Tie a FET current source between +Vcc (unreg) and PIC AD pin and a
LM385-2.5 between the PIC AD pin and your negative input. The FET current
source should supply 200uA or so.

There are other ways, one of them is an inverting opamp with gain 1.
+input to gnd, -input to input, +output to PIC. Feedback and input R's
e.g. 10K and 10K, Be sure to limit the opamp Vo, it can exceed the 5V
supply.

If you are very short on money and board space make a resistor divider
between +Vdd and your -Vin with two resistors to put the voltage in range
(zero) for -Vin at maximum. This will reduce the ADC range and resolution
and depend on Vdd stability (but the ADC depends on it anyway). But it
uses 2 $0.01 parts.

Peter

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