Section 4: Basic measurements Model 6517B Electrometer Reference Manual
4-6 6517B-901-01 Rev. C / August 2015
Voltage measurement considerations
Some considerations for making accurate voltage measurements are summarized in the following
paragraphs. For comprehensive information on precision measurements, refer to the Low Level
Measurements Handbook, which is available from Keithley Instruments.
Loading effects
Circuit loading can be detrimental to high-impedance voltage measurements. To see how meter
loading can affect accuracy, refer to the following figure. R
S
represents the resistance component of
the source, while R
IN
represents the input resistance of the meter. The percent error due to loading
can be calculated using the formula in the illustration. To keep the error under 0.1 %, the input
resistance (R
IN
) must be about 1000 times the value of the source resistance (R
S
). The input
resistance of the Model 6517B is >2 10E
14
. Thus, to keep the error under 0.1 %, the source
resistance of the measured voltage must be <2 10E
11
.
Figure 31: Meter loading
Cable leakage resistance
In an unguarded voltage measurement, leakage current occurs in the input triaxial cable between the
center conductor (HI) and the inner shield (LO). This leakage resistance shunts the voltage source to
be measured. If the resistance of the source is not significantly less than the leakage resistance of the
cable, then measurement errors occurs.
The effects of leakage resistance can be eliminated by using guard to make high impedance voltage
measurements (refer to Guarding (on page 4-16)). In general, guarding should be used when the
resistance of the voltage source is 10
9
or greater.
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