SLAS370C − APRIL 2002 − REVISED OCTOBER 2004
FEATURES
DPGA Gains: 1, 2, 4, 5, 8, 10, 16, 20 V/V
DProgrammable Input (Up to 4-Channel
Differential/Up to 8-Channel Single-Ended or
Some Combination)
D1.15-V, 2.048-V, or 2.5-V Internal Reference
DSPI/DSP Compatible Serial Interface
(≤20 MHz)
DThroughput Rate: 48 kSamples/sec
DError Overload Indicator
DProgrammable Output 2s Complement/Binary
D2.7-V to 5.5-V Single Supply Operation
D4-Bit Digital I/O Via Serial Interface
DPin-Compatible With ADS7870
DSSOP-28 Package
APPLICATIONS
DPortable Battery-Powered Systems
DLow-Power Instrumentation
DLow-Power Control Systems
DSmart Sensor Applications
DESCRIPTION
The ADS7871 (US patents 6140872, 6060874) is a
complete low power data acquisition system on a single
chip. It consists of a 4-channel differential/8-channel
single-ended multiplexer, precision programmable gain
amplifier, 14-bit successive approximation analog-to-
digital (A/D) converter, and a precision voltage reference.
The programmable-gain amplifier provides high input
impedance, excellent gain accuracy, good common-mode
rejection, and low noise.
For many low-level signals, no external amplification or
impedance b u ffering is needed between the signal source
and the A/D input.
The of fset voltage of the PGA is auto-zeroed. Gains of 1,
2, 4, 5, 8, 10, 16, and 20 V/V allow signals as low as 125
mV to produce full-scale digital outputs.
The ADS7871 contains an internal reference, which is
trimmed for high initial accuracy and stability vs
temperature. Drift is typically 10 ppm/°C. An external
reference can be used in situations where multiple
ADS7871s share a common reference.
The serial interface allows the use of SPI, QSPI,
Microwire, and 8051-family protocols, without glue logic.
14-BIT
A/D
MUX
Serial
Interface SCLK
CS
Digital
I/O
I/O0
I/O2
I/O3
I/O1
LN0
LN2
LN3
LN1
LN4
LN6
LN7
LN5
Registers
PGA
_
+
RESET
RISE/FALL
DIN
DOUT
BUSY
CONVERT
BUFOUT/REFINBUFIN
Oscillator CCLK
OSC ENABLE
REF
VREF
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)#!"#$% '# (,+)")&'#!( ,+$ '1+ '+$%( #" +2&( !('$*%+!'( ('&!/&$/ 3&$$&!'40
$#/*)'#! ,$#)+((!5 /#+( !#' !+)+((&$.4 !).*/+ '+('!5 #" &.. ,&$&%+'+$(0
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 2002−2004, Texas Instruments Incorporated
All trademarks are the property of their respective owners.
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2
ORDERING INFORMATION(1)
PRODUCT PACKAGE-LEAD PACKAGE
DESIGNATOR
SPECIFIED
TEMPERATURE
RANGE
PACKAGE
MARKING ORDERING
NUMBER TRANSPORT MEDIA,
QUANTITY
ADS7871
SSOP-28 Surface Mount
DB
−40°C to +85°C
ADS7871 ADS7871IDB Rails, 48
ADS7871 SSOP-28 Surface Mount DB −40°C to +85°CADS7871 ADS7871IDBR Tape and Reel, 1000
(1) For the most current package and ordering information, see the package option addendum located at the end of this data sheet.
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.
ABSOLUTE MAXIMUM RATINGS
over o p e r ating free-air temperature range unless otherwise noted(1)
UNIT
Supply voltage, VDD 5.5 V
Input current
Momentary 100 mA
Analog inputs
Input current
Continuous 10 mA
Analog inputs
Input voltage VDD + 0.5 V to GND − 0.5 V
Operating free-air temperature range, TA−40°C to 85°C
Storage temperature range, TSTG −65°C to 150°C
Junction temperature (TJ max) 150°C
Lead temperature, (10 sec) 300°C
(1) Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. These are stress ratings only, and
functional operation of the device at these or any other conditions beyond those indicated under recommended operating conditions is not implied.
Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability.
SLAS370C − APRIL 2002 − REVISED OCTOBER 2004
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3
ELECTRICAL CHARACTERISTICS
For the Total System (1), −40°C ≤ TA ≤ 85°C, VDD = 5 V, BUFIN = 2.5 V (using external reference), 2.5-MHz CCLK and 2.5-MHz SCLK (unless
otherwise noted)
PARAMETER TEST CONDITIONS MIN TYP MAX UNIT
Analog Input
Input voltage (LNx inputs) Linear operation −0.2 VDD + 0.2 V
Input capacitance (2) 4 9.7 pF
Input impedance (2)
Common mode 6
MΩ
Input impedance (2)
Differential 7
M
Ω
Channel-to-channel crosstalk VI = 2 VPP, 60 Hz (3) 100 dB
Maximum leakage current 100 pA
Static Accuracy
Resolution 14 Bits
No missing codes G = 1 to 20 V/V 13 Bits
Integral linearity G = 1 to 20 V/V −4 ±2 4 LSB
Differential linearity G = 1 to 20 V/V −2 ±0.5 4 LSB
Offset error G = 1 to 20 V/V −24 ±1 24 LSB
Ratiometric configuration or
(4)
G = 1 to 10 V/V −0.2 0.2 %FSR
Full-scale gain error
Ratiometric configuration or
external reference (4) G = 16 and 20 V/V −0.25 0.25 %FSR
Full-scale gain error
Internal reference
G = 1 to 10 V/V −0.35 0.35 %FSR
Internal reference
G = 16 and 20 V/V −0.4 0.4 %FSR
DC common-mode rejection ratio, RTI VI = −0.2 V to 5.2 V, G = 20 V/V 80 dB
Power supply rejection ratio, RTI VDD = 5 V ±10%, G = 20 V/V 88 dB
Dynamic Characteristics
Throughput rate
Continuous mode One channel 48
ksample/s
Throughput rate
Address mode Different channels 48
ksample/s
External clock, CCLK (5) 0.1 20 MHz
Internal oscillator frequency 2.5 MHz
Serial interface clock, SCLK 20 MHz
Data setup time 10 ns
Data hold time 10 ns
Digital Inputs
Low-level input voltage, VIL 0.8 V
High-level input voltage, VIH
VDD ≤ 3.6 V 2 V
Logic levels
High-level input voltage, V
IH VDD > 3.6 V 3 V
Logic levels
Low-level input current, IIL 1
A
High-level input current, IIH 1µ
A
(1) The specifications for the total system are overall analog input to digital output specifications. The specifications for internal functions indicate
the performance of the individual functions in the ADS7871.
(2) The ADS7871 uses switched capacitor techniques for the programmable gain amplifier and A/D converter. A characteristic of such circuits is that
the input capacitance at any selected LNx pin changes during the conversion cycle.
(3) One channel on with its inputs grounded. All other channels off with sinewave voltage applied to their inputs.
(4) Ratiometric configuration exists when the input source is configured such that changes in the reference cause corresponding changes in the input
voltage. The same accuracy applies when a perfect external reference is used.
(5) The CCLK is divided by the DF value specified by the contents of register 3, A/D Control register, bits D0 and D1 to produce DCLK. The maximum
value of DCLK is 2.5 MHz.
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4
ELECTRICAL CHARACTERISTICS
For the Total System (1), −40°C ≤ TA ≤ 85°C, VDD = 5 V, BUFIN = 2.5 V (using external reference), 2.5-MHz CCLK and 2.5-MHz SCLK (unless
otherwise noted)
PARAMETER TEST CONDITIONS MIN TYP MAX UNIT
Digital Outputs
Data coding Binary 2s complement
Low-level output voltage, VOL
ISINK = 5 mA 0.4
V
Logic levels
Low-level output voltage, V
OL ISINK = 16 mA 0.8
V
Logic levels
High-level output voltage, VOH
ISOURCE = 0.5 mA VDD − 0.4
V
High-level output voltage, V
OH ISOURCE = 5 mA 4.6
V
Leakage current Hi-Z state, VO = 0 V to VDD 1µA
Output capacitance 5 pF
Voltage Reference
Bandgap voltage
VREF = 2.048 V, 2.5 V Pin 26 used as output,
Use internal OSC or external
−0.25 ±0.05 0.25 %FSR
Bandgap voltage
reference VREF = 1.15 V
Use internal OSC or external
CCLK as conversion clock 1.15 V
Output drive ±0.6 µA
Reference Buffer
Input voltage, BUFIN 0.9 VDD − 0.2 V
Input impedance, BUFIN At pin 27 1000||3 GΩ||pF
Input offset −10 ±1 10 mV
Output voltage accuracy vs temperature,
(2) (3)
Pin 28 used as output,
−0.25 ±0.05 0.25 %FSR
Output voltage accuracy vs temperature,
BUFOUT/REFIN (2) (3)
Pin 28 used as output,
VREF = 2.048 V and 2.5 V 10 50 ppm/°C
Output drive, BUFOUT/REFIN 20 mA
Power Supply Requirements
Supply voltage 2.7 5.5 V
(2)
1-kHz Sample rate REF and BUF on, Internal os-
cillator on 1.2 mA
Power supply current
(2)
48-kHz Sample rate REF and BUF on, External
CCLK 1.7 2 mA
Power down REF, BUF, Internal
oscillator off 1µA
(2)
1-kHz Sample rate REF and BUF on, Internal
oscillator on 6 mW
Power dissipation
(2)
48-kHz Sample rate REF and BUF on, External
CCLK 8.5 11 mW
Power down REF and BUF off 5µW
Temperature Range
Operating free-air −40 85 °C
Storage range −65 150 °C
Thermal resistance, QJA 65 °C/W
(1) The specifications for the total system are overall analog input to digital output specifications. The specifications for internal functions indicate
the performance of the individual functions in the ADS7871.
(2) REF and BUF contribute 190 µA and 150 µA (950 µW and 750 µW) respectively. At initial power up the default condition for both REF and BUF
functions is power off. They can be turned on under software control by writing a 1 to D3 and D2 of register 7, REF/OSCILLAT OR CONTROL
register.
(3) For VDD < 3 V, VREF = 2.5 V is not usable.
