
+VCC + 0.2
–VREF
–0.2
–0.2
When used with Internal Clock
Data Transfer Only
DYNAMIC CHARACTERISTICS
Total Harmonic Distortion(2)
Signal-to-(Noise + Distortion)
Spurious-Free Dynamic Range
Channel-to-Channel Isolation
MAX
16
Capacitance
Leakage Current
SYSTEM PERFORMANCE
No Missing Codes
Integral Linearity Error
Bipolar Error
Bipolar Error Match
Gain Error
Gain Error Match
Noise
Power-Supply Rejection
TYP
ADS8345EB, NB
✻
✻
✻
✻
✻
✻
V
mA
µA
µA
mW
✻
°C
✻ Same specifications as ADS8345E, N.
NOTES: (1) LSB means Least Significant Bit. With VREF equal to +1.25V, one LSB is 38µV. (2) First nine harmonics of the test frequency. (3) Auto power-down
mode (PD1 = PD0 = 0) active or SHDN = GND.
4
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ADS8345
SBAS177C
TYPICAL CHARACTERISTICS: +5V
At TA = +25°C, +VCC = +5V, VREF = +2.5V, fSAMPLE = 100kHz, and fDCLK = 24 • fSAMPLE = 2.4MHz, unless otherwise noted.
FREQUENCY SPECTRUM
(4096 Point FFT; fIN = 9.985kHz, –0.2dB)
0
0
–20
–20
–40
–40
Amplitude (dB)
–60
–80
–100
–60
–80
–100
–120
–120
–140
–140
0
10
20
30
40
0
50
20
30
40
Frequency (kHz)
SIGNAL-TO-NOISE RATIO AND
SIGNAL-TO-(NOISE + DISTORTION)
vs INPUT FREQUENCY
SPURIOUS-FREE DYNAMIC RANGE
AND TOTAL HARMONIC DISTORTION
vs INPUT FREQUENCY
50
–110
110
100
90
SFDR
100
SNR
SFDR (dB)
SNR and SINAD (dB)
10
Frequency (kHz)
80
SINAD
70
–100
–90
90
(1)
THD
–80
80
–70
70
NOTE: (1) First Nine Harmonics
of the Input Frequency
1
10
1
100
10
100
Frequency (kHz)
Frequency (kHz)
EFFECTIVE NUMBER OF BITS
vs INPUT FREQUENCY
CHANGE IN SIGNAL-TO-(NOISE + DISTORTION)
vs TEMPERATURE
15.0
0.4
fIN = 4.956kHz, –0.2dB
14.5
0.2
14.0
Delta from +25°C (dB)
Effective Number of Bits
–60
60
60
13.5
13.0
12.5
12.0
0.0
–0.2
–0.4
–0.6
11.5
11.0
–0.8
1
10
100
ADS8345
SBAS177C
–50
–25
0
20
50
75
100
Temperature (°C)
Frequency (kHz)
www.ti.com
5
THD (dB)
Amplitude (dB)
FREQUENCY SPECTRUM
(4096 Point FFT; fIN = 1.001kHz, –0.2dB)
TYPICAL CHARACTERISTICS: +5V (Cont.)
At TA = +25°C, +VCC = +5V, VREF = +2.5V, fSAMPLE = 100kHz, and fDCLK = 24 • fSAMPLE = 2.4MHz, unless otherwise noted.
DIFFERENTIAL LINEARITY ERROR vs CODE
3
2
2
1
1
DLE (LSB)
ILE (LSB)
INTEGRAL LINEARITY ERROR vs CODE
3
0
0
–1
–1
–2
–2
–3
8000H
0000H
C000H
4000H
–3
8000H
7FFFH
C000H
7FFFH
Output Code
SUPPLY CURRENT vs TEMPERATURE
CHANGE IN BPZ vs TEMPERATURE
1.7
4
3
Delta from 25°C (LSBs)
1.6
Supply Current (mA)
4000H
0000H
Output Code
1.5
1.4
1.3
2
1
0
–1
1.2
–2
–50
–25
0
25
50
75
100
–50
–25
0
25
50
75
100
Temperature (°C)
Temperature (°C)
CHANGE IN GAIN vs TEMPERATURE
WORST-CASE CHANNEL-TO-CHANNEL
BPZ MATCH vs TEMPERATURE
1.0
5.0
BPZ Match (LSBs)
Delta from 25°C (LSBs)
4.5
0.5
0
4.0
3.5
3.0
2.5
–0.5
2.0
–50
–25
0
25
50
75
100
–50
Temperature (°C)
6
–25
0
25
50
75
100
Temperature (°C)
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ADS8345
SBAS177C
TYPICAL CHARACTERISTICS: +5V (Cont.)
