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Model 6220/6221 User's Manual
When to use Cable Guard
When to use Cable Guard is a judgement call that must be made by the user. In
general, Cable Guard is used to reduce leakage current for high-impedance DUT,
and reduce capacitance in the triax cable and at the DUT to improve output
response (rise time). Lower capacitance results in faster output response.
While Cable Guard will greatly reduce capacitance in the triax cable and at the
DUT, there are other possible sources of high capacitance that can greatly
degrade the overall effectiveness of guarding.
The DUT (or test circuit) may inherently have high capacitance, and/or the voltmeter
used in the test system may have high capacitance. These capacitances, which
cannot be guarded out, may negate the need for Cable Guard (especially for low-
impedance DUT). The Keithley Model 2182/2182A Nanovoltmeter has relatively
high input capacitance (~300pF, and ~500pF in special mode only).
Floating the current source
Using an external source in the test system may require that the Model 622x
current source float off Earth Ground. Depending on the test circuit configuration,
the external source could possibly be shorted through Earth Ground if the Model
622x is not floating off Earth Ground. Details on "Floating the current source" are
provided in Section 2 of the Reference Manual.
Connections to DUT
WARNING
Supplied triax cable
The Model 237-ALG-2 triax cable is supplied with the Model 622x. This 6.6ft (2m)
cable mates to the OUTPUT triax connector and is terminated with alligator clips
that are covered with color-coded boots. Terminal identification for the cable
depends on the triax connector configuration (see
WARNING
To prevent electric shock, all power must be removed from the
test system before making or breaking connections. Turn off
all equipment and sources, and disconnect their power cords.
The Model 237-ALG-2 triax cable can allow exposed voltages
when used in certain applications.
This triax cable is intended for use only by qualified personnel
who recognize shock hazards and are familiar with the safety
precautions required to avoid possible injury. If this product is
to be used by an operator, a qualified person must ensure the
operator is protected from electric shock and contact with haz-
ardous live circuits.
Return to
Section 2 topics
Output Connections
Figure 2-4
and
Figure
2-9
2-5).
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