2606B System SourceMeter® Instrument Reference Manual
Section 4: Theory of operation
2606B-901-01 Rev. B / May 2018
4-17
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. Thus, the SourceMeter instrument only
measures the current through the DUT.
Figure 95: Unguarded measurements
Figure 96: Guarded measurements