At TA = +25°C, +VCC = +2.5V, VREF = +2.5V, fSAMPLE = 100kHz, and fDCLK = 24 • fSAMPLE = 2.4MHz, unless otherwise noted.
WORST-CASE CHANNEL-TO-CHANNEL
GAIN MATCH vs TEMPERATURE
COMMON-MODE REJECTION vs FREQUENCY
0.5
100
90
CMRR (dB)
Gain Match (LSBs)
0.4
0.3
80
70
0.2
60
VCM = 2Vp-p Sinewave Centered Around VREF
0.1
50
–50
–25
0
25
50
75
100
0.1
1
Temperature (°C)
10
100
Frequency (kHz)
TYPICAL CHARACTERISTICS: +2.7V
At TA = +25°C, +VCC = +2.7V, VREF = +1.25V, fSAMPLE = 100kHz, and fDCLK = 24 • fSAMPLE = 2.4MHz, unless otherwise noted.
FREQUENCY SPECTRUM
(4096 Point FFT; fIN = 9.985kHz, –0.2dB)
0
0
–20
–20
–40
–40
Amplitude (dB)
–60
–80
–60
–80
–100
–100
–120
–120
–140
–140
0
10
20
30
40
0
50
20
30
40
Frequency (kHz)
Frequency (kHz)
SIGNAL-TO-NOISE RATIO AND
SIGNAL-TO-(NOISE + DISTORTION)
vs INPUT FREQUENCY
SPURIOUS-FREE DYNAMIC RANGE
AND TOTAL HARMONIC DISTORTION
vs INPUT FREQUENCY
SNR
50
–100
100
95
–90
90
85
SFDR
SFDR (dB)
SNR and SINAD (dB)
10
75
–80
80
–70
70
THD (dB)
Amplitude (dB)
FREQUENCY SPECTRUM
(4096 Point FFT; fIN = 1.001kHz, –0.2dB)
THD(1)
SINAD
65
–60
60
NOTE: (1) First Nine Harmonics
of the Input Frequency
–50
50
55
1
10
100
Frequency (kHz)
ADS8345
SBAS177C
1
10
100
Frequency (kHz)
www.ti.com
7
TYPICAL CHARACTERISTICS: +2.7V (Cont.)
At TA = +25°C, +VCC = +2.7V, VREF = +1.25V, fSAMPLE = 100kHz, and fDCLK = 24 • fSAMPLE = 2.4MHz, unless otherwise noted.
EFFECTIVE NUMBER OF BITS
vs INPUT FREQUENCY
CHANGE IN SIGNAL-TO-(NOISE + DISTORTION)
vs TEMPERATURE
0.4
14.0
fIN = 4.956kHz, –0.2dB
0.2
13.0
Delta from +25°C (dB)
Effective Number of Bits
13.5
12.5
12.0
11.5
11.0
10.5
10.0
0
–0.2
–0.4
–0.6
–0.8
9.5
–1.0
9.0
1
10
–50
100
–25
0
2
2
1
1
0
–1
–2
–2
4000H
0000H
–3
8000H
7FFFH
C000H
100
7FFFH
Output Code
SUPPLY CURRENT vs TEMPERATURE
CHANGE IN BPZ vs TEMPERATURE
1.0
Delta from 25°C (LSBs)
1.3
Supply Current (mA)
4000H
0000H
Output Code
1.2
1.1
1.0
0.9
0.5
0
–0.5
–1.0
–50
–25
0
25
50
75
100
–50
Temperature (°C)
8
75
0
–1
C000H
50
DIFFERENTIAL LINEARITY ERROR vs CODE
3
DLE (LSB)
ILE (LSB)
INTEGRAL LINEARITY ERROR vs CODE
3
–3
8000H
20
Temperature (°C)
Frequency (kHz)
–25
0
25
50
75
100
Temperature (°C)
www.ti.com
ADS8345
SBAS177C
TYPICAL CHARACTERISTICS: +2.7V (Cont.)
