AC Loading Errors In the ac voltage function, the input of the
internal
DMM appears as a 1 MΩ resistance in parallel with 150 pF
of capacitance. The cabling that you use to connect signals to the
instrument will also add additional capacitance and loading. The table
below shows the approximate input resistance at various frequencies.
Input Frequency Input Resistance
100 Hz
1 kHz
10 kHz
100 kHz
700 k
Ω
600 kΩ
100 kΩ
10 kΩ
For low frequencies:
Error (%) =
− 100 x R
s
R
s
+ 1 M Ω
Additional error for high frequencies:
Error (%) = 100 x
1
√
1 + (
2 π x F x R
s
x C
in
)
2
− 1
F = Input frequency
R
s
= Source resistance
C
in
= Input capacitance (150 pF) + Cable capacitance
Use low-capacitance cable when measuring high-frequency ac signals
(see page 336).
Chapter 8 Tutorial
Measurement Fundamentals
364
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