Why AC+DC TRMS?
Make Accurate RMS Measurements With a DMM
Many Digital multimeter labeled “True RMS Multimeter” do not measure the True RMS value. They just measure the AC true RMS of a periodic waveform only under special conditions–namely, if the waveform is symmetric about the zero axis.
As a matter of fact, many AC signal measurements usually have DC offset, for example:
According to the three kinds of waveform on the fisrt line, here attached 2 result which is AC TRMS and AC+DC TRMS.
Wave form

AC TRMS DMM  Result

AC+DC TRMS DMM Result

Sine Wave 1V Peak DC1V

0.707V

1.225V

Sine Wave 1V Peak DC0.5V

0.707V

0.866V

Square Wave 1V Peak DC1V

1.000V

1.414V

It's clear that AC+DC TRMS's result is correct.
Most True RMS DMMs in the market are AC coupled, which gives the RMS only the value of the AC component of a waveform. So, if you are measuring RMS value containing with DC signal, but the meter is only AC coupled, the results can be dramatically wrong. By the way, if you need to measure an AC signal riding on a DC offset but your meter doesn't provide AC + DC directly, you cannot get the correct result, either. You must measure the AC component using AC coupling and measure the DC component separately. Then add the two using RMS addition. That is
Why there’s less DMM that can provide AC + DC TRMS in the market now? It’s because normally an AC/DC converter cannot do it. It needs a higher technique for the realization of AC + DC TRMS. Digitek owns such technique. Moreover, such excellent meters will be provided at a very low price!
We have launched a series of lowcost AC + DC True RMS multimeter adopting the DG838 RMS converter developed by ourselves. This is difficult at other multimeter suppliers’. DG838 is a True RMS converter which is an analog True RMS converter that has a good frequency response and high stability. A true RMS multimeter built with DG838 RMS converter can not only provide pure AC signal true RMS measurement, but also provide true RMS measurement on the signal with both AC and DC. It’s undoubtedly more useful in the signal measurement for a variety of occasions today, comparing to the traditional true RMS multimeters that can measure only pure AC signals. These products can be found on our website:
DT105, DT106, DT107, DT108, DT701, DT702, DT704, DT2843R, DT2844R, DT2865R, DT2866R, DT9405R+, DT9602R+ and Clampon meter DT6920R, etc.
Besides, Digitek also provides the meters that have only AC TRMS.