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5
ELECTRICAL CHARACTERISTICS
For Internal Functions (1), −40°C ≤ TA ≤ 85°C, VDD = 5 V, BUFIN = 2.5 V (using external reference), 2.5-MHz CCLK and 2.5-MHz SCLK (unless
otherwise noted)
PARAMETER TEST CONDITIONS MIN TYP MAX UNIT
Multiplexer
On resistance 100 Ω
Off resistance 1 GΩ
Off channel leakage
On channel = 5.2 V,
Off channel = 0 V VLNx = 5.2 V 100 pA
Off channel leakage
current On channel = 0 V,
Off channel = 5.2 V 100 pA
On channel leakage
On channel = 5.2 V,
Off channel = 0 V 100 pA
On channel leakage
current On channel = 0 V,
Off channel = 5.2 V 100 pA
PGA Amplifier
Offset voltage 100 µV
Small signal bandwidth 5/Gain MHz
Settling time
G = 1 0.3 µs
Settling time
G = 20 6.4 µs
Analog-To-Digital Converter DC Characteristics
Resolution 14 Bits
Integral linearity error ±2 LSB
Differential linearity error ±0.5 LSB
No missing codes 14 Bits
Offset error REFIN = 2.5 V ±2 LSB
Full-scale (gain) error ±0.02 %
Common mode rejection, RTI of A/D 60 dB
Power supply rejection, RTI of ADS7871 External reference, VDD = 5 V ±10% 60 dB
PGA Plus A/D Converter Sampling Dynamics fCCLK = 2.5 MHz, DF = 1
Throughput rate 50 CCLK cycles 50 kHz
Conversion time 14 CCLK cycles 5.6 µs
Acquisition time 28 CCLK cycles 9.6 µs
Auto zero time 8 CCLK cycles 3.2 µs
Aperture delay 36 CCLK cycles 12.8 µs
Small signal bandwidth 5 MHz
Step response 1 Complete Conversion Cycle
(1) The specifications for the total system are overall analog input to digital output specifications. The specifications for internal functions indicate
the performance of the individual functions in the ADS7871.
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6
PIN ASSIGNMENTS SSOP-28 PACKAGE
(TOP VIEW)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
28
27
26
25
24
23
22
21
20
19
18
17
16
15
LN0
LN1
LN2
LN3
LN4
LN5
LN6
LN7
RESET
RISE/FALL
I/O0
I/O1
I/O2
I/O3
BUFOUT/REFIN
BUFIN
VREF
GND
VDD
CS
DOUT
DIN
SCLK
CCLK
OSC ENABLE
BUSY
CONVERT
GND
Terminal Functions
TERMINAL
NO. NAME I/O DESCRIPTION
1−8 LN0−LN7 AI MUX input lines 0−7
9 RESET DI Master reset, zeros all registers
10 RISE/FALL DI Sets the active edge for SCLK. 0 sets SCLK active on falling edge. 1 sets SCLK active on rising edge.
11−14 I/O0−I/O3 DIO Digital input or output signal
15 GND − Connect to ground. (This pin is grounded internally on the ADS7871. It has a weak pulldown on the
ADS7870).
16 CONVERT DI 0 to 1 transition starts a conversion cycle.
17 BUSY DO 1 indicates converter is busy
18 OSC ENABLE DI 0 sets CCLK as an input, 1 sets CCLK as an output and turns the oscillator on.
19 CCLK DIO If OSC ENABLE = 1, then the internal oscillator is output to this pin. If OSC ENABLE = 0, then this is the input
pin for an external conversion clock.
20 SCLK DI Serial data input/output transfer clock. Active edge set by the RISE/FALL pin. If RISE/FALL is low, SCLK is
active o n the falling edge.
21 DIN DIO Serial data input. In the 3-wire mode, this pin is used for serial data input. In the 2-wire mode, serial data
output appears on this pin as well as the DOUT pin.
22 DOUT DO Serial data output. This pin is driven when CS is low and is high impedance when CS is high. This pin
behaves the same in both 3-wire and 2-wire modes.
23 CS DI Chip select. When CS is low, the serial interface is enabled. When CS is high, the serial interface is disabled,
the DOUT pin is high impedance, and the DIN pin is an input. The CS pin only af fects the operation of the
serial interface. It does not directly enable/disable the operation of the signal conversion process.
24 VDD −Power supply voltage, 2.7 V to 5.5 V
25 GND − Power supply ground
26 VREF AO 2.048-/2.5-V on-chip voltage reference
27 BUFIN AI Input to reference buffer amplifier
28 BUFOUT/REFIN AIO Output from reference buffer amplifier and reference input to ADC
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7
TYPICAL PERFORMANCE CURVES
Figure 1
−16
−12
−8
−4
0
4
8
12
16
−50 −25 0 25 50 75 100 125
Gain = 1
Gain = 8
Gain = 20
Gain Error − LSB
GAIN ERROR
vs
FREE-AIR TEMPERATURE
TA − Free-Air T emperature − °C
VDD = 5 V,
CCLK = 2.5 MHz,
REFIN = 2.5 V (ext)
Figure 2
−16
−12
−8
−4
0
4
8
12
16
−50 −25 0 25 50 75 100 125
Gain = 20
Gain = 1
Gain = 8
Offset Error − LSB
OFFSET ERROR
vs
FREE-AIR TEMPERATURE
TA − Free-Air T emperature − °C
VDD = 5 V,
CCLK = 2.5 MHz,
REFIN = 2.5 V (ext)
Figure 3
−0.0025
−0.0015
−0.0005
0.0005
0.0015
0.0025
−50 −25 0 25 50 75 100 125
BufVBG 1.15 V
VBG 1.15 V
BufVREF 2.048 V
VREF 2.048 V
BufVREF 2.5 V
VREF 2.5 V
Voltage Reference Error − V
VOLTAGE REFERENCE ERROR
vs
FREE-AIR TEMPERATURE
TA − Free-Air T emperature − °C
0
VDD = 5 V,
CCLK = 2.5 MHz
Figure 4
2.30
2.35
2.40
2.45
2.50
2.55
2.60
2.65
2.70
−50 −25 0 25 50 75 100 125
Oscillator
−3 sigma
+3 sigma
Internal Oscillator Frequency − MHz
INTERNAL OSCILLATOR FREQUENCY
vs
FREE-AIR TEMPERATURE
TA − Free-Air T emperature − °C
VDD = 5 V
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8
Figure 5
−24
−16
−8
0
8
16
24
012345
Gain = 20
Output Offset Error − LSB
OUTPUT OFFSET ERROR
vs
COMMON-MODE VOLTAGE
Common-Mode Voltage − V
Gain = 8
Gain = 1
VDD = 5 V,
TA = 25°C,
CCLK = 2.5 MHz
Figure 6
−8
−6
−4
−2
0
2
4
6
8
2.5 3 3.5 4 4.5 5 5.5
Gain = 20,
TA = 25°C,
CCLK = 2.5 MHz
VREF = 2.5 VVREF = 2.048 V
Output Offset Error − LSB
Power Supply Voltage − V
OUTPUT OFFSET ERROR
vs
POWER SUPPLY VOLTAGE
Figure 7
2.45
2.475
2.5
2.525
2.55
2.575
2.6
2.625
2.65
−20 −18 −16 −14 −12 −10 −8 −6 −4 −2 0
BUFFER OUTPUT CHARACTERISTIC
− Output Voltage − V
VO
IO − Output Current − mA
ZO = 2Ω Sourcing Current,
VDD = 5 V,
REFIN = 2.5 V,
TA = 25°C
Figure 8
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
5.5
−2 −1.5 −1 −0.5 0 0.5 1 1.5 2
REFERENCE OUTPUT CHARACTERISTIC
− Output Voltage − V
VO
IO − Output Current − µA
VDD = 5 V,
CCLK = 2.5 MHz,
TA = 25°C
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9
Figure 9
−1
0
1
2
3
4
5
6
−6−5−4−3−2−10123456
VDD
VI x GAIN <VDD −1.4
Gain = 1
Gain = 20
Common-Mode Input Voltage − V
Differential Input Voltage − V
TYPICAL INPUT RANGE
VDD = 5 V,
TA = 25°C
Figure 10
0
1
2
3
4
5
6
012345
VDD
Upper Compliance Limit
Lower Compliance Limit
Level-Shift Error
Input to A/D Valid Bit = 1
− Output Voltage − V
PGA OUTPUT
VO
| VI | x Gain
VDD = 5 V,
TA = 25°C
Figure 11
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
0 5 10 15 20 25 30 35 40 45 50
I
QUIESCENT CURRENT
vs
SAMPLING RATE
Sampling Rate − ks/s
SCLK = CCLK = 52 x Sampling Rate,
VDD = 5 V,
VREF and Buf on, OSC off,
TA = 25°C
Q− Quiescent Current − mA
Figure 12
0
5
10
15
20
0 5 10 15 20
Peak-To-Peak Output Code Range
NOISE AND EFFECTIVE NUMBER OF BITS
vs
PGA GAIN
PGA Gain
Effective Number of Bits
15
14
13
12
11
CCLK = 2.5 MHz,
VDD = 5 V,
TA = 25°C
VREF = 2.5 V,
Internal Ref + Buf,
DC Input
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10
INL
−3
−2
−1
0
1
2
3
0 2048 4096 6144 8192 10240 12288 14336 16384
Code
INL − Integral Nonlinearity − LSB
1 µF Cap on,
BUFOUT/REFIN,
VDD = 5 V, REFIN = 2.5 V,
CCLK = 2.5 MHz, TA = 25°C
Figure 13
DNL
−2
−1
0
1
2
3
4
DNL − Differential Nonlinearity − LSB
0 2048 4096 6144 8192 10240 12288 14336 16384
Code
Figure 14
VDD = 5 V,
CCLK = 2.5 MHz,
VREF = 2.5 V,
TA = 25°C
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11
OVERVIEW
The ADS7871 is a complete data acquisition device composed of an input analog multiplexer (MUX), a
programmable gain amplifier (PGA) and an analog-to-digital converter (A/D). Four lines of digital input/output
(I/O) are also provided. Additional circuitry provides support functions including conversion clock, voltage
reference, and serial interface for control and data retrieval.
The ADS7871 is based on the ADS7870 and shares the same interface, functionality, and pinout. The
exceptions are: pin 15 is now hard-wired to ground; the SAR conversion cycle takes 14 clocks rather than 12;
and the output data has only one defined zero at bit D1 (of Register 0), as opposed to three defined zeroes at
bits D1, D2, and D3 (of Register 0) as in the ADS7870.
Control and configuration of the ADS7871 is accomplished by command bytes written to internal registers
through the serial port. Command Register device control includes MUX channel selection, PGA gain, A/D start
conversion command, and I/O line control. Command register configuration control includes internal voltage
reference setting and oscillator control.
Operational modes and selected functions can be activated by digital inputs at corresponding pins. Pin settable
configuration options include SCLK active-edge selection, master reset, and internal oscillator clock enable.
The ADS7871 has eight analog signal input pins, LN0 through LN7. These pins are connected to a network of
analog switches (the MUX block). The inputs can be configured as 8 single-ended or 4 differential inputs, or
some combination.
The four general-purpose digital I/O pins (I/O3 through I/O0) can be made to function individually as either digital
inputs or digital outputs. These pins give the user access to four digital I/O pins through the serial interface
without having to run additional wires to the host controller.
The programmable gain amplifier (PGA) provides gains of 1, 2, 4, 5, 8, 10, 16, and 20 V/V.
The 14-bit A/D converter in the ADS7871 is a successive approximation type. The default output of the
converter is 2s complement format and can be read in a variety of ways depending on the program
configuration.
The ADS7871 internal voltage reference can be software configured for output voltages of 1.15 V, 2.048 V, or
2.5 V. The reference circuit is trimmed for high initial accuracy and low temperature drift. A separate buffer
amplifier is provided to buffer the high impedance VREF output.
The voltage reference, PGA, and A/D converter use the conversion clock (CCLK) and signals derived from it.
CCLK can be either an input or output signal. The ADS7871 can divide the CCLK signal by a constant before
it is applied to the A/D converter and PGA. This allows a higher frequency system clock to be used to control
the A/D converter operation. Division factors (DF) of 1, 2, 4, and 8 are available. The signal that is actually
applied to the PGA and A/D converter is DCLK, where DCLK = CCLK/DF.
