2-10 Measurement Concepts Model 6485 Picoammeter Instruction Manual
Test fixture chassis
• The chassis of the test fixture should be metal so that it can function as a shield for
the DUT or test circuit.
• The test box must have a lid that closes to prevent contact with live circuitry.
• The test fixture must have a screw terminal that is used exclusively for connection
to safety earth ground.
WARNING To provide protection from shock hazards, the test fixture chassis must
be properly connected to safety earth ground. A grounding wire (#18
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 safety earth ground.
Guard plate
A metal guard plate will provide guarding or noise shielding for the DUT or test circuit. It
will also serve as a mounting panel for DUT or test circuits. The guard plate must be insu-
lated with appropriate spacing from the chassis of the test fixture commensurate with the
external source used.
Connectors, terminals, and internal wiring
Basic connector requirements include a female BNC connector and two banana jacks. The
banana jacks provide for connection to an external power supply. The banana jacks must
be insulated from the chassis of the test fixture.
DUT and test circuits are to be mounted on the guard plate using insulated terminals. To
minimize leakage, select terminals that use virgin Teflon insulators.
Inside the test fixture, use an insulated wire to connect the shell of the BNC connector to
the guard plate (the guard plate will serve as a noise shield).
Handling and cleaning test fixtures
Dust, body oil, solder flux, and other contaminants on connector and terminal insulators
can significantly decrease the leakage resistance resulting in excessive leakage currents.
Contaminants on DUT and test circuit components can create a leakage path. The leakage
currents may be large enough to corrupt low-level measurements.
Handling tips:
• Do not touch the bodies of DUT or test circuit components. If you can not handle
them by their leads, use clean cotton gloves to install them in the test fixture.
• Do not touch any connector or terminal insulator.
• If installing a test circuit that is on a PC board, handle the board by the edges. Do not
touch any board traces or components.
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