The PIC18F26K80 errata sheet mentions that the ADC has an offset error as high as 25 bits and discusses one workaround copied from the errata here:
A/D Offset The A/D may have high offset error, up to a maximum of ±25 LSBs; it can be used if the A/D is calibrated for the offset.
Work around Calibrate for offset in Single-Ended mode by connecting A/D positive input to ground and taking the A/D reading. This will be the offset of the device and can be used to compensate for the subsequent A/D readings on the actual inputs.
I have an existing circuit where connecting the AD input to ground is going to be difficult - so I was looking for alternatives.
The ADC has provision for connecting the positive input to an internal bandgap voltage reference (Channel 31)
Pg 358 in datasheet
My Question: Would measuring the bandgap voltage serve as a way of identifying the offset error also or would each channel have a different offset?