AD7707 datasheet(36/40 Pages) AD

36 / 40 page

REV. A

AD7707

–36–

RTD Measurement

Figure 29 shows another temperature measurement application

for the AD7707. 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 AD7707 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 AD7707.

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 GND of the AD7707. In the applica-

tion shown, an external 400

µA current source provides the

excitation current for the PT100 and also generates the refer-

ence voltage for the AD7707 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 radiometrically 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.

AD7707

DVDD

+5V

REF IN(–)

AGND

MCLK IN

MCLK OUT

DRDY

RTD

AVDD

AIN1

LOCOM

DGND

SCLK

DIN

DOUT

CONTROLLER

RL3

6.25k

RL2

RL4

RL1

REF IN(+)

400 A

REF IN(+)

REF IN(–)

Figure 29. RTD Measurement Using the AD7707

Chart Recorders

Another area where both high and low level input channels are

usually found is in chart recorder applications. Circular chart

recorders generally have two requirements. The first which

would utilize the low level input channels of the AD7707 to

measure inputs from thermocouples, RTDs and pressure sen-

sors. The second requirement is to be able to measure dc input

voltage ranges up to

±10 V. The high level input channel is

ideally suited to this measurement as there is no external signal

conditioning required to accommodate these high level input

signals.

AD7707 Datasheet(PDF) 36 Page - Analog Devices

AD7707 Datasheet(HTML) 36 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

Datasheet Download

Link URL