Model 6220/6221 Reference Manual
Output Connections
2-9
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When to use Cable Guard
When to use Cable Guard is a judgement call that must be made by the user. The
information on “
” will provide a basic understanding on the prin-
ciples of guarding.
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 voltme-
ter 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 rel-
atively high input capacitance (~300pF, and ~500pF in special mode only).
Triax Cable Guard
A triax cable has insulation resistance and capacitance between its conductors. A
typical triax cable could typically have 1T
Ω
/ft. of insulation resistance and 100pF/
ft. of cable capacitance.
Unguarded triax cable example – inner shield connected to Output Low
The example in
shows the effects of using an unguarded output config-
uration (as shown in
) to source 100nA to a 1G
Ω
DUT (R
DUT
). As
shown, a 100G
Ω
insulator resistance (R
L1
) and a 100pF (C
L1
) cable capacitance
exist between output high and output low.
Cable capacitance
– Cable capacitance 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 C
L1
. 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
τ
) for the capacitor to charge and allow the settled current to flow through the
DUT.
, one RC time constant (
τ
) is 99ms. Therefore, a set-
tling time of 495ms (5
τ
) is required to allow sufficient settling for the source. The
higher the capacitance and impedance of the DUT, the longer the current rise time
at the DUT.
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Summary of Contents for 6220
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