AD7719 datasheet(38/40 Pages) AD

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REV. 0

AD7719

–38–

3-Wire RTD Configurations

To fully optimize a 3-wire RTD configuration, two identically

matched current sources are required. The AD7719, which

contains two well-matched current sources, is ideally suited to

these applications. One possible 3-wire configuration using the

AD7719 is outlined in Figure 23.

REFIN(–)

IOUT1

DGND

AGND

12.5k

AVDD

AIN2

AIN1

AD7719

RL3

RCM

REFIN(+)

CONTROLLER

IOUT2

DVDD

DRDY

SCLK

DIN

DOUT

XTAL1

XTAL2

RL2

RTD

200 A

RL1

Figure 23. 3-Wire RTD Configuration Using the AD7719

In this 3-wire configuration, the lead resistances will result in

errors if only one current source is used as the 200

µA will flow

through RL1, developing a voltage error between AIN1 and

AIN2. In the scheme outlined below, the second RTD current

source is used to compensate for the error introduced by the

200

µA flowing through RL1. The second RTD current flows

through RL2. Assuming RL1 and RL2 are equal (the leads

would normally be of the same material and of equal length),

and IOUT1 and IOUT2 match, the error voltage across RL2

equals the error voltage across RL1 and no error voltage is

developed between AIN1 and AIN2. Twice the voltage is devel-

oped across RL3 but, since this is a common-mode voltage, it

will not introduce errors. RCM is included so the current flow-

ing through the combination of RL3 and RCM develops enough

voltage that the analog input voltage seen by the AD7719 is

within the common-mode range of the ADC. The reference

voltage for the AD7719 is also generated using one of these

matched current sources. This reference voltage is developed

across the 12.5 k

Ω resistor as shown, and applied to the differen-

tial reference inputs of the AD7719. This scheme ensures that the

analog input voltage span remains ratiometric to the reference

voltage. Any errors in the analog input voltage due to the tem-

perature drift of the RTD current source is compensated for by

the variation in the reference voltage. The typical drift matching

between the two RTD current sources is less than 1 ppm/

°C.

The voltage on either IOUT pin can go to within 0.6 V of the

AVDD supply.

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