User`s guide

60 U2741A User’s Guide
3 Measurement Tutorial
Other Sources of Measurement Error
Loading errors (AC volts)
In the AC voltage function, the input of the digital
multimeter appears as a 1 MΩ resistance in parallel with
100 pF of capacitance. The cabling that you use to connect
signals to the digital multimeter also adds capacitance and
loading.
For low frequencies, the loading error is (Equation 2):
(2)
At high frequencies, the additional loading error is
(Equation 3):
(3)
Measurements below full scale
You can make the most accurate AC measurements when the
digital multimeter is at or near the full scale of the selected
range. Autoranging occurs at 10% (down- range) and 120%
(up- range) of full scale. This enables you to measure some
inputs at full scale on one range and 10% of full scale on the
next higher range. In general, the accuracy is better on the
lower range; for the highest accuracy, select the lowest
manual range possible for the measurement.
Error %()
100– R
s
×
R
s
1 MΩ+
---------------------------=
Error %() 100
1
12π FR
s
C
in
×××()+
------------------------------------------------------------- 1–×=
R
s
= Source resistance
F = Input frequency
C
in
= Input capacitance (100 pF) plus cable capacitance
User's Guide.book Page 60 Friday, August 8, 2008 9:11 AM