At TA = +25°C, +VCC = +2.7V, VREF = +1.25V, fSAMPLE = 100kHz, and fDCLK = 24 • fSAMPLE = 2.4MHz, unless otherwise noted.
WORST-CASE CHANNEL-TO-CHANNEL
BPZ MATCH vs TEMPERATURE
CHANGE IN GAIN vs TEMPERATURE
1.0
0.9
BPZ Match (LSBs)
Delta from 25°C (LSBs)
1.0
0.5
0
0.8
0.7
0.6
0.5
–0.5
–50
–25
0
25
50
75
–50
100
–25
0
Temperature (°C)
WORST-CASE CHANNEL-TO-CHANNEL
GAIN MATCH vs TEMPERATURE
50
75
100
COMMON-MODE REJECTION vs FREQUENCY
0.35
80
70
0.30
CMRR (dB)
Gain Match (LSBs)
25
Temperature (°C)
0.25
60
50
VCM = 1Vp-p Sinewave Centered Around VREF
0.20
40
–50
–25
0
25
50
75
100
0.1
1
Temperature (°C)
POWER-DOWN SUPPLY CURRENT
vs TEMPERATURE
100
SUPPLY CURRENT vs VSS
140
1.5
120
1.4
External Clock Disabled
100
Supply Current (mA)
Supply Current (nA)
10
Frequency (kHz)
80
60
40
fSAMPLE = 100kHz
1.3
1.2
1.1
1.0
20
0.9
0
–50
–25
0
25
50
75
2.5
100
ADS8345
SBAS177C
3.0
3.5
4.0
4.5
5.0
+VSS (V)
Temperature (°C)
www.ti.com
9
THEORY OF OPERATION
determines the range over which the common voltage may
vary (see Figure 3).
The ADS8345 is a classic Successive Approximation
Register (SAR) A/D converter. The architecture is based on
capacitive redistribution which inherently includes a sampleand-hold function. The converter is fabricated on a 0.6µm
CMOS process.
When the input is differential, the amplitude of the input is the
difference between the CHX and COM input (see Figure 4).
A voltage or signal is common to both of these inputs. The
peak-to-peak amplitude of each input is VREF about this
common voltage. However, since the input are 180°C out-ofphase, the peak-to-peak amplitude of the difference voltage is
2 • VREF. The value of VREF also determines the range of the
voltage that may be common to both inputs (see Figure 5).
The basic operation of the ADS8345 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 500mV and
+VCC/2. 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 ADS8345.
In each case, care should be taken to ensure that the output
impedance of the sources driving the CHX and COM inputs
are matched. If this is not observed, the two inputs could
have different settling times. This may result in offset error,
gain error, and linearity error which changes with both
temperature and input voltage. If the impedance cannot be
matched, the errors can be lessened by giving the ADS8345
additional acquisition time.
The analog input to the converter is differential and is
provided via an eight-channel multiplexer. The input can be
provided in reference to a voltage on the COM pin (which is
generally +VCC/2) or differentially by using four of the eight
input channels (CH-CH7). The particular configuration is
selectable via the digital interface.
The input current on the analog inputs depends on a number
of factors: sample rate, input voltage, and source impedance.
Essentially, the current into the ADS8345 charges the internal capacitor array during the sample period. After this
capacitance has been fully charged, there is no further input
current.