The ADS7871 is designed so that its serial interface can be conveniently used with a wide variety of
microcontrollers. It has four conventional serial interface pins: SCLK (serial data clock), DOUT (serial data out),
DIN (serial data in, which may be set bidirectional in some applications), and CS (chip select function).
The ADS7871 has ten internal user accessible registers which are used in normal operation to configure and
control the device (summarized in Figure 18).
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12
FUNCTIONAL DESCRIPTION
Multiplexer
The ADS7871 has eight analog signal input pins, LN0 through LN7. These pins are connected to a network of
analog switches (the MUX block in the block diagram). The switches are controlled by four bits in the Gain/Mux
register.
LN0 through LN7 can be configured as 8 single-ended inputs or 4 differential inputs or some other combination.
Some MUX combination examples are shown in Figure 23. The differential polarity of the input pins can be
changed with the M2 bit in the MUX address. This feature allows reversing the polarity of the conversion result
without having to physically reverse the input connections to the ADS7871.
For linear operation, the input signal at any of the LN0 through LN7 pins can range between GND – 0.2 V and
VDD + 0.2 V. The polarity of the dif ferential signal can be changed through commands written to the Gain/Mux
register, but each line must remain within the linear input common mode voltage range.
Inputs LN0 through LN7 have ESD protection circuitry as the first active elements on the chip. These contain
protection diodes connected to VDD and GND that remain reverse biased under normal operation. If input
voltages are expected beyond the absolute maximum voltage range, it is necessary to add resistance in series
with the input to limit the current to 10 mA or less.
Conversion Clock
The conversion clock (CCLK) and signals derived from it are used by the voltage reference, the PGA, and the
A/D converter. The CCLK pin can be made either an input or an output. For example, one ADS7871 can be
made to be the conversion clock master (CCLK is an output), while the others are slaved to it with their CCLK
pins all being inputs (by default). This can reduce A/D conversion errors caused by multiple clocks and other
systems noise.
When the O SC ENABLE pi n is l ow or zero, the CCLK pin i s an i nput and t he ADS7871 relies on a n applied e xternal
clock for the conversion process. When OSC ENA BLE is high or if the OSCE bit D4 in register 7 is set to a one,
the int ernal oscillator and an internal buffer is enabled, making pin 19 an output. Eit her way the CCLK is sensed
internally at the pin so all ADS7871s see the same clock delays. Capacitive loading on the CCLK pin can draw
significant current com par ed with the supply cur rent to the ADS 7871 (I LOAD = fCCLK × VDD × CLOAD).
The internal reference requires a continuous clock and may be supplied by the internal oscillator independently
of the system clock driving the CCLK pin. Setting OSCR (bit D5 in register 7) and REFE (bit D3 in register 7)
both to one accomplishes this. Figure 11 illustrates all of these relationships.
The ADS7871 utilizes the power saving technique of turning on and off the biasing for the PGA and A/D as
needed. This does not apply to the oscillator , reference, and buffer, these run continuously when enabled. The
buffer output is high impedance when disabled, so for a low power data logging application the filter capacitor
is not discharged when the buffer is turned off and does not require as much settling time when turned on.
The serial interface clock is independent of the conversion clock and can run faster or slower. If it is desirable
to use a faster system clock than the 2.5-MHz nominal rate that the ADS7871 uses then this clock may be
divided to a slower rate ( 1/2, 1/4, 1/8) by setting the appropriate bits in register 3. This clock divider applies
equally t o a n external as well as internal clock to create the internal DCLK for the PGA and A/D conversion cycle.
The ADS7871 has both maximum and minimum DCLK frequency constraints (DCLK = CCLK/DF). The
maximum DCLK is 2.5 MHz. The minimum DCLK frequency applied to the PGA, reference, and A/D is 100 kHz.
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13
Reg.7 D5
OSCR = 1
Reg.7 D5
OSCR = 0
1/4
Internal
Reference
Enabled by
Reg.7D3,
REFE
Enabled by
Reg.7 D2,
BUFE
REF
BUF
Internal Control
Logic
DCLK N Set by
Reg.3 D[1:0],
CFD[1:0]
1/N Divider
Enabled by
Pin 18 OSC Enable
Reg.7 D4, OSCE
Enabled by
Reg.7 D4, OSCE or
Reg.7D5, OSCR or
Pin 18, OSC Enable
Internal Oscillator
(2.5 MHz)
OSC CLK
To ADC
Pin 18
OSC
ENABLE
Pin 19
CCLK
Pin 27
BUFIN
Pin 26
VREF
ADS7871
Pin 28
BUFOUT/REFIN
Figure 15. Block Diagram With Internal and External Clocks and References
Voltage Reference and Buffer Amplifier
The ADS7871 uses a patented switched capacitor implementation of a band-gap reference. The circuit has
curvature correction for drift and can be software configured for output voltages of 1.15 V, 2.048 V, or 2.5 V
(default). The internal reference output (VREF) is not designed to drive a typical load; a separate buffer amplifier
must be used to supply any load current.
The internal reference buffer (REFBUF) can source many tens of milliamps to quickly charge a filter capacitor
tied to its output, but it can only typically sink 200 µA. If there is any significant noise on the REFIN pin, then
a resistor to ground (≥250 Ω) would improve the buffers ability to recover from a positive going noise spike.
This would, of course, be at the expense of power dissipation.
The temperature compensation of the onboard reference is adjusted with the reference buffer in the circuit.
Performance is specified in this configuration.
Programmable Gain Amplifier
The programmable gain amplifier (PGA) provides gains of 1, 2, 4, 5, 8, 10, 16, and 20 V/V. The PGA is a single
supply, rail-to-rail input, auto-zeroed, capacitor based instrumentation amplifier. PGA gain is set by bits G2
through G0 of register 4.
The ability to detect when the PGA outputs are driven to clipping, or nonlinear operation, is provided by the least
significant bit of the output data (register 0) being set to one. This result is the logical OR of fault detecting
comparators within the ADS7871 monitoring the outputs of the PGA. The inputs are also monitored, for
problems, often due to ac common mode or low supply operation and ORed to this OVL bit. Register 2 can b e
read to determine what fault conditions existed during the conversion. An illustration of how the OVL bit could
be set without having reached the maximum output code of the A/D converter is shown in Figure 10. The OVL
bit also facilitates a quick test to allow for an auto-ranging application, indicating to the system controller it should
try reducing the PGA gain.
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14
A/D Converter
The 14-bit A/D converter in the ADS7871 is a successive approximation type. The output of the converter is
2s complement format and can be read through the serial interface MSB first or LSB first. A plot of output codes
vs input voltage is shown in Figure 16. With the input multiplexer configured for differential input, the A/D
transfer function is:
*8192 vCode v8191for *VREF
GvVIN vVREF *1LSB
G
With the input multiplexer configured for single-ended inputs, the A/D transfer function is:
0vCode v8191for 0 vVIN vVREF *1LSB
G
–VREF
+VREF
10 0000 0000 0001 (–8191)
10 0000 0000 0000 (–8192)
Zero Transition
Output Code is 2s Complement
00 0000 0000 0010 (2)
00 0000 0000 0001 (1)
00 0000 0000 0000 (0)
INPUT VOLTAGE
OUTPUT CODE
Negative Full Scale Transition
Positive Full Scale Transition
01 11 11 111 1 1111 (8191)
01 11 11 111 1 1110 (8190)
11 1 111 1111 1111 (–1)
11 1 111 11 11 1 110 (–2)
Figure 16. Output Codes Versus Input Voltage
(1)
(2)
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15
Conversion Cycle
A conversion cycle requires 50 DCLKs, where DCLK = CCLK/DF, the divided-down clock. Operation of the PGA
requires 36 DCLKs: capture the input signal, auto-zero the PGA, level-shift and amplify the input signal. The
period of this cycle makes certain the settling time is sufficient for gain = 20 and (source impedance of 2 kΩ
or less) even if the gain is less than 20. The SAR converter takes the last 14 DCLKs.
For maximum sampling rate the input command and output data must be communicated during this cycle,
although this is not recommended for best performance.
During the conversion cycle the internal capacitive load at the selected MUX input changes between 6 pF and
9.7 pF. When the ADS7871 is not converting, the MUX inputs have a nominal 4-pF load capacitance.
The source impedance of the input causes the voltage to vary on the DCLK transitions as the internal capacitors
are switched in and out. A 10-nF to 100-nF capacitor across the differential inputs helps filter these glitches and
act as an antialias filter in combination with the source impedance. Source impedance greater than 2 kΩ
requires longer settling times and so the CCLK should be reduced accordingly.
For minimum power dissipation, the bias needed for each function is turned on, allowed to settle, and run only
for the duration required for each conversion. Low rate data logging applications can capitalize on this by
utilizing the internal oscillator as needed rather than running a slow system clock.
Starting an A/D Conversion Cycle
There are four ways to cause the ADS7871 to perform a conversion:
1. Send a direct mode instruction.
2. Write to register 4 with the CNV bit = 1
3. Write to register 5 with the CNV bit = 1
4. Assert the CONVERT pin (logic high) —
Serial Interface
The ADS7871 communicates with microprocessors and other external circuitry through a digital serial port
interface. It is compatible with a wide variety of popular microcontrollers and digital signal processors (DSP).
These include TI’s TMS320, MSC1210, and MSP430 product families. Other vendors products such as
Motorola 68HC11, Intel 80C51, and MicroChip PIC Series are also supported.
The serial interface consists of four primary pins, SCLK (serial bit clock), DIN (serial data input), DOUT (serial
data output) and CS (chip select). SCLK synchronizes the data transfer with each bit being transmitted on the
falling o r rising SCLK edge as determined by the RISE/FALL pin. SDIN may also be used as a serial data output
line.
Additional pins expand the versatility of the basic serial interface and allow it to be used with different
microcontrollers. The BUSY pin indicates when a conversion is in progress and may be used to generate
interrupts for the microcontroller. The CONVERT pin can be used as a hardware-based method of causing the
ADS7871 to start a conversion cycle. The RESET pin can be toggled in order to reset the ADS7871 to the
power-on state.
Communication through the serial interface is dependent on the microcontroller providing an instruction byte
followed b y either additional data (for a write operation) or just additional SCLKs to allow the ADS7871 to provide
data (for a read operation). Special operating modes for reducing the instruction byte overhead for retrieving
conversion results are available.
Reset of device (RESET), start of conversion (CONVERT), and oscillator enable (OSC ENABLE) can be done
by signals to external pins or entries to internal registers. The actual execution of each of these commands is
a logical OR function; either pin or register signal TRUE causes the function to execute. The CONVERT pin
signal is an edge-triggered event, with a hold time of two CCLK periods for debounce.
}The next conversion queues up, waiting for the
current conversion to complete
A new conversion cycle starts at the second active edge of
CCLK
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16
Operating Modes
The ADS7871 serial interface operates based on an instruction byte followed by an action commanded by the
contents o f that i nstruction. The 8-bit instruction w ord i s c locked into t he D IN i nput. There are two t ypes o f i nstruction
bytes that may be written to the ADS7871 as determined by bit D7 of the in struction word (see Figure 17). These
two instructions represent two different operat ing modes. In direc t mode (bit D7 = 1), a conversion is started. A
register mode (bit D7 = 0) instruction is followed by a read or writ e operat ion to the spec ified regist er.
