VTI Instruments Ametek EX1200A Series, User Manual

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EX1200A Series Introduction 29
Crest Factor
Although the EX1200A DMM is a true RMS ac multimeter, calibration and all other accuracy
specifications are applicable only to sinusoidal waveforms. If the input waveform is non-
sinusoidal, it will affect the accuracy of the reading. It is important to consider the crest factor of
the input waveforms to determine the percent error contributed by them. The crest factor of a
waveform is the ratio of its peak value to its RMS value. The larger the crest factor, the greater the
energy contained in higher frequency harmonics, and, hence, the larger the error introduced. See
Crest Factor in Section 4 for more information.
Readings per Second
Reading rate is affected by the measurement function and options selected for each measurement.
In general, options that require additional processing, such as ac filtering, increased resolution, use
of
the “auto” modes, decrease the reading rate of a measurement. A tradeoff always exists between
speed and accuracy. To achieve the maximum reading rate, auto-zero should be disabled, the
trigger delay should be set to 0, a fixed range should be used, and the aperture time should be set
to 0.01 PLC.
Common Mode Voltage
It is possible to measure differential voltages using the DMM. This means both input terminals
can have a common voltage level (with reference to ground) and the DMM will still measure the
differential signal between the HI and LO terminals (V
MEAS
). In theory, differential measurements
eliminate the effects of common mode voltage (V
CM
). However, it is not possible to completely
eliminate the effects of common mode voltage. The common-mode rejection ratio (CMRR) of the
DMM measures the tendency of the device to reject input signals common to both input leads.
Common Mode
HI
LO
+
-
+
-
V
MEAS
V
CM
F
IGURE
1-3: R
ECOMMENDED
C
OMMON
M
ODE
C
ONNECTIONS