On Mon, 23 Aug 1999, Todd Conard wrote:

> Low cost isolated A/D
>
> 1.  Each of the four inputs needs isolation from each other.
> 2.  It can be a very slow conversion and only one at a time is necessary.
> 3.  It only needs to be 8 bits.
> 4.  The isolation barier only needs to be 500V.

Several years ago (about 6 or 7) I did a design that had similar
requirements. Though in my case, more analog inputs were required. What I
did was to use CPClare solid state relays (first the LCA then later the
dual LAA series) to isolate the signals from one another and from the A/D
conversion circuitry. At the time only 400V relays were available, but
I've seen higher since (1000 iirc). Now the trick was that the conversion
circuitry had to be isolated from ground. So the input power supply had to
be able to withstand high primary to secondary voltages too. When a signal
was to be converted, the relays were closed and the whole conversion
circuitry would float up to the common mode voltage of the signal. The
'A/D' converter was in fact a 'V/F' converter, the AD652. This is
considerably more linear (and more expensive) than the LM331. I was able
to achieve about 14 or 15 bits of dynamic range and about 30 conversions
per second. (The V/F conversion time was exactly 2 cycles of 60Hz - common
mode noise at 60hz is very common). Low cost? well...

When you speak of 500V isolation, what do you mean exactly. In other
words, how much leakage current can you tolerate? Because for 8-bits you
could design a resistive divider + difference amplifier. I don't have time
to draw the picture... But have you ever seen ground reference opamps
measuring the voltage drop across a high-side current-sense resistor?
These circuits essential use two identical resistor dividers- one on each
side of the sense resistor and use an opamp two measure the voltage
difference at the divided-down junctions. Look at instrument-amplifiers
and difference-amplifiers to get an idea what I'm talking about. A point
of caution though, 5V/500V is a 40db range, and 1/256 is another 48 db.
Which is to say that you're gonna need very well matched resistors to
minimize common-mode voltage errors...