ANALOG INPUT
The analog input is bipolar and fully differential. There are
two general methods of driving the analog input of the
ADS8345: single-ended or differential (see Figure 2). When
the input is single-ended, the COM input is held at a fixed
voltage. The CHX input swings around the same voltage and
the peak-to-peak amplitude is 2 • VREF. The value of VREF
Care must be taken regarding the absolute analog input
voltage. Outside of these ranges, the converter’s linearity
may not meet specifications. Please refer to the Electrical
Characteristics table for min/max ratings.
+2.7V to +5V
ADS8345
Single-ended
or differential
analog inputs.
VREF
0.1µF
1µF to 10µF
1
CH0
+VCC 20
2
CH1
DCLK 19
3
CH2
CS 18
Chip Select
4
CH3
DIN 17
Serial Data In
5
CH4
BUSY 16
6
CH5
DOUT 15
7
CH6
GND 14
8
CH7
GND 13
9
COM
+VCC 12
10 SHDN
VREF 11
Serial/Conversion Clock
Serial Data Out
+1.25V to +2.5V
1µF to 10µF
FIGURE 1. Basic Operation of the ADS8345.
10
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ADS8345
SBAS177C
REFERENCE INPUT
The external reference sets the analog input range. The
ADS8345 will operate with a reference in the range of 500mV
to +VCC/2. Keep in mind that the analog input is the differ-
ence between the CHX input and the COM input, as shown
in Figure 4. For example, in the single-ended mode, a 1.25V
reference with the COM pin at VCC/2, the selected input
channel (CH0-CH7) will properly digitize a signal in the range
of (VCC/2 – 1.25V) to (VCC/2 + 1.25V).
A2-A0
(shown 00oB)(1)
CH0
CH1
CH2
CH3
±VREF(1)
CHX
ADS8345
CH4
COM
CH5
Common-Mode
Voltage
(typically VREF)
CH6
±
–IN
VREF(1)
CHX+
2
ADS8345
(1)
Common-Mode
Voltage
+IN
Converter
CH7
Single-Ended Input
± VREF
2
CHX–
Differential Input
NOTE: (1) Relative to common-mode voltage.
COM
NOTE: (1) See Truth Tables, Table I,
and Table II for address coding.
FIGURE 2. Methods of Driving the ADS8345—Single-Ended
or Differential.
5
SGL/DIF
(shown HIGH)
FIGURE 4. Simplified Diagram of the Analog Input.
VCC = 5V
4.9
5.2
5
4
VCC = 5V
3
2.8
2.1
2
1
0.1
0
–1
0.5
1.0
1.5
2.0
Common Voltage Range (V)
Common Voltage Range (V)
4.2
4
Single-Ended Input
2.5
2
1
0.8
0.2
1.0
1.5
2.0
2.5
VREF (V)
FIGURE 3. Single-Ended Input—Common Voltage Range
vs VREF.
SBAS177C
Differential Input
0
0.0
VREF (V)
ADS8345
3
FIGURE 5. Differential Input—Common Voltage Range vs VREF.
www.ti.com
11
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
(Least Significant Bit) size and is equal to the reference
voltage divided by 65536. Any offset or gain error inherent in
the A/D converter 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, 152.8µV.
Most microprocessors communicate using 8-bit transfers; the
ADS8345 can complete a conversion with three such transfers, for a total of 24 clock cycles on the DCLK input,
provided the timing is as shown in Figure 6.
The first eight clock cycles are used to provide the control
byte via the DIN pin. When the converter has enough information about the following conversion to set the input multiplexer appropriately, it enters the acquisition (sample) mode.
After four more clock cycles, the control byte is complete and
the converter enters the conversion mode. At this point, the
input sample-and-hold goes into the Hold mode. The next
sixteen clock cycles accomplish the actual A/D conversion.
The noise or uncertainty of the digitized output will increase
with lower LSB size. With a reference voltage of 500mV, the
LSB size is 15.3µV. This level is below the internal noise of
the device. As a result, the digital output code will not be
stable and will 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.
Control Byte
Figure 6 shows placement and order of the control bits within
the control byte. Tables I and II give detailed information
about these bits. The first bit, the “S” bit, must always be
HIGH and indicates the start of the control byte. The ADS8345
will ignore inputs on the DIN pin until the START bit is
detected. The next three bits (A2-A0) select the active input
channel or channels of the input multiplexer (see Tables III
and IV and Figure 4).
