10
responds accurately to the total effective RMS value
regardless of the waveform, and is given by the
e x p r e s s i o n :
√d.c.
2
+ (a.c. rms)
2
d.c.+a.c. True RMS voltage is the total effective voltage
having the same heating value corresponding a d.c.
voltage. With d.c.+a.c. 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 and d.c. components. Harmonics
and d.c. components 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
4) Neutrals to overheat due to triple harmonics present
on the neutral
5) Bus bars and electrical panels to vibrate
a.c. only True RMS and Average responding meters can
introduce significant errors in many applications. Table 2
illustrates this.
Bandwidth
Bandwidth of a DMM is the range of frequencies over
which measurements can be made within the specified
accuracy. In other words, a DMM cannot accurately
measure values with frequency spectrums beyond the
frequency response of the DMM. In reality, complex
waveforms, noise and distorted waveforms contain much
higher frequency components than its fundamentals. The
series has bandwidth specifications up to 20kHz in most
ranges, and extended bandwidth specification up to
50kHz on M8037 a.c. 400mV range.
NMRR (Normal Mode Rejection Ratio)
NMRR is the DMM’s ability to reject unwanted a.c. noise
effect which can cause inaccurate d.c. measurements.
NMRR is typically specified in terms of dB (decibels). The
instruments have a NMRR specification of 60dB at 50
and 60Hz, which means the effect of a.c. noise is reduced
more than 1000 times in d.c. measurements.
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