OR
D7 (MSB) D6 D5 D4 D3 D2 D1 D0
G21G1G0M3M2M1M0
R/W
0 16/8 A4 A3 A2 A1 A0
Start Conversion
(Direct Mode)
Read/Write
(Register Mode)
INSTRUCTION BYTE
START CONVERSION INSTRUCTION BYTE (Direct Mode)(1)
BIT SYMBOL NAME VALUE FUNCTION
D7 Mode select 1Starts a conversion cycle (direct mode)
D6−D4 G2−G0 PGA gain select 000
001
010
011
100
101
110
111
PGA Gain = 1 (power up default condition)
PGA Gain = 2
PGA Gain = 4
PGA Gain = 5
PGA Gain = 8
PGA Gain = 10
PGA Gain = 16
PGA Gain = 20
D3−D0 M3−M0 Input channel select See Figure 24 Determines input channel selection for the requested conversion,
differential or single-ended configuration.
(1) The seven lower bits of this byte are also written to register 4, the Gain/Mux register.
READ/WRITE INSTRUCTION BYTE (Register Mode)
BIT SYMBOL NAME VALUE FUNCTION
D7 Mode Select 0Initiates a read or write operation (register mode)
D6 R/W Read/Write Select 0
1W rite operation
Read operation
D5 16/8 Word Length 0
18-Bit word
16-Bit word (2 8-bit bytes)
D4−D0 AS4−AS0 Register Address See Figure 18 Determines the address of the register that is to be read from or written to
Figure 17. Instruction Byte Addressing
Direct Mode
In direct mode a conversion is initiated by writing a single 8-bit instruction byte to the ADS7871 (bit D7 is set
to 1). Writing the direct mode command sets the configuration of the multiplexer, selects the gain of the PGA,
and starts a conversion cycle. After the last bit of the instruction byte is received, the ADS7871 performs a
conversion on the selected input channel with the PGA gain set as indicated in the instruction byte.
The conversion cycle begins on the second falling edge of DCLK after the eighth active edge of SCLK of the
instruction byte. When the conversion is complete, the conversion result is stored in the A/D Output registers
and is available to be clocked out of the serial interface by the controlling device using the READ operation in
the register mode.
The structure of the instruction byte for direct mode is shown in Figure 17.
DD7: This bit is set to 1 for direct mode operation
DD6 through D4 (G2 − G0): These bits control the gain of the programmable gain amplifier . PGA gains of 1,
2, 4, 5, 8, 10,16, and 20 are available. The coding is shown in Figure 17.
DD3 through D0 (M3 − M0): These bits configure the switches that determine the input channel selection.
The input channels may be placed in either differential or single-ended configurations. In the case of
differential configuration, the polarity of the input signal is reversible. The coding is shown in Figure 27.
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17
Note that the seven lower bits of this byte are written to register 4, the Gain/Mux register.
All other controllable ADS7871 parameters are values previously stored in their respective registers. These
values are either the power-up default values (0) or values that were previously written to one of the control
registers in a register mode operation. No additional data is required for a direct mode instruction.
Register Mode
In register mode (bit D7 of the instruction byte is 0) a read or write instruction to one of the ADS7871 registers
is initiated. All of the user determinable functions and features of the ADS7871 can be controlled by writing
information to these registers (see Figure 21). Conversion results can be read from the A/D Output registers.
REGISTER ADDRESS
ADDR
READ/
REGISTER CONTENT
A4 A3 A2 A1 A0
ADDR
NO.
READ/
WRITE D7(MSB) D6 D5 D4 D3 D2 D1 D0 REGISTER NAME
0 0 0 0 0 0 Read ADC5 ADC4 ADC3 ADC2 ADC1 ADC0 0 OVR A/D Output Data, LS Byte
0 0 0 0 1 1 Read ADC13 ADC12 ADC11 ADC10 ADC9 ADC8 ADC7 ADC6 A/D Output Data, MS Byte
0 0 0 1 0 2 Read 0 0 VLD5 VLD4 VLD3 VLD2 VLD1 VLD0 PGA Valid Register
0 0 0 1 1 3 R/W 0 0 BIN 0 RMB1 RBM0 CFD1 CFD0 A/D Control Register
0 0 1 0 0 4 R/W CNV/BSY G2 G1 G0 M3 M2 M1 M0 Gain/Mux Register
0 0 1 0 1 5 R/W CNV/BSY 0 0 0 IO3 IO2 IO1 IO0 Digital I/O State Register
0 0 1 1 0 6 R/W 0 0 0 0 OE3 OE2 OE1 OE0 Digital I/O Control Register
0 0 1 1 1 7 R/W 0 0 OSCR OSCE REFE BUFE R2V RBG Ref/Oscillator Control
Register
1 1 0 0 0 24 R/W LSB 2W/3W 8051 0 0 8501 2W/3W LSB Serial Interface Control
Register
1 1 1 1 1 31 Read 0 0 0 0 0 0 0 1 ID Register
Figure 18. Register Address Map
The instruction byte (see Figure 17) contains the address of the register for the next read/write operation,
determines whether the serial communication is to be done in 8-bit or 16-bit word length, and determines
whether the next operation is read-from or written-to the addressed register.
The structure of the instruction byte for register mode is shown in Figure 17.
DD7: This bit is set to 0 for register mode operation.
DD6 (R/W): Bit 6 of the instruction byte determines whether a read or write operation is performed, 1 for
a read or 0 for a write.
DD5 (16/8): This bit determines the word length of the read or write operation that follows, 1 for sixteen bits
(two eight-bit bytes) or 0 for eight bits.
DD4 through D0 (A4 − A0): These bits determine the address of the register that is to be read from or written
to. Register address coding and other information are tabulated in Figure 18.
For 16-bit operations, the first eight bits are written-to/read-from the address encoded by the instruction byte,
bits A4 through A0 (register address). The address of the next eight bits depends upon whether the register
address for the first byte is odd or even. If it is even, then the address for the second byte is the register address
+ 1. If the register address is odd, then the address for the second byte is the register address – 1.
This arrangement allows transfer of conversion results from the two A/D Output Data registers either MS byte
first or LS byte first (refer to the section Serial Interface Control Register).
Register Summary
A summary of information about the addressable registers is shown in Figure 18. Their descriptions follow , an d
more detailed information is provided later in the section Internal User-accessible Registers.
Registers 0 and 1, the A/D Output Data registers, contain the least significant and most significant bits of the
A/D conversion result (ADC0 through ADC13). Register 0 also has one fixed zero (D1), and a bit to indicate
if the internal voltage limits of the PGA have been over ranged (OVR). This is a read only register. Write an 8-bit
word to register 0 and the ADS7871 is reset.
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18
Register 2, the PGA Valid register, contains information that describes the nature of the problem in the event
that the allowable input voltage to the PGA has been exceeded.
Register 3, the A/D Control register, has two test bits (best left set to zero), a bit to convert the output format
to straight binary (BIN), an unused bit set to zero, two bits to configure an automatic read back mode of the A/D
results (RBM1, RBM0), and two bits that program the frequency divider for the CCLK (CDF1, CDF0).
Register 4, the Gain/Mux register, contains the input channel selection information (M0 through M3) and the
programmable gain amplifier gain set bits (G0 through G2).
Register 5, the Digital I/O State register, contains the state of each of the digital I/O pins (I/O3 through I/O0).
In addition, registers 4 and 5 contain a convert/busy bit (CNV/BSY) that can be used to start a conversion via
a write instruction or sense when the converter is busy with a read instruction.
Register 6, the Digital I/O Control register, contains the information that determines whether each of the four
digital I/O pins is to be an input or an output function (OE3 through OE0). This sets the mode of each I/O pin.
Register 7, the Ref/Oscillator Control register, controls whether the internal oscillator used for the conversion
clock is on or of f (OSCE), whether the internal voltage reference and buffer are on or off (REFE, BUFE), and
whether the reference provides 2.5 V, 2.048 V, or 1.15 V.
Register 24, the Serial Interface Control register, controls whether data is presented MSB or LSB first (LSB bit),
whether the serial interface is configured for 2-wire or 3-wire operation (2W/3W bit), and determines proper
timing control for 8051-type microprocessor interfaces (8051 bit).
Register 31, the ID register, is read only.
Reset
There are three ways to reset.
All register contents and the serial interface are reset on:
1. Cycle power. The power down time must be long enough to allow internal nodes to discharge.
2. Toggle the RESET pin. Minimum pulse width to reset is 50 ns.
3. Write an 8-bit byte to register 0. The ADS7871 does not wait for the data which would normally follow this
instruction.
All of these actions set all internal registers to zero, turns off the oscillator, reference, and buffer . Recovery time
for the reference is dependent on capacitance on the reference and buffer outputs.
Only the serial interface is reset (and disabled) when the CS signal is brought high. If CS is continuously held
low, and the ADS7871 is reset by an 8-bit write to register 0 (even if inadvertently) then the next 1 input to DIN
is the synchronizing bit for the serial interface. The next active edge of SCLK following this 1 latches in the first
bit of the new instruction byte.
For applications where CS cannot be cycled and system synchronization is lost, the ADS7871 must be reset
by writing 39 zeros and a one. The serial interface is then ready to accept the next command byte. This string
length is based on the worst case conditions to ensure that the device is synchronized.
NOTE:
A noisy SCLK, with excessive ringing, can cause the ADS7871 to inadvertently reset. Sufficient capacitance to
correct this problem may be provided by just a scope probe, which would mask this issue during debugging. A
100-Ω capacitor in series with the SCLK pin is usually sufficient to correct this problem. Since the data is changed
on the opposite edge of SCLK, it is usually settled before the active edge of SCLK and would not need its own 100-Ω
resistor, although it would not be detrimental.
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Write Operation
To perform a write operation an instruction byte must first be written to the ADS7871 as described previously
(see Figure 17). This instruction determines the target register as well as the word length (8 bits or 16 bits). The
CS pin must be asserted (0) prior to the first active SCLK edge (rising or falling depending on the state of the
RISE/FALL pin) that latches the first bit of the instruction byte. The first active edge after CS must have the first
bit of the instruction byte. The remaining seven bits of the instruction byte are latched on the next seven active
edges of SCLK. CS must remain low for the entire sequence. Setting CS high resets the serial interface.
When starting a conversion by setting the CNV/BSY bit in the Gain/Mux register and/or the Digital I/O register,
the conversion starts on the second falling edge of DCLK after the last active SCLK edge of the write operation.
Figure 19 shows an example of an eight-bit write operation with LSB first and SCLK active on the rising edge.
The double arrows indicate the SCLK transition when data is latched into its destination register.
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A0 A1 A2 A3 A4 0 0 D0 D1 D2 D3 D4 D7
SCLK
DIN
DOUT
CS
Register is updatedInstruction Latched
0D5D6
Figure 19. Timing Diagram for an 8-Bit Write Operation
Figure 20 shows an example of the timing for a 16-bit write to an even address with LSB first and SCLK active
on the rising edge. Notice that both bytes are updated to their respective registers simultaneously. Also shown
is that the address (ADDR) for the write of the second byte is incremented by one since the ADDR in the
instruction byte was even. For an odd ADDR, the address for the second byte would be ADDR−1.