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 Capacitor Digital-to-Analog Converter (CDAC)
portion of the ADS8345. 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.
BIT 7
(MSB)
BIT 6
BIT 5
BIT 4
BIT 3
BIT 2
BIT 1
BIT 0
(LSB)
S
A2
A1
A0
—
SGL/DIF
PD1
PD0
TABLE I. Order of the Control Bits in the Control Byte.
BIT
NAME
7
S
Start Bit. Control byte starts with first HIGH bit on
DIN.
A2-A0
Channel Select Bits. Along with the SGL/DIF bit,
these bits control the setting of the multiplexer input.
2
SGL/DIF
Single-Ended/Differential Select Bit. Along with bits
A2-A0, this bit controls the setting of the multiplexer
input.
1-0
PD1-PD0
Power-Down Mode Select Bits. See Table V for
details.
6-4
DIGITAL INTERFACE
The ADS8345 has a 4-wire serial interface compatible with
several microprocessor families (note that the digital inputs are
over-voltage tolerant up to +5.5V, regardless of +VCC). Figure 6
shows the typical operation of the ADS8345 digital interface.
DESCRIPTION
TABLE II. Descriptions of the Control Bits within the Control Byte.
CS
tACQ
DCLK
1
8
Idle
DIN
S
A2
8
1
Acquire
A1
A0
1
8
1
8
Conversion
Idle
SGL/ PD1 PD0
DIF
S
(START)
A2
Acquire
A1
A0
1
Conversion
SGL/ PD1 PD0
DIF
(START)
BUSY
DOUT
15
14
13
12
11
10
9
8
7
(MSB)
6
5
4
3
2
1
0
(LSB)
Zero Filled...
15
14
(MSB)
FIGURE 6. Conversion Timing, 24-Clocks per Conversion, 8-Bit Bus Interface. No DCLK delay required with dedicated
serial port.
12
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ADS8345
SBAS177C
The SGL/DIF -bit controls the multiplexer input mode:
either in single-ended mode, where the selected input channel is referenced to the COM pin, or in differential mode,
where the two selected inputs provide a differential input.
See Tables III and IV and Figure 4 for more information. The
last two bits (PD1-PD0) select the Power-Down mode and
Clock mode, as shown in Table V. If both PD1 and PD0 are
HIGH, the device is always powered up. If both PD1 and PD0
are LOW, the device enters a power-down mode between
conversions. When a new conversion is initiated, the device
will resume normal operation instantly—no delay is needed
to allow the device to power up and the very first conversion
will be valid.
A2
A1
A0
0
0
0
1
0
0
0
0
1
1
0
1
0
1
0
1
1
0
0
1
1
1
1
1
CH0 CH1 CH2 CH3 CH4 CH5 CH6 CH7 COM
+IN
–IN
+IN
–IN
+IN
–IN
+IN
–IN
+IN
–IN
+IN
–IN
+IN
–IN
+IN
A1
A0
CH0
CH1
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
+IN
–IN
–IN
CH2
CH3
+IN
–IN
CH4
CH5
+IN
–IN
CH6
CH7
+IN
–IN
+IN
–IN
PD0
0
0
Power-down between conversions. When each
conversion is finished, the converter enters a
low-power mode. At the start of the next conversion, the device instantly powers up to full power.
There is no need for additional delays to assure full
operation and the very first conversion is valid.
DESCRIPTION
1
0
Selects internal clock mode.
0
1
Reserved for future use.
1
1
No power-down between conversions, device always powered. Selects external clock mode.
TABLE V. Power-Down Selection.
Clock Modes
The ADS8345 can be used with an external serial clock or an
internal clock to perform the successive-approximation conversion. In both clock modes, the external clock shifts data in
and out of the device. Internal clock mode is selected when
PD1 is HIGH and PD0 is LOW.
If the user decides to switch from one clock mode to the
other, an extra conversion cycle will be required before the
ADS8345 can switch to the new mode. The extra cycle is
required because the PD0 and PD1 control bits need to be
written to the ADS8345 prior to the change in clock modes.
