Gday D1
Some chips allow you to set Vref+ and Vref-, and as long as the delta is less than the pics supply voltage yr sweet, but you cant do that with the 675.
With the 675, you only have Vref+ or Vdd for the top of the AD range. The bottom is always Vss.
To do what you want with a 675, yr best bet is to power the ADC from Vdd, and bring the input voltage down using a voltage divider. Then multiply the ADC reading to get yr actual voltage. May need a little calibration but thats easy to do.
Andrew