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Model 6517B Electrometer Reference Manual Section 4: Basic measurements

6517B-901-01 Rev. C / August 2015 4-7

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 (on page 4-16)).

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

and C

represent the

leakage resistance and cable capacitance of the triaxial input cable. The equivalent circuit shows the

divider that is formed. If R

is large enough, the divider significantly attenuates the voltage seen at the

input of the Model 6517B (see Cable leakage resistance (on page 4-6)). Also, R

and the cable

capacitance (C

) could create a long RC time constant, resulting in a slow measurement response

(see Input capacitance (on page 4-7)).

Keithley 6517B Reference Manual

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