RIGOL DM3068 - Manual (Page 143)

Chapter 5 Measurement Tutorial RIGOL
User’s Guide for DM3068 5-3
True-RMS AC Measurements
The AC measurement of DM3068 has true RMS response. The average thermal
power of the resistor over a certain period is directly proportional to the square of
the true RMS of the voltage of the resistor over this period of time, regardless of
the waveform shape. DM3068 can accurately measure the true RMS voltage or
current if the energy of the voltage or current waveform outside the effective
bandwidth of the multimeter can be ignored. The effective AC voltage bandwidth
of DM3068 is 800 kHz and the effective AC current bandwidth is 100 kHz.
The multimeter's ACV and ACI functions measure the “AC coupled” true RMS value,
namely the RMS value of the AC component (the DC component is filtered out) of
the input signal. As shown in table 5-1, since the sine, triangle and square (50%
duty cycle) waves do not have DC offset, their AC true RMS values equal the
AC+DC true RMS values.
Table 5-1 True- RMS AC Measurements of Sine, Triangle and Square waves
Waveform Crest Factor (C.F.) AC RMS AC+DC RMS
V
0 2
2
V
2
V
V
0
3
3
V
3
V
V
0
t
T
-V
1
. .F C
V
. .F C
V
Unsymmetrical waveforms such as pulse sequence contain DC component which
will be filtered out during the AC coupled true RMS measurements.
The AC coupled true RMS measurement is especially useful for measuring small AC
signals having DC offset such as the AC ripple from DC power supplies. However, in
some cases you need to measure the AC+DC true RMS value. First you can