National Instruments SCXI-1581, User Manual, Page: 51 / 72

Appendix B Measuring Temperature with Resistive Transducers
© National Instruments Corporation B-17 SCXI-1581 User Manual
Figure B-9. 3-Wire Configuration Using Two Differential Amplifiers
In this configuration, two separate measurements are taken; the first,
labeled
V
1
in Figure B-9, is the sum of the voltage drops across the lead
resistance R
L1
and the resistive device R
T
. If the voltage drop across R
L1
and
R
T
is denoted as V
RL1
and V
RT
respectively, the expression for V
1
becomes:
V
1
= V
RL1
+ V
RT
The second measurement, labeled V
2
in Figure B-9, is equal to the voltage
drop across the lead resistance R
L2
, denoted as V
RL2
; therefore:
V
2
= V
RL2
If the lead resistances R
L1
and R
L2
are assumed equal, you can remove in
software the error voltages due to the lead resistances by subtracting V
2
from V
1
. In most 3-wire device applications the lead wires are all the same
length and made of the same material, therefore substantiating the
assumption of equal lead resistances.
R
L1
R
L2
R
T
EX0+
SCXI-1581
EX0–
SCXI-1102
+
–
CH0+
CH0–
+
–
CH1+
CH1–
V
1 =
V
RL1 +
V
RT
V
2 =
V
RL2
R
L3
Add These
Connections
SCXI-1300
SCXI-1300