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AD7731

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APPLICATIONS

The on-chip PGA allows the AD7731 to handle analog input

voltage ranges from 20 mV to 1.28 V. This makes the AD7731

suitable for a range of application areas from handling signals

directly from a transducer to processing fully-conditioned full-

scale inputs. Some of these applications are discussed in the

following sections.

The AD7731 offers both unipolar and bipolar input ranges. In

many cases, the application is single supply with the bipolar

input voltages referenced to a biased-up differential voltage.

Some applications will, however, require the flexibility of han-

dling true bipolar inputs. Figure 25 shows how to configure the

AD7731 to handle this type of signal.

It should be noted in multiplexed applications that an input

overvoltage (either >AVDD + 0.3 V or <AGND – 0.3 V) on an

unselected channel can affect the conversion result on the se-

lected channel. The system design should ensure that the input

voltage on channels where input leads may be unconnected or

broken be kept within the above limits.

The AD7731 has a variety of different modes aimed at optimiz-

ing the AD7731’s performance across differing application require-

ments. The issue of filtering and settling time and throughput rates

in multichannel applications has previously been discussed in the

Filter Architecture section.

Data Acquisition

The AD7731 with its three differential channels (or five pseudo-

differential channels) is suited to low bandwidth, high resolution

data acquisition systems. In addition, the three-wire digital

interface allows this data acquisition front end to be isolated

with just three optoisolators. The entire system can be operated

from a single +5 V supply provided that the input signals to the

AD7731’s analog inputs are all of positive polarity. Figure 21

shows the AD7731 in an isolated three-channel data acquisition

system.

Programmable Logic Controllers

The AD7731 is also suited to programmable logic controller

applications. In such applications, the ADC is required to

handle signals from a variety of different transducers. The

AD7731’s programmable gain front end allows the part to either

handle low level signals directly from a transducer or full-scale

signals which have already been conditioned. The fast through-

put rate and settling-time of the part is also an important feature

in these applications where loop response time is often critical.

The configuration of the AD7731 in PLC applications is similar

to that outlined in Figure 21 for the data acquisition system.

SIGMA-

DELTA

MODULATOR

CLOCK

GENERATION

AD7731

SERIAL INTERFACE

AND CONTROL LOGIC

DIGITAL

FILTER

SIGMA-DELTA A/D

CONVERTER

BUFFER

PGA

STANDBY

SYNC

MCLK IN

MCLK OUT

SCLK

DIN

DOUT

RESET

RDY

POL

DGND

+5V

100nA

AGND

100nA

AVDD

AIN1

AIN2

AIN3

AIN4

AIN5

AIN6

MICROCONTROLLER

OPTO-ISOLATORS

AGND

REF IN (–)

REF IN (+)

AD780

+5V

VOUT

GND

+VIN

IN1+

IN1–

IN2+

IN2–

IN3+

IN3–

AVDD

DVDD

Figure 21. Data Acquisition Using the AD7731

AD7731 Datasheet(PDF) 39 Page - Analog Devices

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