Amprobe AM-140-A - Manual (Page 10)

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AC True RMS
AC True RMS, normally refers as True RMS, identifies a DMM function that is
AC coupled, and responds accurately only to the effective RMS AC component
value regardless of the waveforms. However, DC component plays an important
role in the distorted non-symmetrical waveforms, and will also be of interest
sometimes. A full wave rectified sine waveform is a good example, and the AC
true RMS function will only give the AC component reading which is at 43.6%
of the total effective DC+AC RMS reading.
DC+AC True RMS
DC+AC True RMS calculates both of the AC and DC components given by the
expression
when making measurement, and 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:
• Overheated transformers, generators and motors to burn out faster than
normal
• Circuit breakers to trip prematurely
• Fuses to blow
• Neutrals to overheat due to the triplen harmonics present on the neutral
• 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 high performance DMMs.In reality, complex waveforms, noise and distorted
waveforms contain 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
is a good and definite 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 effect that can cause digit rolling or offset in voltage