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Model 6517B Electrometer Reference Manual
Input capacitance
At very high resistance levels, the very large time constants created by even a minimal amount of
capacitance can slow down response time considerably. For example, measuring a source with an
internal resistance of 100 G would result in an RC time constant of one second when measured
through a cable with a nominal capacitance of 10 pF. If 1 % accuracy is required, a single
measurement would require at least five seconds.
There are two ways to minimize this problem:
Keep the input cable as short as possible
Use guarding
There is a limit to how short the cable can be. Using guard can reduce these effects by up to a factor
of 1000 (refer to
Guarding
Guarding should be used for high-impedance voltage measurements and for voltage measurements
that use long input cables.
To understand the concept of guarding, review the unguarded circuit shown in the figure below. E
and R
represent the resistance and voltage components of the source, and R
S
leakage resistance and cable capacitance of the triaxial input cable. The equivalent circuit shows the
divider that is formed. If R
input of the Model 6517B (see
capacitance (C
(see
Input capacitance
6517B-901-01 Rev. C / August 2015
Guarding
(on page 4-16)).
is large enough, the divider significantly attenuates the voltage seen at the
S
Cable leakage resistance
) could create a long RC time constant, resulting in a slow measurement response
L
(on page 4-7)).
Section 4: Basic measurements
and C
L
(on page 4-6)). Also, R
and the cable
S
S
represent the
L
4-7
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