** So you missed the schematic in figure 9 entirely ???
It is all new information in relation to your original *dumb* question -
**The data sheet for the AD536 has more detail plus quotes the maths
formulae device operation relies on:
See fig. 9 again and the text around it headed:
" AD536A Principle of Operation ".
Then fuck off.
The 'cover' PDF gives a fairly clear and thorough description of how the
RMS-DC convertor is designed, down to the level of a few basic
Did you expect others to do your homework for you?
** The link I originally supplied him for the AD636 has a full description
of how the device operates INCLUDING a detailed internal schematic.
See the section starting on page 5, " AD636 Principle of Operation ".
** The fool cannot see what is right in front of his eyes.
Usual disability of all wankers.
He obviously did not get the entire document, for whatever technical reason.
Therefore, he didn't see the other information.
Since you apparently don't frequent this newsgroup, you might want to do
some homework yourself before you direct comments like that at an engineer
who can run circles around you in terms of his electrical knowledge.
Benjamin D Miller, PE
From reading this thread, and having conversed with Don on this group over
the past several years, it is easy to see who is the arrogant, foul-mouthed,
whiner, and who is a courteous, sincere individual.
I've got an old AD catalog with "little black boxes" in it. From what I
remember the cat has precision instumentation amps, log converters and ???.
If you have a part number, I'll see if it's in there.
I worked on some true RMS panel meters many years ago, and they used a
heater and a thermocouple inside a glass bulb. There was a considerable
time delay to get an accurate measurement, and there was some ambient
temperature compensation required, but it was very accurate and worked at
DC to RF (at least to several MHz). But these sensors were rather expensive
A friend and I tried to design a true RMS meter using a lamp and a
photocell, but we found that there was a considerable aging effect on the
output of the lamp, and there was also some ambient temperature error that
needed compensation. I think we were finally able to solve the stability
and aging problems by using two lamp/photocell pairs in a sort of bridge
circuit, where one was driven by the measured signal and the other was
driven by a DC signal that also drove the meter. But the current draw for
both elements was more than the allowed specification, as it was a
self-contained meter with a range of about 4-8 VRMS.
For a bench instrument, such a method would be very practical. You just
need to make two well-matched lamp/photocell pairs, and some signal
conditioning, amplification, and limiting for the lamp, and then use an
op-amp or other means to drive the other sensor so that the photocell
outputs are identical. Then the DC current in the DC sensor matches the
true-RMS current in the other, for any waveform or frequency. But it does
have a rather narrow range of operation. You can't get very close to zero.
Probably 20% to 100% would be possible. The lamp must be driven hard enough
to become incandescent and be sensed by the photocell, which has a limited
spectral range. The heater/thermocouple could go lower, but becomes
relatively insensitive, because heat is proportional to I^2.
** It ain't that simple.
The vast majority of so called " true rms " AC ranges fitted to DMMs are
of very limited bandwidth - typically 30Hz to 1kHz within 1% accuracy, a few
more expensive examples go to 20 kHz or even 100kHz.
So, unless the voltage wave under test falls within the particular meter's
bandwidth, the reading will be in error and likely very seriously so.
The RMS voltage of a true and symtrical square wave is just the voltage from
ground of the "top" of the wave.
A "true RMS" meter will measure this accurately.
However, most AC voltmeters don't respond to true RMS but something else
like "average of absolute voltage" or "peak". They are calibrated in RMS
using the assumption that the wave form is a sine wave.
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