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Chapter 2 Using the SCXI-1127/1128
SCXI-1127/1128 User Manual 2-10 ni.com
current and the other for sensing the voltage developed across the resistor,
as shown in Figure 2-9.
Figure 2-9. Signal Connections for a 4-Wire Resistance Measurement
Since ideally the impedance of the voltmeter is infinite, very little or no
current passes through the sense leads of the voltmeter. As a result, the
voltmeter reads the exact voltage developed across R
1
, and no parasitic
resistances are introduced into the measurement:
Therefore, a 4-wire configuration is a more accurate way of performing
resistance measurements than a 2-wire configuration.
Mixed Mode Configuration
The input mode of the SCXI-1127/1128 is configurable on a per channel
basis; therefore, you can connect a variety of signal types in 2-wire, 1-wire,
and 4-wire modes to the same module. An SCXI-1127/1128 can have
2-wire, 1-wire, and 4-wire channels connected to its front at the same time,
as shown in Figure 2-10.
V
SENSE
+
4-Wire
Measurement
R
P
V
R
1
R
1
I
EX
I
P
~0
I
P
~0
R
P
R
P
I
EX
R
P
V
SENSE
–
R
1
measured
V
R
1
I
EX
-------- R
1
==