3: Source-measure hardware Model 4200A-SCS Parameter Analyzer
3-26 4200A-901-01 Rev. C / February 2017
The following figures show how guard can eliminate leakage current through the insulators in a test
fixture.
In the next figure, leakage current (I
L
) flows through the insulators (R
L1
and R
L2
) to COMMON,
adversely affecting the low-current (or high-resistance) measurement of the DUT.
Figure 59: Test fixture - unguarded
In the following figure, the driven GUARD is connected 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. As a result, the SMU or preamplifier measures only the current through the DUT.
Figure 60: Test fixture - guarded
The guard signal has an output impedance of 100 kΩ. Therefore, it is effective only when connected
to high-impedance loads.
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