Keithley 2636 Reference Manual

8-20 Return to Section Topics 2600S-901-01 Rev. C / January 2008
Section 8: Source-Measure Concepts Series 2600 System SourceMeter® Instruments Reference Manual
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 8-15B shows the metal
case of a test fixture being used as a safety shield.
NOTE See Section 3 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 8-15 shows how cable guard can eliminate leakage current through the insulators in a test
fixture. In Figure 8-15A, 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.
In Figure 8-15B, 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 only measures the current
through the DUT.

Keithley 2636 Specifications

General

Brand	Keithley
Model	2636
Category	Measuring Instruments
Language	English