Meter Loading - Keithley 2750 User Manual

E-10 Measurement Considerations

Meter loading

Loading of the voltage source by the Model 2750 becomes a consideration for high source
resistance values. As the source resistance increases, the error caused by meter loading
increases.
Figure E-5
The voltage source, V
Model 2750 is R
Figure E-5
Meter loading
V
S
Source
Voltage
The voltage actually measured by the meter is attenuated by the voltage divider action of
R and R
S
This relationship can be modified to directly compute for percent error:
From the above equation, it is obvious that the input resistance of the Model 2750 must be
at least 999 times the value of source resistance if loading error is to be kept to within
0.1%.
shows the method used to determine the percent error due to meter loading.
, has a source resistance, R
S
, and the voltage measured by the nanovoltmeter is V
I
R
S
+
–
, and it can be calculated as follows:
I
V
M
Percent error = ------------------ -
R
Model 2750 Multimeter/Switch System User's Manual
, while the input resistance of the
S
V
M
Measured
R
I
Voltage
V R
S L
= ------------------ -
R + R
I S
100R
S
+ R
I S
.
M