Section 5: Source-measure concepts
Model 4200A-SCS Source-Measure Unit (SMU) User's Manual
5-4
4200A-SMU-900-01 Rev. A December 2020
Test fixture guarding
GUARD used to drive the inner shields of triaxial connecting cables can be routed within test fixtures.
Inside the test fixture, a triaxial cable can be used to extend the guard near to the DUT, and the guard
can be connected to a guard plate or shield that surrounds the DUT. The center conductor of the
cable is used for FORCE or SENSE, the inner shield is used for GUARD, and the outer shield is
COMMON.
To provide protection from shock hazards, an enclosure should be provided that surrounds
all live parts.
Nonconductive enclosures must be constructed of materials that are suitably rated for
flammability and the voltage and temperature requirements of the test circuit. Connect the
enclosure of all metal test fixtures to protective earth (safety ground). See your specific test
fixture for information. Nonconductive test fixtures must be rated to double the maximum
capability of the test equipment in the system.
For metallic enclosures, the test fixture chassis must be properly connected to protective
earth (safety ground). A grounding wire (16 AWG or larger) must be attached securely to the
test fixture at a screw terminal designed for safety grounding. The other end of the ground
wire must be attached to a known protective earth (safety ground).
The following figures show how guard can eliminate leakage current through the insulators in a
test fixture.
In the following figure, leakage current (I
L
) flows through the insulators (R
L1
and R
L2
) to COMMON,
adversely affecting the low-current (or high-resistance) measurement of the DUT.
Figure 71: Test fixture - unguarded
