Digital AC/DC Voltmeter design

[Marco Ferra]

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

 

[John Larkin]

On Sat, 19 Mar 2005 15:36:38 +0000, Marco Ferra
<mferra_DELETE_@sdf.lonestar.org> wrote:

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

Just take random samples using a fast ADC; square, average, square
root. You can either do it the direct way and get DC-coupled true RMS,
or subtract out the mean value from each sample and effectively AC
couple.

John

 

[Marco Ferra]

Just take random samples using a fast ADC; square, average, square
root. You can either do it the direct way and get DC-coupled true RMS,
or subtract out the mean value from each sample and effectively AC
couple.

John

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

mferra

 

[Fred Bloggs]

John Larkin wrote:

Just take random samples using a fast ADC; square, average, square
root. You can either do it the direct way and get DC-coupled true RMS,
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 know
what you're talking about then why don't you just shut the hell up.

 

[Martin Riddle]

;D




"Fred Bloggs" <nospam@nospam.com> wrote in message news:423C9BA5.4030506@nospam.com...

John Larkin wrote:

Just take random samples using a fast ADC; square, average, square
root. You can either do it the direct way and get DC-coupled true RMS,
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 know
what you're talking about then why don't you just shut the hell up.

 

[John Larkin]

On Sat, 19 Mar 2005 21:37:44 GMT, Fred Bloggs <nospam@nospam.com>
wrote:

John Larkin wrote:

Just take random samples using a fast ADC; square, average, square
root. You can either do it the direct way and get DC-coupled true RMS,
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 know
what you're talking about then why don't you just shut the hell up.

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.

http://www.highlandtechnology.com/DSS/V180DS.html

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

 

[Fred Bartoli]

"Fred Bloggs" <nospam@nospam.com> a écrit dans le message de
news:423C9BA5.4030506@nospam.com...

John Larkin wrote:

Just take random samples using a fast ADC; square, average, square
root. You can either do it the direct way and get DC-coupled true RMS,
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 know
what you're talking about then why don't you just shut the hell up.

Sorry Fred but see the HP3406A.

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


--
Thanks,
Fred.

 

[Fred Bloggs]

Fred Bartoli wrote:

"Fred Bloggs" <nospam@nospam.com> a écrit dans le message de
news:423C9BA5.4030506@nospam.com...


John Larkin wrote:


Just take random samples using a fast ADC; square, average, square
root. You can either do it the direct way and get DC-coupled true RMS,
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 know
what you're talking about then why don't you just shut the hell up.



Sorry Fred but see the HP3406A.

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

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

 

[John Larkin]

On Sun, 20 Mar 2005 10:18:35 +0100, "Fred Bartoli"
<fred._canxxxel_this_bartoli@RemoveThatAlso_free.fr_AndThisToo> wrote:

"Fred Bloggs" <nospam@nospam.com> a écrit dans le message de
news:423C9BA5.4030506@nospam.com...


John Larkin wrote:

Just take random samples using a fast ADC; square, average, square
root. You can either do it the direct way and get DC-coupled true RMS,
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 know
what you're talking about then why don't you just shut the hell up.


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

 

[Fred Bartoli]

"John Larkin" <jjSNIPlarkin@highTHISlandPLEASEtechnology.XXX> a écrit dans
le message de news:kffr31tp6hvlpa2tbirivoccnal9lv49g7@4ax.com...

On Sun, 20 Mar 2005 10:18:35 +0100, "Fred Bartoli"
fred._canxxxel_this_bartoli@RemoveThatAlso_free.fr_AndThisToo> wrote:


"Fred Bloggs" <nospam@nospam.com> a écrit dans le message de
news:423C9BA5.4030506@nospam.com...


John Larkin wrote:

Just take random samples using a fast ADC; square, average, square
root. You can either do it the direct way and get DC-coupled true
RMS,
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 know
what you're talking about then why don't you just shut the hell up.


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

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.


--
Thanks,
Fred.

 

"Marco Ferra" <mferra_DELETE_@sdf.lonestar.org> wrote in message
news:423c4707$0$245$14726298@news.sunsite.dk...

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?

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

 

[John Larkin]

On Sun, 20 Mar 2005 21:24:26 +0100, "Fred Bartoli"
<fred._canxxxel_this_bartoli@RemoveThatAlso_free.fr_AndThisToo> wrote:

"John Larkin" <jjSNIPlarkin@highTHISlandPLEASEtechnology.XXX> a écrit dans
le message de news:kffr31tp6hvlpa2tbirivoccnal9lv49g7@4ax.com...
On Sun, 20 Mar 2005 10:18:35 +0100, "Fred Bartoli"
fred._canxxxel_this_bartoli@RemoveThatAlso_free.fr_AndThisToo> wrote:


"Fred Bloggs" <nospam@nospam.com> a écrit dans le message de
news:423C9BA5.4030506@nospam.com...


John Larkin wrote:

Just take random samples using a fast ADC; square, average, square
root. You can either do it the direct way and get DC-coupled true
RMS,
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 know
what you're talking about then why don't you just shut the hell up.


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

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.

Very cool link... thanks.

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

John