UT8805E Benchtop Digital Multimeter
49
Chapter 4 Measurement Guide
4.1 True RMS AC Measurement
With true RMS response of AC measurement, at a period, the average heating power of resistor is
proportional to the RMS value square of the voltage on resistor, which has nothing to do with waveform.
UT8805E can accurately measure the true RMS value of voltage or current waveform when its energy
outside the effective bandwidth of the multimeter can be ignored.
The AC coupling true RMS value that the AC voltage/current functions measured, is the RMS of signal
AC component (the DC component is filtered out).
Since the sine wave, triangle wave and square wave do not contain DC offset, their AC RMS is equal to
AC+DC RMS. As shown in Table 4-1.
Table 4- 1 The true RMS AC measurement of sine/triangular/square wave
Waveform
Crest Factor C.F.
AC true RMS
AC+DC RMS
Sine wave
Triangular
wave
Square
wave
Asymmetrical waveforms contain DC component (such as pulse train), which will be filtered out by AC
coupling true RMS measurements.
AC coupling true RMS measurement is ideal for measuring small AC signals with DC offset, such as the
AC ripple in the DC power supply output. However, there are cases where AC+DC RMS need to be
measured. By this time, the DC and AC component of the signal can be measured by DC/AC voltage
functions separately, and then the value of AC+DC RMS can be calculated by following formula. Slow
speed mode is necessary for DC voltage measurement to achieve the best AC suppression.
4.2 Crest Factor Error (Non-sinusoidal Input)
The waveform of the input signal will affect the accuracy of the measurement.
Generally, the signal waveform, which is described as crest factor, is the ratio of the crest value to
effective value. The greater the crest factor, the greater the energy contained in the high frequency
harmonics.
Error related to the crest factor exists in all the multimeters. Particularly, the crest factor error is not
suitable for input signals below 100Hz.
The error caused by crest factor can be estimated as follows:
Sum of error = error (sine wave) + error (crest factor) + error (bandwidth).
