I measure negative voltages accurately without any external active components.

All you have to do is connect a resistor from Vref to the A/D input pin, and another resistor from that A/D pin to the voltage to be measured.

For example: To measure -12V, I use a National 4.096V reference on RA3 and a 2K resistor from that pin to the A/D input I want to use. From there, I use a 7.5K resistor to the voltage to be measured.

The code then loops 19 times and adds the results of the conversions (for averaging). The result is then subtracted from 15360. The answer is in tens of millivolts.

I have used this technique for years with good results.