AD7719 datasheet(37/40 Pages) AD

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REV. 0

AD7719

–37–

Temperature Measurement

The AD7719 is also useful in temperature measurement appli-

cations, Figure 21 shows an RTD temperature measurement

application. In this application, the transducer is an RTD (Re-

sistive 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 com-

mon-mode voltage. There is no voltage drop across lead resistances

RL2 and RL3 as the input current to the AD7719 is very low,

looking into a high input impedance buffer. RCM is included to

shift the analog input voltage to ensure that it lies within the

common- mode range (AGND + 100 mV to AVDD – 100 mV) of

the ADC. In the application shown, the on-chip 200

µA cur-

rent source provides the excitation current for the PT100 and

also generates the reference voltage for the AD7719 via the

12.5 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 12.5 k

Ω resistor must have a low temperature coefficient to

avoid errors in the reference voltage over temperature.

REFIN(–)

IOUT1

12.5k

AVDD

AIN2

AIN1

AD7719

REFIN(+)

CONTROLLER

IOUT2

DVDD

DGND

AGND

PWRGND

DRDY

SCLK

DIN

DOUT

XTAL1

XTAL2

RREF

RL1

RL2

RL3

RL4

RCM

RTD

200 A

Figure 21. 4-Wire RTD Temperature Masurement Using

the AD7719

Figure 22 shows a further enhancement to the circuit shown in

Figure 21. Generally dc excitation has been accepted as the

normal method of exciting resistive-based sensors like RTD’s

(Resistance Temperature Detectors) in temperature measure-

ment applications.

With dc excitation, the excitation current through the sensor

must be large enough so that the smallest temperature/resistance

change to be measured results in a voltage change that is larger

than the system noise, offset, and drift of the system. The purpose

of switching the excitation source is to eliminate dc-induced

REFIN(–)

IOUT1

IOUT2

AVDD

AIN2

AIN1

AIN3

AIN4

AD7719

REFIN(+)

MUX1

RREF

BUF

PGA

200 A

EMF1

RESISTIVE

TRANSDUCER

EMF2

Figure 22. Low Resistance Measurement AD7719

errors. DC errors (EMF1 and EMF2) due to parasitic thermo-

couples produced by differential metal connections (solder and

copper track) within the circuit are also eliminated when using

this switching arrangement. This excitation is a form of synchro-

nous detection where the sensor is excited with an alternating

excitation source and the ADC only measures information in

the same phase as the excitation source.

The switched polarity current source is developed using the on-

chip current sources and external phase control switches (A and

A) driven from the controller. During the conversion process the

AD7719 takes two conversion results, one on each phase. During

Phase 1 the on-chip current source is directed to IOUT1 and

flows top to bottom through the sensor and switch controlled by

A. In Phase 2, the current source is directed to IOUT2 and flows

in the opposite direction through the sensor and through switch

controlled by A. In all cases the current flows in the same direc-

tion through the reference resistor to develop the reference voltage

for the ADC. All measurements are ratiometrically derived. The

results of both conversions are combined within the microcon-

troller to produce one output measurement representing the

resistance or temperature of the transducer. For example, if the

RTD output during Phase 1 is 10 mV, a 1 mV circuit-induced

dc error exists due to parasitic thermocouples, the ADC mea-

sures 11 mV. During the second phase, the excitation current is

reversed and the ADC measures –10 mV from the RTD and

again sees 1 mV dc error, giving an ADC output of –9 mV

during this phase. These measurements are processed in the

controller (11 mV – (–9 mV)/2 = 10 mV) thus removing the

dc-induced errors within the system.

In the circuit shown in Figure 22, the resistance measurement is

made using ratiometric techniques. Resistor RREF, which devel-

ops the ADC reference, must be stable over temperature to

prevent reference-induced errors in the measurement output.

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