3-4) NMRR (Normal Mode Rejection Ratio)
NMRR is the DMM's ability to reject unwanted AC noise
effect which can cause inaccurate DC measurements.
NMRR is typically specified in terms of dB (decibel).
KM629 has a NMRR specification of >50dB at 50 and
60Hz, which means a good ability to reject the effect of
AC noise in DC measurements.
3-5) CMRR (Common Mode Rejection Ratio)
Commom mode voltage is voltage present on both the
COM and VOLTAGE input terminals of a DMM, with
respect to ground. CMRR is the DMM's ability to reject
common mode voltage effect which can cause digit
rattle or offset in voltage measurements.
KM629 has a CMRR specifications of >60dB at DC to
60Hz in ACV function; and >100dB at DC, 50 and 60Hz
in DCV function. If neither NMRR nor CMRR
specification is specified, a DMM's performance will be
uncertain.
3-6) Crest Factor
Crest Factor is the ratio of the Crest (instantaneous
peak) value to the True RMS value. That is :
Crest Factor =
Vcrest
Vrms
A pure sinusoidal waveform has a Crest Factor of
1.414. A badly distorted sinusoidal waveform normally
has a much higher Crest Factor. If you are measuring a
signal above the DMM's specified Crest Factor, the
DMM's may not produce accurate measurements.
KM629 can accurately measure the True RMS value of
voltage signal with a Crest Factor up to 3.0 at full scale,
and 6.0 at half scale.
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3-7) Average responding RMS calibrated
RMS (Root-Mean-Square) is the term used to describe
the effective or equivalent DC value of an AC signal.
Most digital multimeters use average responding RMS
calibrated technique to measure RMS values of AC
signals. The technique is to obtain the average value by
rectifying and filtering the AC signal. The average value
is then scaled upward (calibrated) to read the RMS value
of a sine wave.
In measuring pure sinusoidal waveform, this technique is
cost effective and accurate. In measuring non-
sinusoidal waveforms, however, significant errors can be
introduced because of different scaling factors relating
average to RMS values.
3-8) True RMS
True RMS is a term which identifies a DMM that responds
accurately to the effective RMS value regardless of the
waveform.
True RMS voltage is the effective voltage having the
same heating value corresponding a DC voltage. With
True RMS voltage measurement, you can accurately
measure the voltage values regardless of the waveforms
such as: square, sawtooth, triangle, pulse trains, spikes,
as well as distorted waveforms with the presence of
harmonics. Harmonics may cause:
1) Overheated transformers, generators and motors to
burn out faster than their shelf life
2) Circuit breakers to trip prematurely
3)Fuses to blow
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