34405A User’s and Service Guide 63
Measurement Tutorial 3
Capacitance Measurements
The multimeter implements capacitance measurements by applying a
known current to the capacitor as shown below:
Capacitance is calculated by measuring the change in voltage (
D
V) that
occurs over a “short aperture” time, (
D
t). The measurement cycle consists
of two parts: a charge phase and a discharge phase.
The values of capacitance and loss resistance measured with the
multimeter may differ from the values measured using an LCR meter. This
is to be expected, since this is essentially a DC measurement method,
while LCR measurement uses applied frequencies anywhere from 100 Hz
to 100 kHz. In most cases, neither method measures the capacitor at its
exact frequency of application.
For the best accuracy, take a zero null measurement with open probes, to
null out the test lead capacitance, before connecting the probes across the
capacitor to be measured.
Measurement Model
(during charge phase)
Measurement Model
(during discharge phase
C
offset
V
charge
d
C
R'
R
P
C
C
offset
34405A users guide.book Page 63 Saturday, September 2, 2006 3:38 PM
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