Re: VOLTAGE DIFFERENCE BETWEEN SUPPLY GND AND PCB GND
You can make my day by telling me that your power supply is built on a breadboard and/or wired with these fancy pluggable wires usually used in the Arduino context.
What you need is a dsPIC PCB that houses the connectors for the sensors - providing power and power-GND from this PCB. Or - provided the offset is constant - you can correct the readings mathematically. But I assume your sensor's GND is -32 mV wrt the dsPIC's Vss/AVss. So the sensor readings will be a bit too low - not allowing to measure the first 32 mV at the lower end.
PEBKAC / EBKAC / POBCAK / PICNIC (eventually see en.wikipedia.org)