Model 2601B-PULSE System SourceMeter Instrument Reference Manual Section 7: Theory of operation
2601B-PULSE-901-01A April 2020 7-25
Guard connections
Guard is typically used to drive the guard shields of cables and test fixtures. Guard is extended to a
test fixture from the cable guard shield. Inside the test fixture, the guard can be connected to a guard
plate or shield that surrounds the device-under-test (DUT).
A safety shield must be used whenever hazardous voltages (>30 V
RMS
, 42 V
PEAK
) will be
present in the test circuit. To prevent electrical shock that could cause injury or death, never
use the 2601B-PULSE in a test circuit that may contain hazardous voltages without a properly
installed and configured safety shield. The figure in this topic shows the metal case of a test
fixture being used as a safety shield.
See Guarding and shielding (on page 4-20) for details about guarded test connections.
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 123: Unguarded measurements
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