SIG
L
ENT
SDM3055 Digital Multimeter 93
Chapter 4 Measurement Tutorial
True RMS AC Measurement
The AC measurement of the Multimeter has true RMS response. The power
dissipated in a resistor within a time is proportional to the square of the
measured true RMS voltage, independent of wave shape. The instrument can
accurately measure true RMS voltage or current, as long as the wave shape
contains negligible energy above the effective bandwidth.
The AC voltage and AC current functions measure the “AC coupled” true RMS
value, which is to measure the RMS value of the AC component (DC
component is rejected) of the input signal. For sine waves, triangle waves, and
square waves, the AC and AC+DC values are equal since these waveforms do
not contain a DC offset. See the following table 5-1.
Table 5- 1 True RMS AC Measurement of Sine, Triangle and Square waves
Waveform
Crest Factor
（
C.F.
）
AC RMS
AC+DC
RMS
Sine
Triangle
Square
Non-symmetrical waveforms, such as pulse trains, contain DC voltages which
are rejected by AC coupled true RMS measurements.
An AC coupled true RMS measurement is desirable in situations where you
are measuring small AC signals in the presence of DC offsets. For instance,
measuring AC ripple present on DC power supplies. There are situations,
however, where you might want to know the AC+DC true RMS value. You can
determine this value by combining results from DC and AC measurements as
the following shows. You should perform the DC measurement using 5.5 digit
mode for best AC rejection.
RMS
(AC+DC)
= √ AC
2
+ DC
2
