Inside the test fixture, a triaxial cable can be used to extend guard to the DUT. The center conductor
of the cable is used for HI, and the inner shield is used for guard.
The figures below show how cable guard can eliminate leakage current through the insulators in a
test fixture. In this figure, leakage current (I
L
) flows through the insulators (R
L1
and R
L2
) to LO,
adversely affecting the low-current (or high-resistance) measurement of the DUT.
Also in the figures below, the driven guard is connected to the cable shield and extended to the metal
guard plate for the insulators. Since the voltage on either end of R
L1
is the same (0 V drop), no
current can flow through the leakage resistance path, so the instrument only measures the current
through the DUT.
Figure 82: Unguarded measurements
Figure 83: Guarded measurements
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