Fluke NetDAQ 2645A User Manual download pdf (Page 241)

C-1

Appendix C

True-RMS Measurements

Introduction C-1.

The instrument measures the true rms value of ac voltages. In physical terms, the

rms (root-mean-square) value of an waveform is the equivalent dc value that

causes the same amount of heat to be dissipated in a resistor. True rms

measurement greatly implies the analysis of complex ac signals. Since the rms

value is the dc equivalent of the original waveform, it provides a reliable basis for

comparing dissimilar waveforms.

Effects of Internal Noise in AC Measurements C-2.

With the input shorted and the instrument set for ac volts (VAC) measurement,

internal amplifier noise causes a typical display reading of approximately 0.50 mV

ac. Since the instrument is a true rms responding measurement device, this noise

contributes minimally to the readings at the specified floor of each range. When

the rms value of the two signals (internal noise and range floor) is calculated, the

effect of the noise is shown as:

Total rms digits = Square Root of (0.50

+ 15.00

) = 15.008

The display will read 15.01. At the 28.00 mV input level on the 300.00 mV range

in the slow rate, the display will read 28.00 with no observable error.

Waveform Comparison (True RMS vs Average Responding) C-3.

Figure C-1 illustrates the relationship between ac and dc components for common

waveforms and compares readings for true-rms measurements (NetDAQ) and

average-responding measurements. For example, consider the first waveform, a

1.41421V (zero-to-peak) sine wave. Both the instrument and rms-calibrated

average-responding measurement devices display the correct rms reading of

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Fluke NetDAQ 2645A User Manual Page 241