C True-Rms Measurements; True-Rms Measurements - Fluke 2680A User Manual

Introduction
The 2680 Series instruments measure 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 simplifies 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
With the input shorted and the instrument set for ac volts (V ac) measurement,
internal amplifier noise causes a typical display reading of approximately 0.50 mV
ac. Since the instrument is a true rms responding measurement instrument, 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
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.

True-RMS Measurements

2
+ 15.00
Appendix C
2
) = 15.008
C-1