Electronic – PIC18F26K80 ADC calibration using Bandgap

It all depends on what you are trying to calculate - if you are trying to calculate AC RMS values then having your software continually calculating the mean value of the two signals and using that in the RMS calculation gets rid of those pesky dc offsets and offset drift with temperature.

This just leaves you with gain calibration and this can be tricky if you haven't got highly stable potential divider circuits - I'd start by picking resistors with a 10ppm/ºC temperature coefficient - this then gets rid of that as an error but, your ADCs will have gain and non-linearity errors and these might drift with temperature. How you would counter these temperature effects could mean you ditching your current ADC and using a better ADC with the performance you want/need.

If you are wanting to measure dc levels then I'd use precision resistors and much better op-amps of the "zero-drift" variety AND design your hardware so you can do a "zero" measurement - you'll need analogue switches to do this and you'll need to do it on a regular basis.

It all boils down to what you want to measure and how accurate you want to be; neither of which are stated in the question.

On an ADC, the reference voltage Vref is used to establish the full scale voltage, not some middle-ish setpoint. If you're expecting an input voltage of greater than 1.5V, and you'd like the ADC to sample the difference between the input voltage and 1.5V, yes, you would use differential mode, and send 1.5 V to the negative input.

Vref may also need to be set, depending on the ADC (very often there is a default value and the use of the Vref terminal is set through configuration commands). If you want to get the best resolution out of your ADC, you would set Vref to the largest difference between your signal and 1.5V that you expect. If you're not concerned about resolution, you might just use Vcc or the default value (but you should do some napkin calculations to make sure your resolution is good enough)