>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.
>

How much voltage seperates the floating supply from ground?  Is it a limited
range (12v or so) or is it much more than that?  Can your floating circuit
tolerate any leakage to ground?  For that matter, why is it floating?