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0A1 100
D0 D1 D2 D3 D4 D5 D6 D1 D2 D3 D4 D5 D6 D7
SCLK
DIN
DOUT
CS
Both Bytes UpdatedInstruction Latched
Data for ADDR Data for ADDR + 1
A2 A3 A4 D7 D0
Figure 20. Timing Diagram of a 16-Bit Write Operation to an Even Address
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Read Operation
A read operation is similar to a write operation except that data flow (after the instruction byte) is from the
ADS7871 to the host controller. After the instruction byte has been latched (on the eighth active edge of SCLK),
the DOUT pin (and the DIN pin if in two-wire mode) begins driving data on the next nonactive edge of SCLK.
This allows the host controller to have valid data on the next active edge of SCLK.
The data on DOUT (or DIN) transitions on the nonactive edges of SCLK. The DIN pin (two-wire mode) ceases
driving data (return to high impedance) on the nonactive edge of SCLK following the eighth (or sixteenth) active
edge of the read data. DOUT is only high impedance when CS is not asserted. With CS high (1), DOUT (or
DIN) is forced to high impedance mode. In general, the ADS7871 is insensitive to the idle state of the clock
except that the state of SCLK may determine if DIN is driving data or not.
Upon completion of the read operation, the ADS7871 is ready to receive the next instruction byte. Read
operations reflect the state of the ADS7871 on the first active edge of SCLK of the data byte transferred.
Figure 21 shows an example of an eight-bit read operation with LSB first and SCLK active on the rising edge.
The double rising arrows indicate when the instruction is latched.
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A0 A1 A2 1
D0 D1 D2 D6 D7
SCLK
DIN
DOUT
CS
0
A3 A4 0
D3 D4 D5
Figure 21. Timing Diagram for an 8-Bit Read Operation
Figure 22 provides an example of a 16-bit read operation from an odd address with LSB first and SCLK active
on the rising edge. The address (ADDR) for the second byte is decremented by one since the ADDR in the
instruction byte is odd. For an even ADDR, the address for the second byte would be incremented by one.
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1 A2 110
D0 D1 D2 D3 D6 D7 D0 D3 D4
Data from ADDR Data from ADDR−1
SCLK
DIN
DOUT
CS
A1 A3 A4
D4 D5 D1 D2 D5 D6 D7
Figure 22. Timing Diagram for a 16-Bit Read Operation to an Odd Address
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Multiplexer Addressing
The last four bits in the instruction byte (during a start conversion instruction) or the Gain/Mux register
(ADDR = 4) assign the multiplexer configuration for the requested conversion. The input channels may be
placed i n either dif ferential or single-ended configurations. For differential configurations, the polarity of the input
signal is reversible by the state of M2 (Bit D2). In single-ended mode, all input channels are measured with
respect to system ground (pin 25). Figure 23 shows some examples of multiplexer assignments and Figure 24
provides the coding for the input channel selection.
EXAMPLES OF MULTIPLEXER OPTIONS
LN0, LN1
Channel 4 Differential
LN2, LN3
LN4, LN5
LN6, LN7
LN0, LN1
LN2, LN3
LN0
LN1
LN2
LN3
LN4
LN5
LN6
LN7
+
+
+
+
+
+
+
+
+
–
+
–
–
+
–
+
+
–
–
+
+
+
+
+
Channel 8 Single−Ended
LN4
LN5
LN6
LN7
Channel
Differential and
Single−Ended
Figure 23. Examples of Multiplexer Options
CODING FOR DIFFERENTIAL INPUT CHANNEL SELECT CODING FOR SINGLE-ENDED INPUT CHANNEL SELECT
(negative input is ground)
SELECTION BITS INPUT LINES SELECTION BITS INPUT LINES
M3 M2 M1 M0 LN0 LN1 LN2 LN3 LN4 LN5 LN6 LN7 M3 M2 M1 M0 LN0 NL1 LN2 LN3 LN4 LN5 LN6 LN7
0000+− 1000+
0 0 0 1 + − 1 0 0 1 +
0 0 1 0 + − 1 0 1 0 +
0 0 1 1 + − 1 0 1 1 +
0100−+ 1100 +
0 1 0 1 − + 1 1 0 1 +
0 1 1 0 − + 1 1 1 0 +
0 1 1 1 − + 1 1 1 1 +
NOTE: Bit M3 selects either differential or single-ended mode. If differential mode is selected, bit M2 determines the polarity of the input channels.
Bold items are power-up default conditions.
Figure 24. Multiplexer Addressing
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INTERNAL USER-ACCESSIBLE REGISTERS
The registers in the ADS7871 are eight bits wide. Most of the registers are reserved, the ten user-accessible
registers are summarized in the register address map (see Figure 18). Detailed information for each register
follows. The default power-on/reset state of all bits in the registers is 0.
ADC Output Registers
The A/D Output registers are read only registers located at ADDR = 0 and ADDR = 1 that contain the results
of the A/D conversion, ADC13 through ADC0 (see Figure 25). The conversion result is in 2s complement
format. The bits can be taken out of the registers MSB (D7) first or LSB (D0) first, as determined by the state
of the LSB bits (D7 or D0) in the Serial Interface Control register . The ADDR = 0 register also contains the OVR
bit which indicates if the internal voltage limits to the PGA have been exceeded.
ADC OUTPUT REGISTERS
ADDR D7 (MSB) D6 D5 D4 D3 D2 D1 D0
0 ADC5 ADC4 ADC3 ADC2 ADC1 ADC0 0 OVR
1 ADC13 ADC12 ADC11 ADC10 ADC9 ADC8 ADC7 ADC6
ADDR = 0 (LS Byte)
BIT SYMBOL NAME VALUE FUNCTION
D7−D2 ADC5−ADC0 A/D Output (1) Six least significant bits of conversion result
D1 — — 0 This bit is not used and is always 0.
D0 OVR PGA Over-Range 0
1Valid conversion result
An analog over-range problem has occurred in the PGA. Conversion result
may be invalid. Details of the type of problem are stored in register 2, the
PGA Valid register.
ADDR = 1 (MS Byte)
BIT SYMBOL NAME VALUE FUNCTION
D7−D0 ADC13−ADC6 ADC Output (1) Eight most significant bits of conversion result
(1) Value depends on conversion result
Figure 25. ADC Output Registers (ADDR = 0 and ADDR = 1)
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PGA Valid Register
The PGA Valid register (ADDR = 2) is a read only register that contains the individual results of each of the six
comparators for the PGA, VLD5 through VLD0, as shown in Figure 26.
PGA VALID REGISTER
ADDR D7 (MSB) D6 D5 D4 D3 D2 D1 D0
2 0 0 VLD5 VLD4 VLD3 VLD2 VLD1 VLD0
ADDR = 2
BIT SYMBOL NAME VALUE FUNCTION
D7−D6 — — 0 These bits are not used and are always 0.
D5 VLD5 PGA Valid 5 0
1Voltage at minus (−) output from the PGA is within its minimum value.
Voltage at minus (−) output from the PGA has exceeded its minimum value.
D4 VLD4 PGA Valid 4 0
1Voltage at minus (−) output from the PGA is within its maximum value.
Voltage at minus (−) output from the PGA has exceeded its maximum value.
D3 VLD3 PGA Valid 3 0
1Voltage at minus (−) input to the PGA is within its maximum value.
Voltage at minus (−) input to the PGA has exceeded its maximum value.
D2 VLD2 PGA Valid 2 0
1Voltage at plus (+) output from the PGA is within its minimum value.
Voltage at plus (+) output from the PGA has exceeded its minimum value.
D1 VLD1 PGA Valid 1 0
1Voltage at plus (+) output from the PGA is within its maximum value.
Voltage at plus (+) output from the PGA has exceeded its maximum value.
D0 VLD0 PGA Valid 0 0
1Voltage at plus (+) input to the PGA is within its maximum value.
Voltage at plus (+) input to the PGA has exceeded its maximum value.
Bold items are power-up default conditions.
Figure 26. PGA Valid Register (ADDR = 2)
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A/D Control Register
The A/D Control register (ADDR = 3) configures the CCLK divider and read back mode option as shown in
Figure 27.
ADC CONTROL REGISTER
ADDR D7 (MSB) D6 D5 D4 D3 D2 D1 D0
3 0 0 BIN 0 RBM1 RBM0 CFD1 CFD0
ADDR = 3
BIT SYMBOL NAME VALUE FUNCTION
D7−D6 — — 0 These bits are reserved and must always be written 0.
D5 BIN Output Data
Format 0
1Mode 0 − Twos complement output data format
Mode 1 − Binary output data format
D4 — — 0This is a reserve bit and must always be written 0
D3−D2 RBM1−RBM0 Automatic Read
Back Mode 00
01
10
11
Mode 0 − Read instruction required to access ADC conversion result.
Mode 1 − Most significant byte returned first
Mode 2 − Least significant byte returned first
Mode 3 − Only most significant byte returned
D1−D0 CFD1−CFD0 CCLK Divide 00
01
10
11
Division factor for CCLK = 1 (DCLK = CCLK)
Division factor for CCLK = 2 (DCLK = CCLK/2)
Division factor for CCLK = 4 (DCLK = CCLK/4)
Division factor for CCLK = 8 (DCLK = CCLK/8)
Bold items are power-up default conditions.
Figure 27. ADC Control Register (ADDR = 3)
Read Back Modes
RBM1 and RBM0 determine which of four possible modes is used to read the A/D conversion result from the
A/D Output registers.
DMode 0 (default mode) requires a separate read instruction to be performed in order to read the output
of the A/D Output registers
DMode 1, 2 , and 3: Provide for dif ferent types of automatic read-back options of the conversion results from
the A/D Output registers without having to use separate read instructions:
Mode 1: Provides data MS byte first
Mode 2: Provides data LS byte first
Mode 3: Output only the MS byte
For more information refer to the read back mode section.
Clock Divider
CFD1 and CFD0 set the CCLK divisor constant which determines the DCLK applied to the A/D, PGA, and
reference. The A/D and PGA operate with a maximum clock of 2.5 MHz. In situations where an external clock
is used to pace the conversion process it may be desirable to reduce the external clock frequency before it is
actually applied to the PGA and A/D. The signal that is actually applied to the A/D and PGA is DCLK, where
DCLK = CCLK/DF (DF is the division factor determined by the CFD1 and CFD0 bits). For example, if the
external clock applied to CCLK is 10 MHz and DF = 4 (CFD1 = 1, CFD0 = 0), DCLK equals 2.5 MHz.
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Gain/Mux Register
The Gain/Mux register (ADDR = 4) contains the bits that configure the PGA gain (G2 − G0) and the input channel
selection (M3 − M0) as shown in Figure 28. This register is also updated when direct mode is used to start a
conversion so its bit definition is compatible with the instruction byte.
