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From time to time, I have seen requests for information about how a PIC - using only a positive voltage supply, can measure negative voltages.
In order to simplify things, I have created a spreadsheet that does most of the work for you.
Those with better math skills could probably simplify the equations, but nonetheless, you can find it here:
http://www.charleslinquist.com/PICNegativeVoltage.xlsx
Thank you Charles,
This should prove helpful.
I should also add that - when converting voltages, you gain a lot of accuracy from oversampling.
For example, if you need to multiply an A/D reading by 14, rather than take one sample and multiplying it by 14, take 14 samples and add them up. At 40Mhz, a single conversion takes only about 20uSec, so the oversampling doesn't add much overhead.
http://en.wikipedia.org/wiki/Oversampling