Datasheet

AD7714
REV. C–36–
RTD Measurement
Figure 14 shows another temperature measurement application
for the AD7714. 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 R
L1
and R
L4
but these simply shift the common-
mode voltage. There is no voltage drop across lead resistances
R
L2
and R
L3
as the input current to the AD7714 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 AD7714.
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 AGND of the AD7714. In the appli-
cation shown an external 400␣ µA current source provides the
excitation current for the PT100 and it also generates the refer-
ence voltage for the AD7714 via the 6.25 kΩ 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 6.25␣ kΩ resistor must have
a low temperature coefficient to avoid errors in the reference
voltage over temperature.
CLOCK
GENERATION
AUTO-ZEROED
SD
MODULATOR
CHARGE BALANCING A/D
CONVERTER
DIGITAL
FILTER
AV
DD
1mA
BUFFER
AV
DD
SERIAL INTERFACE
REGISTER BANK
STANDBY
SYNC
MCLK IN
MCLK OUT
1mA
REF IN (+)
REF IN (–)
AGND
DGND
BUFFER
DOUT DIN
CS
SCLK
POL
DRDY
RESET
DV
DD
AD7714
+5V
400mA
AGND
R
L4
R
L3
R
L2
R
L1
RTD
6.25kV
AIN1
AIN2
A = 1–128
SWITCHING
MATRIX
PGA
Figure 14. RTD Measurement Using the AD7714