Keysight Technologies B1500A Self-Paced Training Manual page 72

Module 2
Basic Measurement
The guard connection is needed for measurement < 1 nA. Below 1 nA a regular coax cable's
capacitance dominates over the DUT (device under test) capacitance. What you see is cable
charging current. I = C(dv/dt) where dv/dt is the rate of change of SMU voltage from one step to the
next of a coax cable with no guard. C is the total capacitance of the cable.
The above diagram shows how the cable capacitance is eliminated with a triaxial cable. The guard is
driven at the same voltage as the force center conductor. No current can flow between guard and
force when they are held at the same potential. Guard and force are isolated by a buffer amplifier.
They can never be shorted together.
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