AD7714
RTD Measurement
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.
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 RL1 and RL4 but these simply shift the common-
mode voltage. There is no voltage drop across lead resistances
RL2 and RL3 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.
+5V
AV
DV
DD
DD
400A
REF IN (+)
AV
AD7714
DD
6.25k⍀
1A
REF IN (–)
AIN1
CHARGE BALANCING A/D
STANDBY
CONVERTER
R
L1
AUTO-ZEROED
DIGITAL
SYNC
MODULATOR
⌺⌬
R
PGA
FILTER
L2
BUFFER
RTD
AIN2
A = 1–128
MCLK IN
CLOCK
R
L3
GENERATION
SERIAL INTERFACE
REGISTER BANK
1A
MCLK OUT
AGND
R
L4
RESET
AGND
DGND
DRDY
POL
BUFFER
DOUT
DIN
CS
SCLK
Figure 14. RTD Measurement Using the AD7714
–36–
REV. C
ADI [ ADI ]
