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3.6
Guarded, 3-Terminal Measurements
In many cases it is necessary to measure the resistance between two points in the
presence of resistance from each of these points to a third point. This third point can
often be guarded to avoid error caused by the extraneous resistance. This can best be
illustrated as shown in Figure 3-2 below. Here R X is the quantity to be measured in the
presence of R A and R B . If the junction of R A and R B is tied to guard, R A is placed
across the voltage source and has no effect if it is greater than 1.25MΩ. As long as R B is
greater than R S (standard range resistor) the error caused by R B is minimized, thus if
there is a choice its better to have the higher of the two stray resistances connected to R B .
E
Figure 3-2: Guarded Measurement Configuration
The guard terminal can be used whether GUARD or the +UNKNOWN terminal is
grounded, but it's important to note that if the +UNKNOWN terminal is grounded the
GUARD terminal will be at a high negative voltage level. When the GUARD terminal is
connected to a true ground of a device, such as chassis, it is the one that must be
grounded, not the +UNKNOWN.
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R
A
R
X
R
R
S
B
--
+
GUARD
V
Applications
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