Guards; Triax Cable Guard - Keithley 6220 DC User Manual

2-8 Output Connections

Guards

The Model 622x provides two guards: triax Cable Guard and banana jack Guard.
Cable Guard – This guard provides a voltage that is at essentially the same
potential as Output High of the Model 622x. Guarding may greatly reduce leakage
current and capacitance in the test circuit. Effective guarding requires that the
triax Cable Guard configuration be used for the triax cable and a guard plate be
used for the DUT.
Banana Jack Guard – This guard is similar to cable guard in that it provides a
voltage that is essentially the same potential as Output High (1mV accuracy, typi-
cal). This guard should not be used to guard a triax cable. Rather, banana jack
Guard is designed for use with a voltmeter to make measurements on a high-
impedance DUT.
NOTE Details on using the banana jack Guard is provided in

Triax Cable Guard

A triax cable has insulation resistance and capacitance between its conductors. A
typical triax cable could typically have 1TΩ/ft. of insulation resistance and 100pF/ft.
of cable capacitance. Significant insulation resistance and capacitance can also be
present in the test fixture for the DUT.
Capacitance – Any capacitance that exists between the output and the DUT
slows down the response (current rise time) at the DUT. When the Model 622x
output is turned on (or a step change occurs), there is an initial current surge
through the capacitance seen across the load. Current will stop flowing through
the capacitor after it fully charges. A current is considered to be "settled" when it is
within 1% of its final value. It takes approximately five RC time constants (5
the capacitor to charge and allow the settled current to flow through the DUT.
Leakage current – After the capacitance charges, it is effectively removed from
the test circuit (assuming the source level remains constant). Insulation resistance
in parallel with the DUT is left, effectively making a current divider. Leakage cur-
rent (not delivered to the DUT) can be significant if the insulation resistance is not
significanly greater than the resistance of the DUT. For example, if the insulation
resistance is only 10 times the resistance of the DUT, then 10% sourcing error will
occur.
NOTE The principles of guarding are explained in Section 2 of
Section 2 of the Reference Manual.
the Reference Manual (see "Guards").
Return to Section 2 topics
Model 6220/6221 User's Manual
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