2600AS-901-01 Rev. B / September 2008
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4-19
Series 2600A System SourceMeter® Instruments Reference Manual
Section 4: Source-Measure Concepts
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 DUT.
WARNING
To prevent injury or death, a safety shield must be used to prevent
physical contact with a guard plate or guard shield that is at a
hazardous potential (>30Vrms or 42.4V peak). This safety shield
must completely enclose the guard plate or shield and must be
connected to safety earth ground.
shows the metal
case of a test fixture being used as a safety shield.
NOTE See
for details on 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 In/Out HI, the inner shield is used for guard, and the outer shield
is used for In/Out LO and is connected to the safety shield (which is connected to safety earth
ground).
A coaxial cable can be used if the guard potential does not exceed 30Vrms (42.4V peak). The
center conductor is used for In/Out HI, and the outer shield is used for guard. For higher guard
potentials, use a triaxial cable as previously explained.
shows how cable guard can eliminate leakage current through the insulators in a test
fixture. In
, leakage current (I
L
) flows through the insulators (R
L1
and R
L2
) to In/Out
LO, adversely affecting the low-current (or high-resistance) measurement of the DUT.
, 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 (0V drop), no current
can flow through the leakage resistance path. Thus, the SourceMeter instrument only measures
the current through the DUT.
