Datasheet

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a
AD7731
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 617/329-4700 World Wide Web Site: http://www.analog.com
Fax: 617/326-8703 © Analog Devices, Inc., 1997
Low Noise, High Throughput
24-Bit Sigma-Delta ADC
FUNCTIONAL BLOCK DIAGRAM
SIGMA-
DELTA
MODULATOR
AD7731
SERIAL INTERFACE
AND CONTROL LOGIC
CLOCK
GENERATION
PROGRAMMABLE
DIGITAL
FILTER
SIGMA-DELTA A/D CONVERTER
BUFFER
PGA
100nA
AGND
100nA
AV
DD
NC
AIN1
AIN2
AIN3
AIN4
STANDBY
SYNC
MCLK IN
MCLK OUT
SCLK
CS
DIN
DOUT
RESET
RDY
POL
DGNDAGND
AV
DD
DV
DD
REF IN(–)
REF IN(+)
MUX
AIN5
AIN6
REGISTER BANK
CALIBRATION
MICROCONTROLLER
GENERAL DESCRIPTION
The AD7731 is a complete analog front-end for process control
applications. The device has a proprietary programmable gain
front end that allows it to accept a range of input signal ranges,
including low level signals, directly from a transducer. The sigma-
delta architecture of the part consists of an analog modulator
and a low pass programmable digital filter, allowing adjustment
of filter cutoff, output rate and settling time.
The part features three buffered differential programmable gain
analog inputs (which can be configured as five pseudo-differential
inputs), as well as a differential reference input. The part oper-
ates from a single +5 V supply and accepts seven unipolar ana-
log input ranges: 0 to +20 mV, +40 mV, +80 mV, +160mV,
+320 mV, +640 mV and +1.28 V, and seven bipolar ranges:
±20 mV, ±40 mV, ±80 mV, ±160 mV, ±320 mV, ±640 mV and
±1.28 V. The peak-to-peak resolution achievable directly from
the part is 16 bits at an 800 Hz output rate. The part can switch
between channels with 1 ms settling time and maintain a perfor-
mance level of 13 bits of peak-to-peak resolution.
The serial interface on the part can be configured for three-wire
operation and is compatible with microcontrollers and digital
signal processors. The AD7731 contains self-calibration and
system calibration options and features an offset drift of less
than 5 nV/°C and a gain drift of less than 2ppm/°C.
The part is available in a 24-lead plastic DIP, a 24-lead SOIC
and 24-lead TSSOP package.
FEATURES
24-Bit Sigma-Delta ADC
16 Bits p-p Resolution at 800 Hz Output Rate
Programmable Output Rates up to 6.4 kHz
Programmable Gain Front End
60.0015% Nonlinearity
Buffered Differential Inputs
Programmable Filter Cutoffs
FAST
Step™* Mode for Channel Sequencing
Single Supply Operation
APPLICATIONS
Process Control
PLCs/DCS
Industrial Instrumentation
*FASTStep is a trademark of Analog Devices, Inc.
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