True RMS Voltmeter⚡

The digital multi-meter ( DMM ) in the electronics enthusiast's workshop may be specified as measuring the actual RMS voltage on its AC voltage ranges; but that may still only apply to sinusoidal voltage waveforms, or non-sinusoidal are displayed as if sinusoidal. Actually, my DMM only measures the less useful average value, which is not the same. *

Back in 2017 I designed a voltmeter capable of measuring the true RMS voltage of signals having complex waveforms, ( not just sinusoidal ), up to frequencies of several MHz, and even superimposed on a DC level. Central to my voltmeter's operation is a highly accurate RMS-to-DC converter chip. I had made several of them with different display back-lights; orange, green & white.

Measuring a 5V 100KHz square-wave, 12% duty cycle, CF 2.86

I decided to look again at the voltmeter to see if any improvements would be beneficial. Since 2017 I have changed the development tools I use for my embedded projects several times, which would rule out making alterations to the firmware. 

First I fitted a 3-D printed front panel; see image below.

Measuring the output voltage of a 3V DC precision source

I also carried out some comparative tests together with my DMM and oscilloscope using:

(1) a 400Hz sine-wave and (2) a 400Hz square-wave, having a 27% duty cycle.

The results suggested that my DMM is AC coupled and employs a full-wave rectifier circuit for sine-wave voltages. Surprisingly it is almost as accurate as the RMS voltmeter for measuring square-wave voltages. 

* Specification of my own 3½ digit DMM : average value of sine wave, frequency range 40Hz-400Hz. 

RMS = Root Mean Square ( the RMS value is a particularly useful quantity as it relates directly to the power ['heating effect'] of the signal )