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2600AS-901-01 Rev. B / September 2008 Return to Section Topics 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.

Figure 4-13B shows the metal

case of a test fixture being used as a safety shield.

NOTE See Section 2 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.

Figure 4-13 shows how cable guard can eliminate leakage current through the insulators in a test

fixture. In Figure 4-13A, 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.

In Figure 4-13B, 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 2600a series User Manual

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