Hello

Maybe set up a Voltage to Frequency converter powered by the 3-30 Volt i/p, use a standard opto isolator and count the pulses? 


You could look at one of these opto-couplers if you chose an analoug interface & A/D it on the uP's side.

SIEMENS IL300 or IL300
    Gain Stability +/- 0.005%
    Linearity  0.01%
    These analoug opto's have two separate isolated photo diodes,
    one you use as feed back on the input loop side.
    ( You could use a op amp that would accept a wide Supply voltage in (3-36V) to control a linear range of 
      i/p voltage to feed back current.)

HP HCPL-4562
    Wide band   17 MHz
    Low Temp. Coeffiicient  -0.3%/deg. C
    High Linearity at low drive currents
    High Gain Typicaly 2.0

    

>>> Andy Kunz <montana@FAST.NET> 05/27/98 10:33AM >>>
I need to measure a voltage 3-30VDC using a PIC, but the PIC has a totally
different ground reference than the voltage I'm supposed to be measuring
(the two circuits _are_ connected, but I have no way of knowing how because
each install will probably be different).

I was thinking that this is an ideal situation for using an opto-isolator,
and was wondering if anybody knew in a practical way if this would provide
reasonable accuracy with varied temps, etc.

Anybody have any other suggestions?