–IN
TABLE III. Single-Ended Channel Selection (SGL/DIF HIGH).
A2
PD1
When power is first applied to the ADS8345, the user must
set the desired clock mode. It can be set by writing PD1 = 1
and PD0 = 0 for internal clock mode or PD1 = 1 and PD0
= 1 for external clock mode. After enabling the required clock
mode, only then should the ADS8345 be set to power-down
between conversions (i.e., PD1 = PD0 = 0). The ADS8345
maintains the clock mode it was in prior to entering the
power-down modes.
External Clock Mode
+IN
–IN
In external clock mode, the external clock not only shifts data
in and out of the ADS8345, it also controls the A/D conversion steps. BUSY will go HIGH for one clock period after the
last bit of the control byte is shifted in. Successive-approximation bit decisions are made and appear at DOUT on each
of the next 16 DCLK falling edges (see Figure 6). Figure 7
shows the BUSY timing in external clock mode.
+IN
–IN
+IN
TABLE IV. Differential Channel Control (SGL/DIF LOW).
CS
tCSS
tCL
tCH
tBD
tBD
tD0
tCSH
DCLK
tDS
DIN
tDH
PD0
tBDV
tBTR
BUSY
tDV
tTR
DOUT
15
14
FIGURE 7. Detailed Timing Diagram.
ADS8345
SBAS177C
www.ti.com
13
Since one clock cycle of the serial clock is consumed with
BUSY going HIGH (while the MSB decision is being made),
16 additional clocks must be given to clock out all 16 bits of
data; thus, one conversion takes a minimum of 25 clock
cycles to fully read the data. Since most microprocessors
communicate in 8-bit transfers, this means that an additional
transfer must be made to capture the LSB.
If CS is LOW when BUSY goes LOW following a conversion,
the next falling edge of the external serial clock will write out
the MSB on the DOUT line. The remaining bits (D14-D0) will
be clocked out on each successive clock cycle following the
MSB. If CS is HIGH when BUSY goes LOW then the DOUT
line will remain in tri-state until CS goes LOW, as shown in
Figure 9. CS does not need to remain LOW once a conversion has started. Note that BUSY is not tri-stated when CS
goes HIGH in internal clock mode.
There are two ways of handling this requirement. One is
where the beginning of the next control byte appears at the
same time the LSB is being clocked out of the ADS8345 (see
Figure 6). This method allows for maximum throughput and
24 clock cycles per conversion.
Data can be shifted in and out of the ADS8345 at clock rates
exceeding 2.4MHz, provided that the minimum acquisition
time tACQ, is kept above 1.7µs.
Digital Timing
The other method is shown in Figure 8, which uses 32 clock
cycles per conversion; the last seven clock cycles simply
shift out zeros on the DOUT line. BUSY and DOUT go into a
high-impedance state when CS goes HIGH; after the next
CS falling edge, BUSY will go LOW.
Figure 7 and Tables VI and VII provide detailed timing for the
digital interface of the ADS8345.
SYMBOL
DESCRIPTION
MIN
tACQ
tDS
tDH
tDO
tDV
tTR
tCSS
tCSH
tCH
tCL
tBD
tBDV
tBTR
Acquisition Time
DIN Valid Prior to DCLK Rising
DIN Hold After DCLK HIGH
DCLK Falling to DOUT Valid
CS Falling to DOUT Enabled
CS Rising to DOUT Disabled
CS Falling to First DCLK Rising
CS Rising to DCLK Ignored
DCLK HIGH
DCLK LOW
DCLK Falling to BUSY Rising
CS Falling to BUSY Enabled
CS Rising to BUSY Disabled
1.5
100
10
Internal Clock Mode
In internal clock mode, the ADS8345 generates its own
conversion clock internally. This relieves the microprocessor
from having to generate the SAR conversion clock and
allows the conversion result to be read back at the processor’s
convenience, at any clock rate from 0MHz to 2.0MHz. BUSY
goes LOW at the start of a conversion and then returns HIGH
when the conversion is complete. During the conversion,
BUSY will remain LOW for a maximum of 8µs. Also, during
the conversion, DCLK should remain LOW to achieve the
best noise performance. The conversion result is stored in an
internal register; the data may be clocked out of this register
any time after the conversion is complete.
