VTI Instruments Ametek EX1200A Series, User Manual

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EX1200A Triggering 89
NOTE Although the DMM can measure up to 300 V rms (200 V from LO or Sense LO to Guard), this
may exceed the maximum switching value of the plug-in module installed in the system. Ensure
that voltage, current, and power specification for the plug-in modules are not exceeded when
measurements are taken.
True RMS
The EX1200A DMM measures
the “heating” potential of an applied signal. Unlike an “average
responding” measurement, true RMS measurement
s can be used to determine the power dissipated
in a resistance, even by a non-sinusoidal signal. The power is proportional to the square of the
measured true RMS voltage independent of wave shape. Readings of this type are more accurate
and closer to the expected reading of an analog instrument. A complex algorithm calculates the
true RMS voltage of the applied waveforms. When measuring non-sinusoidal waveforms, crest
factor error can occur. For more information on these errors, refer to the
Crest Factor discussion
later in this section.
AC Filters
Theoretically, a true RMS DMM is insensitive to the frequency of the applied signal. This means
that it can measure the RMS value of signals that have both ac and dc components. This, however,
is not the case for the EX1200A DMM, as it measures only the RMS of the of ac component of the
signal. Here, the unwanted frequencies (including DC) are rejected by the signal conditioning
stage before the RMS value is evaluated. This is similar to an ac coupled input system with a
selectable cut-off frequency.
Low frequency accuracy is severely impacted by harmonics generated during the RMS
conversion. To get better low frequency accuracy in SLOW/MEDIUM mode, the DMM employs
low pass digital decimation filters.
1) Low Filter: This filter guarantees frequency accuracy between 3 Hz to 300 kHz. This filter
has a sharp cutoff and hence a large settling time. The ADC integration time is fixed at
0.01 PLC and auto-zero is forced on. The resultant resolution is always 6½ digits
2) Medium Filter: This filter guarantees frequency accuracy between 20 Hz and 300 kHz. This
filter does not have as sharp a cutoff as the SLOW filter, hence the settling time is smaller
compared to the SLOW mode. The ADC integration time is fixed at 0.01 PLC and auto-zero
is forced on. The resultant resolution is always 6½ digits
3) High Filter: This filter allows frequencies between 200 Hz and 300 kHz. This mode uses the
ADC’s frequency response to reject the RMS convertor harmonics. The integration time and
auto-zero settings can be manipulated in this mode allowing for flexible reading rates.
Because of the correlation between filter bandwidth and reading rate, choosing the correct filter to
satisfy the bandwidth requirements for an application is important for measurement optimization.
Crest Factor
It is often thought that the sine wave accuracy specification of a true RMS DMM apply to all
waveforms. This, however, is not the case. The input signal can dramatically affect measurement
accuracy of EX1200A DMM. Because of this, calibration and accuracy specifications are only
applicable to sinusoidal waveforms. Crest factor is a common way to describe the shape of a
waveform.
Crest factor is a waveform
’s
ratio between its peak value and its RMS value. For a time-
continuous repetitive signal, the crest factor is approximately equal to the square root of the
inverse of the duty cycle. The larger the crest factor, the greater the energy contained in the higher
frequency harmonics, resulting in greater error. It is important to consider the crest factor of input
waveforms to determine the percent error contributed by them (see
Crest Factor for typical crest
factors for most common waveforms).