Chapter 2 Connecting Signals
SCXI-1503 User Manual 2-4 ni.com
2-Wire Configuration
The basic 2-wire configuration is shown in Figure2-2. In this configuration
an error voltage (VE) is introduced into the measurement equal to the
excitation current (IEX) times the sum of the two lead resistances, RL1 and
RL2. If we assume equal lead resistances, RL1 =RL2 =RL, the magnitude of
the error voltage is:
This is the most commonly used configuration for connecting thermistors
to a signal conditioning system because the large sensitivity of thermistors
usually results in the introduction of a negligible error by the lead
resistances.
RTDs typically have a much smaller sensitivity and nominal resistance than
thermistors, therefore a 2-wire configuration usually results in the
introduction of larger errors by the lead resistance.
Figure 2-2. 2-Wire Resistive Sensor Connected in a 2-Wire Configuration
VE2RLIEX
=
ON
CH X
+
–
SCXI-1306External Sensor SCXI-1503
Channel X
IEX+
AI+
AI–
IEX–
I = 100 µA
RL1
RL2
RT