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Theory of Operation
The two-wire RTD measurement is least accurate with longer lead wires resistance. Two-wire RTD
measurement variation with temperature adds significant measurement error. The two-wire RTD
measurement is used only when the sensor is located at a very short distance. Higher accuracy is not a
concern in this case of short distance. Otherwise (in the case of long distance), the effect of lead wires
cannot be eliminated.
5.2.6 Three-Wire RTD Application
Line 3
R BIAS
REFOP
REFON
AVSS
MUX
IDAC
Current
RTD
Line 2
Line 1
AIN0
AIN1
PGA
1:128
24-Bit
ΔΣ ADC
Digital
Filter
Serial
Interface
and
Control
SCLK
DIN
DOUT
DRDY
CS
START
RESET
Line Resistance
IDAC
Current
CLK
A
NOTE: R BIAS should be as close to the ADC as possible.
Figure 35. Three-Wire RTD Circuit
For optimum performance in three-wire RTD applications, the two current sources of ADS1220 have been
extremely closely matched. Figure 35 shows a typical implementation of a ratiometric 3-wire RTD
measurement using two excitation-current sources integrated in the ADS1220, which excites the RTD as
well as implements automatic RTD lead-resistance compensation. In order to implement a ratiometric 3-
wire RTD measurement using the ADS1220, IDAC1 is routed to one of the excitation leads of the RTD,
while IDAC2 is routed to the second excitation lead. Both currents have the same value, which is
programmable by IDAC [2:0]: IDAC current setting bits in configuration register 2. The design of the
ADS1220 ensures that both IDAC values are closely matched, even across temperature.
R L
I
SOURCE1
R RTD
V + MEAS
-
I
SOURCE2
R L
R L
Figure 36. Simplified Three-Wire RTD Configuration and Excitation
Assuming that I SOURCE1 = I SOURCE2 = I and all lead wire resistances are equal.
VMEAS I RL I RRTD 2 I RL I RRTD 3 I RL
V I R 2 I R 3 I R
MEAS L L L
V V I R
MEAS MEAS RTD
(21)
TIDU271–May 2014
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Temperature Sensor Interface Module for Programmable Logic Controllers
(PLC)
Copyright © 2014, Texas Instruments Incorporated
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