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Figure 2-4. Connections for Voltage Measurements

Voltage Measurement Considerations: Two primary con-

siderations come to mind when making voltage

measurements. especially for voltage sources with high out-

put resistances. For one thing, the loading effects of the

measuring instrument come into play at the high resistance

levels involved. Secondly, the distributed capacitance of the

source, the input cable, and the input circuit of the instrument

itself come into play when making these measurements.

To see how meter loading can affect accuracy, refer to Figure

2-5. In this figure there is a voltage source with a value ES and

an output RS connected to the input of the electrometer,

which has its input resistance represented by RIN. The percent

error due to loading can be calculated as follows:

100 RS

% ERROR = -

Rs + RIN

Thus, to keep the error under 0.1%. the input resistance must

be about 1000 times the value of the source resistance. R.

At very high resistance levels, the very large time contants

created by even a minimal amount of capacitance can slow

down response time considerably. For example, measuring a

wurce with an internal resistance of 1OOGQ would result in an

RC time constant of one second when measured through a

cable with a nominal capacitance of 1OpF. If 1% accuracy is

required, a single measurement would require at least five

seconds.

Basically, there are two ways to minimize this problem: (1)

keep the input cable as short as possible, and (2) use guarding.

With the first method, there is a limit as to how short the

cable can be. Using guarding can reduce these effects by up to

a factor of 1000. The Model 617 has a rear panel switch to

allow guarding to be easily applied to the input circuit: see the

next paragraph for details.

2-9

Keithley 617 User Manual

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