GAIN/MUX REGISTER
ADDR D7 (MSB) D6 D5 D4 D3 D2 D1 D0
4 CNV/BSY G2 G1 G0 M3 M2 M1 M0
ADDR = 4
BIT SYMBOL NAME VALUE FUNCTION
D7 CNV/BSY Convert/Busy 0
1Idle Mode
Busy Mode; write = start conversion
D6−D4 G2−G0 PGA Gain Select 000
001
010
011
100
101
110
111
PGA Gain = 1
PGA Gain = 2
PGA Gain = 4
PGA Gain = 5
PGA Gain = 8
PGA Gain = 10
PGA Gain = 16
PGA Gain = 20
D3−D0 M3−M0 Input Channel Select see
Figure 24 Determines input channel selection for the requested conversion, differential or
single-ended configuration.
Bold items are power-up default conditions.
Figure 28. Gain/Mux Register (ADDR = 4)
Input Channel Selection
Bits M3 through M0 configure the switches that determine the input channel selection. The input channels may
be placed in either differential or single-ended configurations. In the case of differential configuration, the
polarity o f the input pins is reversible by the state of the M2 bit. The coding for input channels is given in Figure 24
and examples of different input configurations are shown in Figure 23.
Convert/Busy
If the CNV/BSY bit is set to a 1 during a write operation, a conversion starts on the second falling edge of DCLK
after the active edge of SCLK that latched the data into the Gain/Mux register . The CNV/BSY bit may be read
with a read instruction. The CNV/BSY bit is set to 1 in a read operation if the ADS7871 is performing a conversion
at the time the register is sampled in the read operation.
Gain Select
Bits G2 through G0 control the gain of the programmable gain amplifier . PGA gains o f 1, 2 , 4, 5, 8, 10, 16, and
20 are available. The coding is shown in Figure 28.
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Digital Input/Output State Register
The Digital I/O State register (ADDR = 5) contains the state of each of the four digital I/O pins. Each pin can
function as a digital input (the state of the pin is set by an external signal connected to it) or a digital output (the
state of the pin is set by data from a serial input to the ADS7871). The input/output functional control is
established by the digital I/O mode control bits (OE3−OE0) in the Digital I/O Control register. In addition, the
convert/busy bit (CNV/BSY) can be used to start a conversion via a write instruction or determine if the converter
is busy by executing a read instruction.
Digital I/O State Bits
Bits D3 through D0 (I/O3−I/O0) of the Digital I/O State register are the state bits. If the corresponding mode
bit makes the pin a digital input, the state bit indicates whether the external signal connected to the pin is a 1
or a 0, and it is not possible to control the state of the corresponding bit with a write operation. The state of the
bit is only controlled by the external signal connected to the digital I/O pin. Coding is shown in Figure 29.
DIGITAL I/O STATE REGISTER
ADDR D7 (MSB) D6 D5 D4 D3 D2 D1 D0
5 CNV/BSY 0 0 0 IO3 IO2 IO1 IO0
ADDR = 5
BIT SYMBOL NAME VALUE FUNCTION
D7 CNV/BSY Convert/Busy 0
1Idle Mode
Busy Mode; write = start conversion
D6−D4 — — 0 These bits are not used and are always 0.
D3 IO3 State for I/O3 0
1Input or Output State = 0
Input or Output State = 1
D2 IO2 State for I/O2 0
1Input or Output State = 0
Input or Output State = 1
D1 IO1 State for I/O1 0
1Input or Output State = 0
Input or Output State = 1
D0 IO0 State for I/O0 0
1Input or Output State = 0
Input or Output State = 1
Bold items are power-up default conditions.
NOTE: When the mode control makes a pin a digital input, it is not possible to control the state of the corresponding bit in the Digital I/O State register
with a write operation. The state of the bit is only controlled by the external signal connected to the digital I/O pin.
Figure 29. Digital I/O State Register (ADDR = 5)
The four digital I/O pins allow control of external circuitry, such as a multiplexer, or allow the digital status lines
from other devices to be read without using any additional microcontroller pins. Reads from this register always
reflect the state of the pin, not the state of the latch inside the ADS7871.
Convert/Busy
If CNV/BSY is set to a 1 during a write operation, a conversion starts on the second falling edge of DCLK after
the active edge of SCLK that latched the data into the Digital I/O register. The CNV/BSY bit may be read with
a read instruction. The CNV/BSY bit is set to 1 in a read operation if the ADS7871 is performing a conversion
at the time the register is sampled in the read operation.
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Digital I/O Control Register
The Digital I/O Control register (ADDR = 6) contains the information that determines whether each of the four
digital I/O lines is configured as an input or output. Setting the appropriate OE bit to 1 enables the corresponding
I/O pin as an output. Setting the appropriate OE bit to 0 enables the corresponding I/O pin as an input (see
Figure 30).
DIGITAL I/O CONTROL REGISTER
ADDR D7 (MSB) D6 D5 D4 D3 D2 D1 D0
6 0 0 0 0 OE3 OE2 OE1 OE0
ADDR = 6
BIT SYMBOL NAME VALUE FUNCTION
D7−D4 — — 0 These bits are reserved and must always be set to 0.
D3 OE3 State for I/O3 0
1Digital I/O 1 − digital input
Digital I/O 1 = digital output
D2 OE2 State for I/O2 0
1Digital I/O 2− digital input
Digital I/O 2 − digital output
D1 OE1 State for I/O1 0
1Digital I/O 3− digital input
Digital I/O 3 − digital output
D0 OE0 State for I/O0 0
1Digital I/O 4− digital input
Digital I/O 4 − digital output
Bold items are power-up default conditions.
Figure 30. Digital I/O Control Register (ADDR = 6)
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Reference/Oscillator Configuration Register
The Reference/Oscillator Configuration register (ADDR = 7) determines whether the internal oscillator is used
(OSCE and OSCR), whether the internal voltage reference and buffer are on or off (REFE and BUFE), and whether
the reference is 2.5 V, 2.048 V, or 1.15 V as shown in Figure 31.
REFERENCE/OSCILLATOR REGISTER
ADDR D7 (MSB) D6 D5 D4 D3 D2 D1 D0
7 0 0 OSCR OSCE REFE BUFE R2V RBG
ADDR = 7
BIT SYMBOL NAME VALUE FUNCTION
D7−D6 — — 0 These bits are reserved and must always be set to 0.
D5 OSCR Oscillator Control 0
1Source of clock for internal VREF is CCLK pin.
Clocking signal comes from the internal oscillator.
D4 OSCE Oscillator Enable 0
1CCLK is configured as an input.
CCLK outputs a 2.5-MHz signal (70 µA).
D3 REFE Reference Enable 0
1Reference is powered down.
Reference is powered up.
D2 BUFE Buffer Enable 0
1Buffer is powered down and draws no current.
Buffer is powered up and draws 150 µA of current.
D1 R2V 2-V Reference 0
1VREF = 2.5 V (RBG bit = 0)
VREF = 2.048 V (RBG bit = 0)
D0 RBG Bandgap Reference 0
1Bit R2V determines the value of the reference voltage.
VREF = 1.15 V
Bold items are power-up default conditions.
Figure 31. Reference/Oscillator Configuration Register (ADDR = 7)
Oscillator Control
The internal voltage reference uses a switched capacitor technique which requires a clocking signal input.
When OSCR = 1, the clocking signal for the reference comes from the internal oscillator. When OSCR = 0, the
clocking signal for the reference is derived from the signal on the CCLK pin and affected by the frequency divider
controlled by the CFD0 and CFD1 bits in the A/D Control register.
The OSCE bit is the internal oscillator enable bit. When it is set to 1, power is applied to the internal oscillator
causing i t t o produce a 2.5-MHz output and causing the signal to appear at the CCLK pin. The internal oscillator
is also enabled when the OSCR bit and the REFE bit are set to 1, but does not make CCLK an output pin.
The internal oscillator is also enabled when the OSC ENABLE pin is set to 1. The power-up default condition
is 0 for OSCE and OSCR. If either the OSC ENABLE pin is held high, or either of these control register bits
are 1, then the oscillator is turned on.
Voltage Reference and Buffer Enable
When the REFE bit = 0 (power-up default condition), the reference is powered down and draws no current.
When REFE is set to 1, the reference is powered up and draws approximately 190 µA of current. When the
BUFE bit = 0 (power-up default condition), the buffer amplifier is powered down and draws no current. When
the buffer amplifier is set to 1, it is powered up and draws approximately 150 µA of current.
Selecting the Reference Voltage
When the RBG bit is set to 1, the voltage on the VREF pin is 1.15 V and the R2V bit has no effect. When this
bit is set to 0 (power-up default condition), the R2V bit determines the value of the reference voltage.
When R2V = 0 and RBG = 0 (power-up default condition), the voltage at the VREF pin is 2.5 V. When R2V =
1 and RBG = 0, the reference voltage is 2.048 V.
A 14-bit bipolar input A/D converter has 16384 states and each state corresponds to 305 µV with the 2.5-V
reference. With a 2.048-V reference, each A/D bit corresponds to 250 µV.
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Serial Interface Control Register
The Serial Interface Control register (ADDR = 24), see Figure 32, allows certain aspects of the serial interface
to be controlled by the user. It controls whether data is presented MSB or LSB first and whether the serial
interface is configured for 2-wire or 3-wire operation, and it determines proper timing control for 8051-type
microprocessor interfaces.
The information in this register is formatted with the information symmetric about its center. This is done so that
it may be read or written either LSB (bit D7) or MSB (bit D0) first. Each control bit has two locations in the register .
If either of the two is set, the function is activated. This arrangement can potentially simplify microcontroller
communication code.
The instruction byte to write this configuration data to register 24 is itself symmetric. From Figure 17, a register
mode write instruction of 8 bits to address 24 is 0001 1000 in binary form. Therefore, this command to write
to this register is valid under all conditions.
SERIAL INTERFACE CONTROL REGISTER
ADDR D7 (MSB) D6 D5 D4 D3 D2 D1 D0
24 LSB 2W/3W 8051 0 0 8051 2W/3W LSB
ADDR = 7
BIT SYMBOL NAME VALUE FUNCTION
D7 LSB LSB or MSB first 0
1Serial interface receives and transmits MSB first.
Serial interface receives and transmits LSB first.
D6 2W/3W 2 Wire or 3 Wire 0
13-Wire mode
2-Wire mode
D5 8051 Serial Interface 0
1DIN high impedance on the next inactive edge or when CS goes inactive.
DIN pin is high impedance on last active SCLK edge of the bye of data transfer
D4−D3 — — 0 These bits are reserved and must always be set 0.
D2 8051 Serial Interface 0
1DIN high impedance on the next inactive edge or when CS goes inactive.
DIN pin is high impedance on last active SCLK edge of the byte of data transfer
D1 2W/3W 2 Wire or 3 Wire 0
13-Wire mode
2-Wire mode
D0 LSB LSB or MSB first 0
1Serial interface receives and transmits MSB first.
Serial interface receives and transmits LSB first.
Bold items are power-up default conditions.
Figure 32. Serial Interface Control Register (ADDR = 24)
LSB or MSB
The LSB bit determines whether the serial interface receives and transmits either LSB or MSB first. Setting the
LSB bit (1) configures the interface to expect all bytes LSB first as opposed to the default MSB first (LSB = 0).
