Model 4200A-SCS Parameter Analyzer Reference Manual
Section 3: Source-measure hardware
4200A-901-01 Rev. C / February 2017
3-25
In the guarded circuit of the next figure, however, the cable shield is driven by a unity-gain, low-
impedance amplifier (GUARD). Since the voltage across R
L
is nearly 0 V, the leakage current is
effectively eliminated. Current through any leakage resistance (R
G
) between the shield and
COMMON may be considerable, but it is of little consequence because it is supplied by the unity-gain
amplifier rather than the FORCE terminal of the SMU or preamplifier.
Figure 58: Guarded circuit
Test fixture guarding
GUARD used to drive the inner shields of triax 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).
