Measurement Considerations C-9
Meter loading
Loading of the voltage source by the Model 2182 becomes a consideration for high source
resistance values. As the source resistance increases, the error caused by meter loading
increases.
Figure C-5 shows the method used to determine the percent error due to meter loading. The
voltage source, V
S
, has a source resistance, R
S
, while the input resistance of the Model 2182 is
R
I
, and the voltage measured by the nanovoltmeter is V
M
.
Figure C-5
Meter loading
The voltage actually measured by the meter is attenuated by the voltage divider action of R
S
and R
I
, and it can be calculated as follows:
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 2182 must be at
least 999 times the value of source resistance if loading error is to be kept to within 0.1%.
V
S
Source
Voltage
+
–
R
S
R
I
V
M
Measured
Voltage
V
M
V
S
R
L
R
I
R
S
+
-------------------=
Percent error
100R
S
R
I
R
S
+
-------------------=
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