2-Wire or 3-Wire Operation
The 2W/3W bit configures the ADS7871 for 2-wire or 3-wire mode. In two-wire mode (2W/3W = 1), the DIN pin
is enabled as an output during the data output portion of a read instruction. The DIN pin accepts data when the
ADS7871 is receiving and it outputs data when the ADS7871 is transmitting. When data is being sent out of
the DIN pin, it also appears on the DOUT pin as well. In three-wire mode (2W/3W = 0), data to the ADS7871
is received on the DIN pin and is transmitted on the DOUT pin. The power-up default condition is three-wire
mode.
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Serial Interface Timing (8051 Bit)
The 8051 bit changes the timing of when the DIN pin goes to high impedance at the end of an operation. When
the bit is a 1, the pin goes to high impedance on the last active SCLK edge of the last byte of data transfer i nstead
of waiting for the next inactive edge, or CS to go inac tive. This allows the ADS7871 to disconnec t from the data
lines s oon e nough t o a void contention with a n 8 0C51-type interface. The 8 0C51 drives data four CPU cycles before
an inactive SCLK edge and for two CPU cycles after an active SCLK edge. When the 8051 bit is a 0, the DIN pin
goes high impedance on the next inactive SCLK edge or when CS goes inactive (1).
Figure 33 and Figure 34 show the timing of when the ADS7871 sets the DIN pin to high impedance at the end
of a read operation when the 2W/3W bit is set. The behavior of DOUT does not depend of the state of 2W/3W.
The 8051 bit is not set for these two examples.
ÓÓÓ
ÓÓÓ
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ÓÓÓ
A0 A1 A2 A3 A4 010 D0D1D2D3D4D5D6D7
D3 D4 D5 D6 D7
SCLK
DIN
DOUT
CS
Micro drives DIN ADS7871 drives DIN DIN high−impedance on CS
D0 D1 D2
Figure 33. Timing for High Impedance State on DIN/DOUT (CS = 1)
ÓÓ
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A0 A3 1 0D0D1D2D3 D7
D0 D1 D4 D5 D6 D7
Micro drives DIN ADS7871 drives DIN DIN high−impedance on inactive edge
SCLK
DIN
DOUT
CS
D4 D5 D6
A4 0
D2 D3
A1 A2
Figure 34. Timing for High Impedance State on DIN/DOUT (Inactive SCLK Edge)
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Figure 35 shows the timing for entering the high impedance state when the 8051 bit is set. Notice that on the
last bit of the read operation the DIN (and DOUT) pin goes to the high impedance state on the active edge of
SCLK instead of waiting for the inactive edge of SCLK or CS going high as shown in Figure 33 and Figure 34.
This is for compatibility with 80C51 mode 0 type serial interfaces. An 80C51 forces DIN valid before the SCLK
falling edge and holds it valid until after the SCLK rising edge. This can lead to contention but setting the 8051
bit fixes this potential problem without requiring CS to be toggled high after every read operation.
ÓÓÓ
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ÓÓÓ
A0 A1 A2 A3 0 D0 D5
D0 D1 D2 D3 D4 D7
Micro drives DIN ADS7871 drives DIN DIN high−impedance on active SCLK edge
SCLK
DIN
DOUT
CS
D6 D7
D5 D6
A4 0 1D1 D2 D3 D4
Figure 35. Timing for High-Impedance State on DIN/DOUT (8051 Bit = 1)
ID Register
The ADS7871 has an ID register (at ADDR = 31) to allow the user to identify which revision of the ADS7871
is installed. This is shown in Figure 36.
ID REGISTER
ADDR D7 (MSB) D6 D5 D4 D3 D2 D1 D0
3100000001
ADDR = 31
BIT SYMBOL NAME VALUE FUNCTION
D7−D0 — — — The contents of this register identify the revision of the ADS7871
Figure 36. ID Register (ADDR = 31)
Remaining Registers
The remaining register addresses are not used in the normal operation of the ADS7871. These registers return
random values when read and nonzero writes to these registers cause erratic behavior. Unused bits in the
partially used registers must always be written low.
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STARTING A CONVERSION THROUGH THE SERIAL INTERFACE
There are two methods of starting a conversion cycle through the serial interface. The first (nonaddressed or
direct mode) is by using the start conversion byte as described earlier. The second (addressed mode) is by
setting the CNV/BSY bit of register 4 or register 5 by performing a write instruction.
The conversion starts on the second falling edge of DCLK after the eighth active edge of SCLK (for the
instruction in nonaddressed mode or the data in addressed mode). The BUSY pin goes active (1) one DCLK
period (1, 2, 4, or 8 CCLK periods depending on CFD1 and CFD0) after the start of a conversion. This delay
is to allow BUSY to go inactive when conversions are queued to follow in immediate succession. BUSY goes
inactive at the end of the conversion.
If a conversion is already in progress when the CNV/BSY bit is set on the eighth active SCLK edge, the
CNV/BSY bit is placed in the queue and the current conversion is allowed to finish. If a conversion is already
queued, the new one replaces the currently queued conversion. The queue is only one conversion long.
Immediately upon completion of the current conversion, the next conversion starts. This allows for maximum
throughput through the A/D converter. Since BUSY is defined to be inactive for the first DCLK clock period of
the conversion, the inactive (falling) edge of BUSY can be used to mark the end of a conversion (and start of
the next conversion).
Figure 37 shows the timing of a conversion start using the convert start instruction byte. The double rising arrow
on SCLK indicates when the instruction is latched. The double falling arrow on CCLK indicates where the
conversion cycle actually starts (second falling edge of CCLK after the eighth active edge of SCLK). This
example is for LSB first, CCLK divider = 1, and SCLK active on rising edge. Notice that BUSY goes active one
CCLK period later since CCLK divider = 1.
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M0 M1 M2 M3 G0 G1 G2 1
Conversion Starts
SCLK
DOUT
CS
CCLK
BUSY
DIN
Figure 37. Timing Diagram for a Conversion Start Using Serial Interface Convert Instruction
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Figure 38 shows an example of a conversion start using an 8-bit write operation to the Gain/Mux register with
the CNV/BSY bit set to 1. The double rising arrow on SCLK indicates where the data is latched into the Gain/Mux
register and the double arrow on CCLK indicates when the conversion starts. The example is for LSB first, CCLK
divider = 1, and SCLK active on the rising edge.
ÓÓ
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A0 A1 A2 A3 A4 0 0 0 M0 M1 M2 M3 G0 G1
SCLK
DIN
DOUT
CS
CCLK
BUSY
Conversion Starts
G2 1
Figure 38. Timing Diagram for a Conversion Start Using 8-Bit Write to the Gain/Mux Register
Figure 39 shows the timing of a conversion start using the convert start instruction byte when a conversion is
already in progress (indicated by BUSY high). The double rising arrow on SCLK indicates when the instruction
is latched. The second falling arrow on CCLK indicates when the conversion cycle would have started had a
conversion not been in progress. The double falling arrow on CCLK indicates where the conversion cycle
actually starts (immediately after completion of the previous conversion). This example is for LSB first, CCLK
divider = 2, and SCLK active on the rising edge. Notice that BUSY is low for two CCLK periods because the
CCLK divider = 2.
ÓÓÓÓÓÓÓÓÓÓÓÓÓÓ
ÓÓÓÓÓÓÓÓÓÓÓÓÓÓ
y
z
ÓÓÓ
ÓÓÓ
ÓÓÓÓÓÓÓÓÓÓÓÓÓÓ
ÓÓÓÓÓÓÓÓÓÓÓÓÓÓ
M0 M1 G0 1
Normal Start Delayed Start
SCLK
DIN
DOUT
CS
CCLK
BUSY
M2 G1 G2
M3
Figure 39. Timing Diagram of Delayed Conversion Start with Serial Interface
SLAS370C − APRIL 2002 − REVISED OCTOBER 2004
www.ti.com
34
STARTING A CONVERSION USING THE CONVERT PIN
A conversion can also be started by an active (rising) edge on the CONVERT pin. Similar to the CNV/BSY
register bit, the conversion starts on the second falling edge of CCLK after the CONVERT rising edge.
The CONVERT pin must stay high for at least two CCLK periods. CONVERT must also be low for at least t w o
CCLK periods before going high. BUSY goes active one DCLK period after the start of the conversion.
Contrary to the CNV/BSY bit in the register, the CONVERT pin aborts any conversion in process and
restart a new conversion. BUSY goes low at the end of the conversion. CS may be either high or low when
the CONVERT pin starts a conversion.
Figure 40 shows the timing of a conversion start using the CONVERT pin. The double falling arrow on CCLK
indicates when the conversion cycle actually starts (the second active CCLK edge after CONVERT goes
active). This example is for CCLK divider = 4. Notice that BUSY goes active four CCLK periods later.
ÓÓÓÓÓÓÓÓÓÓÓÓÓÓ
ÓÓÓÓÓÓÓÓÓÓÓÓÓÓ
Conversion Starts
CCLK
BUSY
CONV
Figure 40. Timing Diagram of Conversion Start Using CONVERT Pin
READ BACK MODES
There are four modes available to read the A/ D conv ersion res ult from the A/D Output regist er s . The RB M 1 and
RBM0 bits in the A/D Control register (ADDR = 3) contr ol whic h mode is used by the ADS 7871.
Read Back Mode 0 (default mode) requires a separate read instruction to retrieve the conversion result
Read Back Mode 1 (automatic) provides the output most significant byte first
Read Back Mode 2 (automatic) provides the output least significant byte first
Read Back Mode 3 (automatic) provides only the most significant byte
Mode 3 does not short cycle the A/D. Automatic read back mode is only triggered when starting a conversion
using the serial interface. Conversions started using the CONVERT pin do not trigger the read back mode.
The first bit of data for an automatic read back is loaded on the first active SCLK edge of the read portion of
THE instruction. The remaining bits are loaded on the next inactive SCLK edge (the first one after the first active
edge). To avoid getting one bit from one conversion and the remainder of the byte from another conversion,
a conversion should not finish between the first active SCLK edge and the next inactive edge.
Mode 0
Mode 0 (default operating mode) requires a read instruction to be performed to retrieve a conversion result.
MS byte first format is achieved by performing a sixteen bit read from ADDR = 1. LS byte first format is achieved
by performing a sixteen bit read from ADDR = 0. Reading only the most significant byte can be achieved by
performing an eight bit read from ADDR = 1.
To increase throughput it is possible to read the result of a conversion while a conversion is in progress. The
last conversion completed prior to the first active SCLK edge of the conversion data word (not the instruction
byte) is retrieved. This overlapping allows a sequence of start conversion N, read conversion N – 1, start
conversion N +1, read conversion N, etc. For conversion 0, the result of conversion –1 would need to be
discarded.
SLAS370C − APRIL 2002 − REVISED OCTOBER 2004
www.ti.com
35
Mode 1
In this mode, the serial interface configures itself to clockout a conversion result as soon as a conversion is
started. This is useful since a read instruction is not required so eight SCLK cycles are saved. This mode
operates like an implied sixteen bit read instruction byte for ADDR = 1 was sent to the ADS7871 after starting
the conversion.
It is not necessary to wait for the end of the conversion to start clocking out conversion results. The last
completed conversion at the sampling edge of SCLK is read back (whether a conversion is in progress or not).
Mode 2
This mode is similar to Mode 1 except that the conversion result is provided LS byte first (equivalent to a sixteen
bit read from ADDR = 0).