TYP
MAX
UNITS
µs
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
200
200
200
100
0
200
200
200
200
200
TABLE VI. Timing Specifications (+VCC = +2.7V to 3.6V,
TA = –40°C to +85°C, CLOAD = 50pF).
CS
tACQ
DCLK
1
8
Idle
DIN
S
A2
1
8
Acquire
A1
A0
1
1
8
8
Conversion
Idle
SGL/
DIF PD1 PD0
(START)
BUSY
DOUT
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
Zero Filled...
0
(MSB)
(LSB)
FIGURE 8. External Clock Mode, 32 Clocks Per Conversion.
CS
tACQ
DCLK
1
8
Idle
DIN
S
A2
Acquire
A1
A0
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
Conversion
SGL/ PD1 PD0
DIF
(START)
BUSY
DOUT
15
14
13
12
11
10
(MSB)
9
8
7
6
5
4
3
2
1
0
Zero Filled...
(LSB)
FIGURE 9. Internal Clock Mode Timing.
14
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ADS8345
SBAS177C
SYMBOL
DESCRIPTION
MIN
tACQ
tDS
tDH
tDO
tDV
tTR
tCSS
tCSH
tCH
tCL
tBD
tBDV
tBTR
Acquisition Time
DIN Valid Prior to DCLK Rising
DIN Hold After DCLK HIGH
DCLK Falling to DOUT Valid
CS Falling to DOUT Enabled
CS Rising to DOUT Disabled
CS Falling to First DCLK Rising
CS Rising to DCLK Ignored
DCLK HIGH
DCLK LOW
DCLK Falling to BUSY Rising
CS Falling to BUSY Enabled
CS Rising to BUSY Disabled
1.7
50
10
TYP
MAX
UNITS
µs
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
100
70
70
50
0
150
150
100
70
70
Operating the ADS8345 in auto power-down mode will result
in the lowest power dissipation, and there is no conversion
time “penalty” on power-up. The very first conversion will be
valid. SHDN can be used to force an immediate power-down.
NOISE
TABLE VII. Timing Specifications (+VCC = +4.75V to +5.25V,
TA = –40°C to +85°C, and CLOAD = 50pF).
Data Format
The output data from the ADS8345 is in Binary Two’s
Complement format, as shown in Table VIII. This table
represents the ideal output code for the given input voltage
and does not include the effects of offset, gain error, or noise.
DESCRIPTION
ANALOG VALUE
DIGITAL OUTPUT
Full-Scale Range
2 • VREF
Least Significant
Bit (LSB)
2 • VREF/65536
BINARY CODE
HEX CODE
+Full-Scale
+VREF – 1LSB
0111 1111 1111 1111
7FFF
0V
0000 0000 0000 0000
0000
0V – 1LSB
1111 1111 1111 1111
FFFF
–VREF
1000 0000 0000 0000
8000
Midscale
Midscale – 1LSB
–Full-Scale
If DCLK is active and CS is LOW while the ADS8345 is in
auto power-down mode, the device will continue to dissipate
some power in the digital logic. The power can be reduced
to a minimum by keeping CS HIGH.
BINARY TWO’S COMPLEMENT
TABLE VIII. Ideal Input Voltages and Output Codes.
The noise floor of the ADS8345 itself is rather low (see
Figures 10 and 11). The ADS8345 was tested at both 5V and
2.7V, and in both the internal and external clock modes. A
low-level DC input was applied to the analog-input pins and
the converter was put through 5000 conversions. The digital
output of the A/D converter will vary in output code due to the
internal noise of the ADS8345. This is true for all 16-bit, SARtype, A/D converters. Using a histogram to plot the output
codes, the distribution should appear bell-shaped with the
peak of the bell curve representing the nominal code for the
input value. The ±1σ, ±2σ, and ±3σ distributions will represent the 68.3%, 95.5%, and 99.7%, respectively, of all codes.
