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Chapter 8 Tutorial
Measurement Fundamentals
AC Loading Errors In the ac voltage function, the input of the
internal DMM appears as a 1 M 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
For low frequencies:
100 x R
s
Error (%) =
R
1 M
s
Additional error for high frequencies:
Error (%) = 100 x
F = Input frequency
R
= Source resistance
s
C
= Input capacitance (150 pF) + Cable capacitance
in
Use low-capacitance cable when measuring high-frequency ac signals
(see page 336).
364
700 k
600 k
100 k
10 k
1
2
1
2 x
F x R
x C
s
in
1
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