dear Gavin,

farnell catalog may/october shows a "precision voltage to frequency
convertor" by EXAR code 4151, dil 8 pin. farnell code 562-701, $3.78 NZ
1 up price. if you have a small difference in supply voltage of your
floating PSU to the PIC's GND then you may be able to output this to an
input to the pic (with suitable resistance) if a large difference then
you will need an optoisolator.

note that the TMRO/PA4 pin only has one internal protection diode due to
single sided output driver.

of course this way of testing the voltage would be applicable to a
pic12c508 or 16c84 etc as uses any pin for "tacho" input.

if you want to skip tacho S/W routine then the frequency can be back
converted to a voltage with a resistor/capacitor on the opto output.
(lesser accuracy)

regards,
Graham Daniel.

Gavin Jackson wrote:
>
> Hi there
>
> I need to measure the output from a circuit
> that has a floating power supply. The circuit
> that does the measurement has a 16C71
> and its supply is referenced to ground.
>
> The circuit with the floating power supply
> produces a voltage in the range of 3V to 9V
> and I need to isolate it from the PIC and
> bring it down to within the limits of the PIC's
> AD converter.
>
> Is there a simple way of using OP-Amps to
> provide the isolation or another method that
> does not cost much?
>
> Regards
>
> Gavin
> --------------------------
> vulcan@ihug.co.nz
> http://www.geocities.com/TheTropics/Cabana/2625
> --------------------------