Figure 41 and Figure 42 show timing examples of an automatic read back operation using mode 2. In Figure 41,
the result of the previous conversion is retrieved. This example is for LSB first, CCLK divider = 2, and SCLK
active on the rising edge. The data may be read back immediately after the start conversion instruction. It is
not necessary to wait for the conversion to actually start (or finish).
Ó
Ó
Ó
ÓÓ
ÓÓ
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ÓÓ
ÓÓ
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ÓÓÓ
M0 M1 M2 M3 G0 G1 G2 1
OVR 0 B0 B1 B6 B7 B10 B13
SCLK
CS
CCLK
DIN
DOUT
BUSY
The remaining output bits loaded in the output registerFirst output bit loaded in the output register
B2 B3 B4 B5 B8 B9 B11 B12
Figure 41. Timing Diagram for Automatic Read Back of Previous Conversion Result Using Mode 2
SLAS370C − APRIL 2002 − REVISED OCTOBER 2004
www.ti.com
36
In Figure 42 the result of the just requested conversion is retrieved. The microcontroller must wait for BUSY
to go inactive before clocking out the ADC Output register. CS must stay low while waiting for BUSY. This
example is for LS byte first, CCLK divider = 1, and SCLK active on the falling edge. Notice that the DOUT pin
is not driven with correct data until the appropriate active edge of SCLK.
Ó
Ó
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ÓÓ
1G2 M0
B5 B4 B3 B2 B1 B0 0 OVR B13 B12 B11 B10 B8
SCLK
CS
CCLK
DIN
DOUT
BUSY
B9 B7 B6
G1 G0 M3 M2 M1
Figure 42. Timing Diagram for Automatic Read Back of Current Conversion Result Using Mode 2
Mode 3
This mode only returns the most significant byte of the c onversion. It is equi valent to an eig ht bit read from
ADDR = 1.
SLAS370C − APRIL 2002 − REVISED OCTOBER 2004
www.ti.com
37
APPLICATION INFORMATION
REQUIRED SUPPORT ELEMENTS
As with any precision analog integrated circuit, good power supply bypassing is required. A low ESR ceramic
capacitor in parallel with a large value electrolytic capacitor across the supply line furnishes the required
performance. Typical values are 0.1 µF and 10 µF respectively. Noise performance of the internal voltage
reference circuit is improved if a ceramic capacitor of approximately 0.01 µF is connected from VREF to ground.
Increasing the value of this capacitor may bring slight improvement in the noise on VREF but increases the time
required to stabilize after turn on.
If the internal buffer amplifier is used, it must have an output filter capacitor connected to ground to ensure
stability. A nominal value of 0.47 µF provides the best performance. Any value between 0.1 µF and 10 µF is
acceptable. In installations where one ADS7871 buffer is used to drive several devices, an additional filter
capacitor of 0.1 µF should be installed at each of the slave devices.
The circuit in Figure 43 shows a typical installation with all control functions under control of the host embedded
controller. The SCLK is active on the falling edge. If the internal voltage reference and oscillator are used, they
must be turned on by setting the corresponding control bits in the device registers. These registers must be
set on power up and after any reset operation.
18
19
16
17
26
27
28
15
RESET
RISE/FALL
CS
SCLK
DIN
DOUT
OSC_CTRL
CCLK
CONVERT
BUSY
VREF
BUFIN
BUFOUT/REFIN
GND
VDD
GND
D100
D101
D102
D103
LN0
LN1
LN2
LN3
LN4
LN5
LN6
LN7
23
20
21
22
24
25
11
12
13
14
1
2
3
4
5
6
7
8
ADS7871
VDD
Digital I/O − 4 Lines
Analog In − 8 Lines
0.01 µF
0.47 µF
0.01 µF 10 µF
Serial Interface
Figure 43. Typical Operation with Recommended Capacitor Values
SLAS370C − APRIL 2002 − REVISED OCTOBER 2004
www.ti.com
38
APPLICATION INFORMATION
MICROCONTROLLER CONNECTIONS
The ADS7871 is quite flexible in interfacing to various microcontrollers. Connections using the hardware mode
of two types of controllers (Motorola M68HC11, Intel 80C51) are described below.
Motorola M68HC11 (SPI)
The Motorola M68HC11 has a three-wire (four if you count the slave select) serial interface that is commonly
referred to as SPI (serial peripheral interface), where the data is transmitted MSB first. This interface is usually
described as the microcontroller and the peripheral each having two 8-bit shift registers (one for receiving and
one for transmitting).
The transmit shift register of the microcontroller and the receive shift register of the peripheral are connected
together and vice versa. SCK controls the shift registers. SPI is capable of full duplex operation (simultaneous
read and write). The ADS7871 does not support full duplex operation. The ADS7871 can only be written to
or read from. It cannot do both simultaneously.
Since the M68HC11 can configure SCK to have either rising or falling edge active, the RISE/FALL pin on the
ADS7871 can be in whichever state is appropriate for the desired mode of operation of the M68HC11 for
compatibility with other peripherals.
In the Interface Control register (see Figure 32), the 2W/3W bit should be cleared (default). The LSB bit should
be clear (default). The 8051 bit should also be clear (default). Since the ADS7871 defaults to SPI mode, the
M68HC11 should not need to initialize the ADS7871 Interface Configuration register after power-on or reset.
Figure 44 shows a typical physical connection between an M68HC11 and a ADS7871. A pull-up resister on
DOUT may be needed to keep DOUT from floating during write operations. CS may be permanently tied low
if desired, but then the ADS7871 must be the only peripheral.
M68HC11 ADS7871
MISO
MOSI
SCK
SS
DIN
DOUT
SCLK
CS
10 kΩ typ
VDD
Figure 44. Connection of a M68HC11 to an ADS7871
SLAS370C − APRIL 2002 − REVISED OCTOBER 2004
www.ti.com
39
APPLICATION INFORMATION
Intel 80C51
The Intel 80C51 operated in serial port mode 0 has a two-wire (three-wire if an additional I/O pin is used for CS)
serial interface. The TXD pin provides the clock for the serial interface and RXD serves as the data input and
output. The data is transferred LSB first. Best compatibility is achieved by connecting the RISE/FALL pin of the
ADS7871 high (rising edge of SCLK active). In the Interface Configuration register, the LSB bit and the 8051
bit should be set. The 2W/3W bit should also be set. The first instruction after power-on or reset should
be a write operation to the Interface Configuration register.
Figure 45 shows a typical physical connection between an 80C51 and an ADS7871. CS may be permanently
tied low if desired, but then the ADS7871 must be the only peripheral.
80C51 ADS7871
RXD
TXD
Px.x
DIN
DOUT
SCLK
CS
10 kΩ typ
VDD
Figure 45. Connection of an 80C51 to an ADS7871
PACKAGING INFORMATION
Orderable Device Status (1) Package
Type Package
Drawing Pins Package
Qty Eco Plan (2) Lead/Ball Finish MSL Peak Temp (3)
ADS7871IDB ACTIVE SSOP DB 28 50 Green (RoHS &
no Sb/Br) CU NIPDAU Level-1-260C-UNLIM
ADS7871IDBG4 ACTIVE SSOP DB 28 50 Green (RoHS &
no Sb/Br) CU NIPDAU Level-1-260C-UNLIM
ADS7871IDBR ACTIVE SSOP DB 28 1000 Green (RoHS &
no Sb/Br) CU NIPDAU Level-1-260C-UNLIM
ADS7871IDBRG4 ACTIVE SSOP DB 28 1000 Green (RoHS &
no Sb/Br) CU NIPDAU Level-1-260C-UNLIM
(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) Eco Plan - The planned eco-friendly classification: Pb-Free (RoHS), Pb-Free (RoHS Exempt), or Green (RoHS & no Sb/Br) - please check
http://www.ti.com/productcontent for the latest availability information and additional product content details.
TBD: The Pb-Free/Green conversion plan has not been defined.
Pb-Free (RoHS): TI's terms "Lead-Free" or "Pb-Free" mean semiconductor products that are compatible with the current RoHS requirements
for all 6 substances, including the requirement that lead not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered
at high temperatures, TI Pb-Free products are suitable for use in specified lead-free processes.
Pb-Free (RoHS Exempt): This component has a RoHS exemption for either 1) lead-based flip-chip solder bumps used between the die and
package, or 2) lead-based die adhesive used between the die and leadframe. The component is otherwise considered Pb-Free (RoHS
compatible) as defined above.
Green (RoHS & no Sb/Br): TI defines "Green" to mean Pb-Free (RoHS compatible), and free of Bromine (Br) and Antimony (Sb) based flame
retardants (Br or Sb do not exceed 0.1% by weight in homogeneous material)
(3) MSL, Peak Temp. -- The Moisture Sensitivity Level rating according to the JEDEC industry standard classifications, and peak solder
temperature.
Important Information and Disclaimer:The information provided on this page represents TI's knowledge and belief as of the date that it is
provided. TI bases its knowledge and belief on information provided by third parties, and makes no representation or warranty as to the
accuracy of such information. Efforts are underway to better integrate information from third parties. TI has taken and continues to take
reasonable steps to provide representative and accurate information but may not have conducted destructive testing or chemical analysis on
incoming materials and chemicals. TI and TI suppliers consider certain information to be proprietary, and thus CAS numbers and other limited
information may not be available for release.
In no event shall TI's liability arising out of such information exceed the total purchase price of the TI part(s) at issue in this document sold by TI
to Customer on an annual basis.
PACKAGE OPTION ADDENDUM
www.ti.com 9-Oct-2007
Addendum-Page 1
TAPE AND REEL INFORMATION
*All dimensions are nominal
Device Package
Type Package
Drawing Pins SPQ Reel
Diameter
(mm)
Reel
Width
W1 (mm)
A0 (mm) B0 (mm) K0 (mm) P1
(mm) W
(mm) Pin1
Quadrant
ADS7871IDBR SSOP DB 28 1000 330.0 16.4 8.2 10.5 2.5 12.0 16.0 Q1
PACKAGE MATERIALS INFORMATION
www.ti.com 30-Jan-2009
Pack Materials-Page 1
*All dimensions are nominal
Device Package Type Package Drawing Pins SPQ Length (mm) Width (mm) Height (mm)
ADS7871IDBR SSOP DB 28 1000 346.0 346.0 33.0
PACKAGE MATERIALS INFORMATION
www.ti.com 30-Jan-2009
Pack Materials-Page 2
MECHANICAL DATA
MSSO002E – JANUARY 1995 – REVISED DECEMBER 2001
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
DB (R-PDSO-G**) PLASTIC SMALL-OUTLINE
4040065 /E 12/01
28 PINS SHOWN
Gage Plane
8,20
7,40
0,55
0,95
0,25
38
12,90
12,30
28
10,50
24
8,50
Seating Plane
9,907,90
30
10,50
9,90
0,38
5,60
5,00
15
0,22
14
A
28
1
2016
6,50
6,50
14
0,05 MIN
5,905,90
DIM
A MAX
A MIN
PINS **
2,00 MAX
6,90
7,50
0,65 M
0,15
0°–ā8°
0,10
0,09
0,25
NOTES: A. All linear dimensions are in millimeters.
B. This drawing is subject to change without notice.
C. Body dimensions do not include mold flash or protrusion not to exceed 0,15.
D. Falls within JEDEC MO-150
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