One thing you can do is to take away the resistor network on your analog inputs and just put a 1k in place of the 100k(for isolation and input protection). That resistor network is scaling a 0-70v input to a 0-5v in to the PIC.
Then connect a 2 volt source to AN3 to serve as the high voltage ref. To do this you need to set register bits PCFG3-0 to '0001'. This changes the range of the ADC to 0-2V, instaed of 0-5v. You could use a simple resistor divider like 3k/2k or a precision voltage reference like a LMV431 or even a zener diode.
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