AD7731 datasheet(40/44 Pages) AD

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AD7731

–40–

REV. 0

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

EXCITATION VOLTAGE = +5V

100nA

AGND

100nA

AVDD

AIN1

AIN2

AIN3

AIN4

AIN5

AIN6

AGND

REF IN (–)

REF IN (+)

AVDD

DVDD

IN+

OUT+

IN–

OUT–

Figure 22. Pressure Measurement Using the AD7731

Pressure Measurement

One typical application of the AD7731 where it is connected

directly to a transducer is in pressure measurement. Figure 22

shows the AD7731 with a pressure transducer in a bridge

arrangement. The differential output from the transducer is

connected directly to the AIN1/AIN2 input channel. The entire

circuit is powered from a single +5 V supply that generates the

excitation voltage for the transducer and the power supply, and

reference voltage for the AD7731. The application is ratiometric

and variations in the excitation voltage do not introduce errors

in the measurement.

Temperature Measurement

Another application area where the transducer can be connected

directly to the AD7731 is in temperature measurement. Figure

23 outlines a connection between a thermocouple and the

AD7731. In order to place the differential voltage from the

AD7731 on a suitable common-mode voltage, the AIN2 input

of the AD7731 is biased up at the reference voltage, +2.5 V.

Figure 24 shows another temperature measurement application

for the AD7731. In this case, the temperature transducer is an

RTD (Resistive Temperature Device), a PT100. The arrange-

ment is a four-lead RTD configuration. There are voltage drops

across lead resistances RL1 and RL4 and across resistor R2 but

these simply shift the common-mode voltage. Resistor R2 is

required to set the common-mode voltage within the allowable

range for the AD7731. The voltage differential caused by RL2

and RL3 and the AD7731’s offset current is negligible.

In the application shown, the external 400

µA current source

provides the excitation current for the PT100 and it also gener-

ates the reference voltage for the AD7731 via resistor R1. Varia-

tions in the excitation current do not affect the circuit as the

input voltage and the reference voltage vary ratiometrically with

the excitation current. Resistor R1, however, must have a low

temperature coefficient to avoid errors in the reference voltage

over temperature.

AD7731 Datasheet(PDF) 40 Page - Analog Devices

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