3: Source-measure hardware Model 4200A-SCS Parameter Analyzer
3-24 4200A-901-01 Rev. C / February 2017
The next figure shows the triaxial cable connections to the device under test (DUT). Note that
GUARD is not connected in this example, but it can be routed internally to a test fixture, as described
in Test fixture guarding (on page 3-25
).
Figure 56: GUARD connections
Guarding concepts
Guarding is especially important with high-impedance circuits. Consider the comparison of the
unguarded and guarded circuits shown in the next figures. In both cases, FORCE is connected to
DUT HI, while COMMON is connected to DUT LO.
In the unguarded circuit of the following figure, the cable leakage resistance, R
L
, is effectively in
parallel with the DUT, creating an unwanted leakage current, I
L
. This leakage current may seriously
affect readings, particularly at low current levels.
Figure 57: Guarded circuit
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