AD7705 datasheet(39/44 Pages) AD

39 / 44 page

AD7705/AD7706

Rev. C | Page 39 of 44

TEMPERATURE MEASUREMENT

Another application of the AD7705 is temperature measure-

ment. Figure 26 outlines a connection between a thermocouple

and the AD7705. For this application, the AD7705 is operated in

buffered mode to allow large decoupling capacitors on the front

end to eliminate any noise pickup from the thermocouple leads.

When the AD7705 operates in buffered mode, it has a reduced

common-mode range. To place the differential voltage from the

thermocouple on a suitable common-mode voltage, the

AIN1(−) input of the AD7705 is biased up at the reference

voltage, 2.5 V.

AD7705

VDD

AIN1(+)

AIN1(–)

THERMOCOUPLE

JUNCTION

REF IN(+)

REF IN(–)

GND

MCLK IN

MCLK OUT

RESET

DRDY

REF192

GND

OUTPUT

DOUT DIN

SCLK

Figure 26. Temperature Measurement Using the AD7705

Figure 27 shows another example of a temperature measure-

ment application for the AD7705. In this case, the transducer is

a resistive temperature device (RTD), a PT100, and the

arrangement is a 4-lead RTD configuration. There are voltage

drops across lead resistances RL1 and RL4, which shift the

common-mode voltage. There is no voltage drop across lead

resistances RL2 and RL3, because the input current to the

AD7705 is very low. The lead resistances present a small source

impedance; therefore, it is not generally necessary to use the

buffer of the AD7705. If the buffer is required, the common-

mode voltage should be set accordingly by inserting a small

resistance between the bottom end of the RTD and the GND

of the AD7705. In the application shown, an external 400 μA

current source provides the excitation current for the PT100

and generates the reference voltage for the AD7705 via the

6.25 kΩ resistor. Variations in the excitation current do not

affect the circuit, because both the input voltage and the

reference voltage vary ratiometrically with the excitation

current. However, the 6.25 kΩ resistor must have a low

temperature coefficient to avoid errors in the reference

voltage over temperature.

AD7705

VDD

AIN1(+)

AIN1(–)

REF IN(–)

GND

MCLK IN

MCLK OUT

RESET

DRDY

DOUT DIN

SCLK

REF IN(+)

6.25k

400

μA

RL1

RL2

RL3

RL4

RTD

Figure 27. RTD Measurement Using the AD7705

AD7705 Datasheet(PDF) 39 Page - Analog Devices

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