+AD4- I've been struggling with a way to do a low cost isolated A/D
 +ADwAJA-2/
+AD4- channel) for a PIC 16C877.  I need a total of 4 isolated inputs at
500V
+AD4- of isolation with 8bits of resolution.  I've looked at a lot of
+AD4- different possibilities, so far, a voltage to frequency converter
coupled
+AD4- through an optocoupler seems to be the most cost effective solution.


Todd,

What conversion time do you need to achieve?

I won't discuss power supply here in any detail.
Sigma Delta / Delta modulator / Variable mark space could be attractive
here if speed is not critical.
Similar to VCO but POSSIBLY easier to get a good result.

Effectively you have a delta modulator which outputs a data stream whose
mean value is proportional to the input voltage.
You can think of it as an oscillator which is mark/space modulated by the
input voltage.
A comparator acts as the decision maker.
A capacitor is fed by a resistor from the input voltage AND a feedback
hi/lo level controlled by the comparator. The comparator compares (what
else :-)?) it's input to a reference voltage and drives the feedback
resistor to +ACI-correct+ACI- and error. When this is done with a
microprocessor a
digital delay is effectively inserted by the computation time. In an all
analog version you use either hysteresis on the comparator OR a delay
between comparator response and the feedback. This could be a second
comparator with a simple RC delay so there is a delay between when the 1st
and 2nd comparators toggle.

Output of comparator fed via opto or magnetic coupling or whatever.

LM339 comparators cost under +ACQ-NZ0.50 for a pack of 4. There are newer
better and dearer devices available. You will need either 1 or 2 per
+ACI-channel+ACI-

The main disadvantage is that it is comparatively slow. Depends on uP loop
time to read values and sum hi/lo results. Vertical counters mentioned here
previously may apply.
Say you can get 10uS per loop then you need about 2x 2+AF4-N  samples so
for 8
bit this is 512 samples +AD0- 5.12 mS. Say 10mS and 1000 samples.

There is more to this than meets the eye but if this is a volume
application this may be a good way to do it. If time needed is much slower
than this you could do one channel at a time making processing very easy.

You COULD easily put a PIC on the +ACI-hot+ACI- side and do 4 channels
there (16C71
or newer variants) and then use opto-serial to send data to cold side.

Same applies to a Sigma delta version. Scott Datallo was getting 10 bits
plus with ease with a PIC and NO comparators at all. (Code on his website).
I'm using a Z8 with its internal comparator to get similar results. PIC
+ACQ-NZ3 (anything will do), 2N resistors, N capacitors for N Sigma Delta
channels, Opto and resistor, resonator, power source. Cheap. Needs 2N pins
for SD done all at once or N pins done one at a time (all can share
common feedback pin).
If pressed a 12C508 could do 4 channels :-) (4 input, 1 feedback, 1 opto
output).



Russell McMahon

+AD0APQA9AD0APQA9AD0APQA9AD0APQA9AD0APQA9AD0APQA9AD0APQA9AD0APQA9AD0APQA9AD
0APQA9AD0APQA9AD0APQA9AD0APQA9-

-----Original Message-----
From: Lance Allen +ADw-l.allen+AEA-auckland.ac.nz+AD4-
To: PICLIST+AEA-MITVMA.MIT.EDU +ADw-PICLIST+AEA-MITVMA.MIT.EDU+AD4-
Date: Monday, 23 August 1999 14:42
Subject: Re: low cost isolated A/D inputs?


+AD4-
+AD4- I've been struggling with a way to do a low cost isolated A/D
 +ADwAJA-2/
+AD4- channel) for a PIC 16C877.  I need a total of 4 isolated inputs at
500V
+AD4- of isolation with 8bits of resolution.  I've looked at a lot of
+AD4- different possibilities, so far, a voltage to frequency converter
coupled
+AD4- through an optocoupler seems to be the most cost effective solution.

I haven't done the full costs but my first reaction would be to do the
A2D on the isolated side with a chip like the MAX1111 and optocouple
the serial interface. Power would be via an isolated capacitor drive
(cheapest method) and since the I.C. only draws 6uA+AEA-1kS/S and 85uA at
50 kS/S the 1 MHz drive wouldn't have to be too grunty.
Its also single supply and costsUS +ACQ-1.69 (last 1000up price I have)
+AF8AXwBfAF8AXwBfAF8AXwBfAF8AXwBfAF8AXwBfAF8AXwBfAF8AXwBfAF8AXwBfAF8AXwBfAF
8AXw-

Lance Allen
Technical Officer
Uni of Auckland
Psych Dept
New Zealand
+AF8AXwBfAF8AXwBfAF8AXwBfAF8AXwBfAF8AXwBfAF8AXwBfAF8AXwBfAF8AXwBfAF8AXwBfAF
8AXw-