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Page 78 of 87 Applications
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 RX is the quantity to be measured in the
presence of RA and RB. If the junction of RA and RB is tied to guard, RA is placed
across the voltage source and has no effect if it is greater than 1.25M
Ω
. As long as RB is
greater than RS (standard range resistor) the error caused by RB is minimized, thus if
there is a choice its better to have the higher of the two stray resistances connected to RB.
E
R
R
R
A
B
X
+
--
GUARD
R
S
V
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.