>> Don't these all require a separately-derived power supply 
>> on the
>> measurement side?

> Not if the voltage is about a volt or more(an IR LED 
> drop). Opto #1 LED
> sees the measured side. Basically the output of the two 
> optos fight it
> out for the opamp's input - the opamp drives opto #2 LED.

> There are examples on the web, many from opto makers that 
> supply matched
> pairs. Russell was suggesting doing your own matching, I 
> think.

I suggested both purpose-built parts and the possibility of 
"roll your own" with multi unit COTS optos - the latter will 
be rather cheaper than purpose built parts.

Even with a LED only input you'd probably want some sort of 
buffer.

An isolated supply can be made reasonably easily. eg

- A two winding coil with suitably high frequency AC feed 
and rectification.

- Or even two capacitors passing AC.

- Extra points :-). A photovoltaic panel and optical power 
input - LED or bulb.
Lessee. White LED 50 l/W, 20 mA.
About 66 mW in. 50 l/W ~= 15% light out = say 10 mW light.
Couple half this to panel and panel = 5% (amorphous) = 0.5 
mW. Maybe 1 to 2 mW with good coupling, good panel etc.
Enough to do the job with care. Some calculators have small 
multi-cell panels that may do. Vout ~= cells/2 max.

As the main requirement for isolation is to float supplies 
that are not formally connected the isolation system 
(probably) need not have an overly capable isolation spec. 
Of the above options the two winding coil would probably be 
the easiest, possibly the cheapest and probably the highest 
power transfer per $. PV panel method is the most elegant 
[tm] but probably dearest.



        Russell


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