Extech Instruments MultiMaster MM560 User Manual page 12

DC+AC True RMS
DC+AC True RMS calculates both of the AC and DC components given
by the expression
can responds accurately to the total effective RMS value regardless of
the waveform. Distorted waveforms with the presence of DC
components and harmonics may cause:
1) Overheated transformers, generators and motors to burn out faster
than normal
2) Circuit breakers to trip prematurely
3) Fuses to blow
4) Neutrals to overheat due to the triplen harmonics present on the
neutral
5) Bus bars and electrical panels to vibrate
AC Bandwidth
AC bandwidth of a DMM is the range of frequencies over which AC
measurements can be made within the specified accuracy. It is not the
frequency measurement function, and is the frequency response of the
AC functions. A DMM cannot accurately measure the AC value with
frequency spectrums beyond the AC bandwidth of the DMM. Therefore,
wide AC bandwidth plays an important role in a high performance DMM.
In reality, complex waveforms, noise and distorted waveforms contain a
much higher frequency spectrum than its fundamental.
NMRR (Normal Mode Rejection Ratio)
NMRR is the DMM's ability to reject unwanted AC noise effect that can
cause inaccurate DC measurements. NMRR is typically specified in
terms of dB (decibel). This series has a NMRR specification of >60dB at
50 and 60Hz, which means a good ability to reject the effect of AC noise
when making DC measurements.
CMRR (Common Mode Rejection Ratio)
Common 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 effects that can cause
digit rattle or offset in voltage measurements. This series has a CMRR
specifications of >90dB at DC to 60Hz in ACV function; and >120dB at
DC, 50 and 60Hz in DCV function. If neither a NMRR nor a CMRR
specification is specified, a DMM's performance will be uncertain.
when making measurement, and
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