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Section 4: Theory of operation Model 2651A High Power System SourceMeter® Instrument Reference Manual

4-18 2651A-901-01 Rev. A / March 2011

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).

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 (>30 V RMS or 42.4 V peak). This

safety shield must completely enclose the guard plate or shield and must be connected to

safety earth ground. The figure in this topic shows the metal case of a test fixture being used

as a safety shield.

NOTE

See Guarding and shielding (on page 2-53) for details about guarded test connections.

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 input/out HI, and the inner shield is used for guard. The

outer shield, which is connected to the safety earth ground through the safety shield, is used for

input/output LO.

A coaxial cable can be used if the guard potential does not exceed 30 V RMS (42.4 V peak). The

center conductor is used for input/output HI, and the outer shield is used for guard. For higher guard

potentials, use a triaxial cable.

The figure below shows 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 In/Out LO,

adversely affecting the low-current (or high-resistance) measurement of the DUT.

Also in the figure 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 (0V drop), no current

can flow through the leakage resistance path. Thus, the SourceMeter instrument only measures the

current through the DUT.

Keithley 2651A Reference Manual

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