Paging John Larkin - Re: Digital AC/DC Voltmeter design

>"Fred Bloggs" a =E9crit dans le message de
> >news: snipped-for-privacy@nospam.com... > > >> John Lark> > >> >JusttakerandomsamplesusingafastADC; square, average, square > >> > root. You can either do it the direct way and get DC-coupled true RM=

>> > or subtract out the mean value from each sample and effectively AC > >> > couple. > > >> That's not how it's done in practice- not even close. If you don't kno=

>> what you're talking about then why don't youjustshut the hell up. > > >Sorry Fred but see the HP3406A. > > >BTW, you don't needfastADC.Justfastenough sampling. > > Actually, you don't even need to samplefast; there's certainly no > Nyquist issue here, as we'rejustgathering statistics on a waveform, > not trying to reproduce it. The reason to sample randomly (or at least > at a not-exactly-periodically rate) is to avoid aliasing the signal or > its harmonics. If the signal is of a known frequency (say, 60 Hz) you > can sample at some fixed rate that dances betweeen the harmonics > safely; the math gets interesting. My old survey meter sampled at some > magic rate close to 27 Hz, as I recall. > > Theadcs/h does have to have bandwidth compatible with all the signal > components. So you can wind upusinga wide-bandwidthADCfired > slowly, or mux'd between a lot of channels. > > The 3406 used a veryfasts/h, essentially the full-bridge sampler > like in the 1 GHz 1810 sampling scope plugin, fired at a relatively > low rate. Anybody got details? Was the 3406 true RMS? A schematic > would be fun. > > John

I apologize for resurrecting an old topic, but Google led me here and this deals exactly with what I need and all I think I need is a follow- up question and answer.

John, you say you can take RMS voltages using that method -- i.e. taking random samples and then running the RMS calculation on them.

What I need is a DC voltage read of a DC voltage signal. The signal is not a constant voltage, but for the most part is a rectified 60Hz signal.

Is there any reason the same method of taking random samples but performing an average instead of an RMS wouldn't work for taking DC readings?

My concern is that when taking an average you will get a different subset of readings than you would at other times, and so the DC reading will change without the signal changing.

For example, if you take the average of a rectified 1Vp-p sinusoidal signal over a single time period (i.e. a single sine hump), you would get an average of 2/pi V =3D 0.637 V. If, however, you took the average reading over a slightly longer time period of say 6/5*T, the average would then be 0.581 V. So the DC reading you get changes significantly depending on the length of time you take readings, even though the actual DC reading (the original signal) doesn't change at all.

Is that problem mitigated by taking random samples?

You can average the samples to get the DC value. Or square, sum, root to get dc-coupled RMS. Or subtract the average from all the samples and then do the RMS thing to get "ac coupled RMS", what old voltmeters called "output."

If you know that the signal is 50/60 Hz, you can pick a sample rate and total time to span N cycles of the line, and get stable readings. Take, say, 1000 samples at 1 KHz, spanning 1 second. You can do block summing, producing a new output every second. Even 100 samples taken over an integral number of line cycles should be pretty good.

Or you can do continuous sampling and lowpass filtering with a suitable time constant, to get continuous output readings. The sample rate shouldn't have any chance of heterodyning against any harmonic component of the input, or you can get creepie-crawlies in the displayed values. It gets to be fun picking sampling rates.

If you random sample a strongly-varying waveform, like a rectifier output, there's no heterodyning but the result will be noisy and you'll need to mash a lot of samples to get a low-noise result, N being roughly the square of the desired s/n ratio. That may not be a problem if you have a fast sample rate.

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John