The transition noise can be calculated by dividing the number
of codes measured by 6 and this will yield the ±3σ distribution, or 99.7%, of all codes. Statistically, up to 3 codes could
fall outside the distribution when executing 1000 conversions. The ADS8345, with 5 output codes for the ±3σ
distribution, will yield a < ±0.83LSB transition noise at 5V
operation. Remember, to achieve this low-noise performance,
the peak-to-peak noise of the input signal and reference
must be < 50µV.
POWER DISSIPATION
3544
There are three power modes for the ADS8345: full-power
(PD1-PD0 = 11B), auto power-down (PD1-PD0 = 00B), and
shutdown (SHDN LOW). The effects of these modes varies
depending on how the ADS8345 is being operated. For
example, at full conversion rate and 24-clocks per conversion, there is very little difference between
full-power mode and auto power-down; a shutdown will not
lower power dissipation.
When operating at full-speed and 24-clocks per conversion
(see Figure 6), the ADS8345 spends most of its time acquiring or converting. There is little time for auto power-down,
assuming that this mode is active. Thus, the difference
between full-power mode and auto power-down is negligible.
If the conversion rate is decreased by simply slowing the
frequency of the DCLK input, the two modes remain approximately equal. However, if the DCLK frequency is kept at the
maximum rate during a conversion, but conversions are
simply done less often, then the difference between the two
modes is dramatic. In the latter case, the converter spends
an increasing percentage of its time in power-down mode
(assuming the auto power-down mode is active).
ADS8345
SBAS177C
701
568
122
FFFEH
65
FFFFH
0000H
0001H
0002H
Code
FIGURE 10. Histogram of 5000 Conversions of a DC Input at
the Code Transition, 5V operation external clock
mode. VREF = VCOM = 2.5V.
www.ti.com
15
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 highpower 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.
2305
938
780
436
6
435
64
FFFCH FFFDH FFFEH FFFFH
0000H
0001H
0002H
28
8
0003H
0004H
With this in mind, power to the ADS8345 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.
Code
FIGURE 11. Histogram of 5000 Conversions of a DC Input at
the Code Center, 2.7V operation external clock
mode. VREF = VCOM = 1.25V.
AVERAGING
The noise of the A/D converter can be compensated by
averaging the digital codes. By averaging conversion results,
transition noise will be reduced by a factor of 1/√n, where n
is the number of averages. For example, averaging 4 conversion results will reduce the transition noise by 1/2 to
±0.25LSBs. Averaging should only be used for input signals
with frequencies near DC.
For AC signals, a digital filter can be used to low-pass filter
and decimate the output codes. This works in a similar
manner to averaging: for every decimation by 2, the
signal-to-noise ratio will improve 3dB.
LAYOUT
For optimum performance, care should be taken with the
physical layout of the ADS8345 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
16
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 ADS8345 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 DCLK during a conversion).
The ADS8345 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.
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ADS8345
SBAS177C
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)
ADS8345E
ACTIVE
SSOP
DBQ
20
50
RoHS & Green
Call TI
Level-2-260C-1 YEAR
-40 to 85
ADS8345E
ADS8345E/2K5
ACTIVE
SSOP
DBQ
20
2500
RoHS & Green
Call TI
Level-2-260C-1 YEAR
-40 to 85
ADS8345E
ADS8345EB
ACTIVE
SSOP
DBQ
20
50
RoHS & Green
Call TI
Level-2-260C-1 YEAR
-40 to 85
ADS8345E
B
ADS8345N
ACTIVE
SSOP
DB
20
70
RoHS & Green
Call TI
Level-2-260C-1 YEAR
-40 to 85
ADS8345N
B
ADS8345N/1K
ACTIVE
SSOP
DB
20
1000
RoHS & Green
Call TI
Level-2-260C-1 YEAR
-40 to 85
ADS8345N
B
ADS8345NB
ACTIVE
SSOP
DB
20
70
RoHS & Green
Call TI
Level-2-260C-1 YEAR
-40 to 85
ADS8345N
B
(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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