Datasheet

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
V
DD
5V
AIN1(+)
AIN1(–)
THERMOCOUPLE
JUNCTION
REF IN(+)
REF IN(–)
GND
MCLK IN
MCLK OUT
RESET
DRDY
REF192
GND
OUTPUT
DOUT DIN
CS
SCLK
5V
01166-026
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 R
L1
and R
L4
, which shift the
common-mode voltage. There is no voltage drop across lead
resistances R
L2
and R
L3
, 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
V
DD
5V
AIN1(+)
AIN1(–)
REF IN(–)
GND
MCLK IN
MCLK OUT
RESET
DRDY
DOUT DIN CS SCLK
REF IN(+)
6.25kΩ
400μA
R
L1
R
L2
R
L3
R
L4
RTD
01166-027
Figure 27. RTD Measurement Using the AD7705