AD7714YN datasheet(36/40 Pages) AD

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AD7714

REV. C

–36–

RTD Measurement

Figure 14 shows another temperature measurement application

for the AD7714. In this case, the transducer is an RTD (Resis-

tive Temperature Device), a PT100. The arrangement is a 4-

lead RTD configuration. There are voltage drops across the lead

resistances RL1 and RL4 but these simply shift the common-

mode voltage. There is no voltage drop across lead resistances

RL2 and RL3 as the input current to the AD7714 is very low. The

lead resistances present a small source impedance so it would

not generally be necessary to turn on the buffer on the AD7714.

If the buffer is required, the common-mode voltage should be

set accordingly by inserting a small resistance between the bot-

tom end of the RTD and AGND of the AD7714. In the appli-

cation shown an external 400

µA current source provides the

excitation current for the PT100 and it also generates the refer-

ence voltage for the AD7714 via the 6.25 k

Ω resistor. Variations

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

input voltage and the reference voltage vary ratiometrically with

the excitation current. However, the 6.25 k

Ω resistor must have

a low temperature coefficient to avoid errors in the reference

voltage over temperature.

CLOCK

GENERATION

AUTO-ZEROED

MODULATOR

CHARGE BALANCING A/D

CONVERTER

DIGITAL

FILTER

AVDD

1 A

BUFFER

AVDD

SERIAL INTERFACE

STANDBY

SYNC

MCLK IN

MCLK OUT

1 A

REF IN (+)

REF IN (–)

AGND

DGND

BUFFER

DOUT

DIN

SCLK

POL

DRDY

RESET

DVDD

AD7714

+5V

400 A

AGND

RL4

RL3

RL2

RL1

RTD

6.25k

AIN1

AIN2

A = 1–128

PGA

Figure 14. RTD Measurement Using the AD7714

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