Digital AC/DC Voltmeter design

Hi to all

I'm trying to build a digital AC/DC voltmeter in the most efficient manner I can, and although I can easily convert one DC voltage using an ADC, I can't find a reliable way (ie. getting the RMS value) of an AC signal.

This signal could be perfectly sinusoidal, but it can also be a signal that doesn't, for example, crosses zero. Thus how can I *remove* the DC component of an AC signal (if indeed it has it) and how can I get a digital value from this signal?

Sincere regards mferra

Well, I did design a line of end-use power survey meters that used this principle. I used a 6803 uP to sample and digitize 16 separate AC circuits (voltage:current pairs) and compute/log everything: trms volts, amps, power, pf, kwh; it did temperatures and humidity and waveform acquisition and stuff like that, too. Over 4000 were sold, and I received something like $1.2 million in royalties over the product lifetime. The 6803, running at 1.2 MHz, had no trouble doing the math, logging the data, managing a display, and talking 9600 baud.

Here's an updated version, in VME. It's used mostly for production test of aircraft power systems and big diesel backup generators.

I really don't understand why anybody would buy the Analog Devices metering chips; a 90-cent CPU with on-chip ADC will do a lot more.

What's your experience along these lines?

John

I see, and I'll use your approach, thanks for the idea. I'll keep in touch about the possible failure/success.

mferra

That's not how it's done in practice- not even close. If you don't know what you're talking about then why don't you just shut the hell up.

;D

"Fred Bloggs" a écrit dans le message de news: snipped-for-privacy@nospam.com...

Sorry Fred but see the HP3406A.

BTW, you don't need fast ADC. Just fast enough sampling.

Actually, you don't even need to sample fast; there's certainly no Nyquist issue here, as we're just gathering 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.

The adc s/h does have to have bandwidth compatible with all the signal components. So you can wind up using a wide-bandwidth ADC fired slowly, or mux'd between a lot of channels.

The 3406 used a very fast s/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

A random sample ADC is not the same as an ADC sampling randomly.

"John Larkin" a écrit dans le message de news: snipped-for-privacy@4ax.com...

RMS,

Sorry, bad wording again. I meant that the sampler had to have enough BW so as to see all the signal components.

For the full operating & service manual see: ftp://bama.edebris.com/hp/3406a/

Lots of other manuals there.

Strangely enough, that was my last design, just before I retired 12 years ago. I designed a calculating RMS-DC converter that could be integrated on a BiCMOS LSI chip. It used the identity that Vrms = average value of (V^2/Vrms) If I had it to do today, I'm sure a different method would be used.

You might look at some of the Analog Devices app notes. They make a stand-alone LSI chip for the purpose.

Norm Strong

Very cool link... thanks.

I checked an old HP catalog, and the 3406 is indeed a sample averaging instrument, not true RMS.

John