Keithley 2601B Reference Manual page 250

Section 4: Theory of operation Series 2600B System SourceMeter® Instrument Reference Manual
Inside the test fixture, a triaxial cable can be used to extend guard to the device under test (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 ) flows through the insulators (R and R ) to LO,
L L1 L2
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 is the same (0 V drop), no
L1
current can flow through the leakage resistance path. Thus, the SourceMeter instrument only
measures the current through the DUT.
Figure 107: Unguarded measurements
Figure 108: Guarded measurements
4-24 2600BS-901-01 Rev. B / May 2013