The ADC internal full-scale calibration is said to only be available with a gain of 1. How is one supposed to calibrate the gain at other settings?
If I do an internal zero calibration the ADC1OF register ends up with a value around 80. If I do an internal full-scale calibration with a gain of 1 the ADC1INTGN register most often gets set to 512 but occasionally gets a zero value. If I don't do a full-scale calibration at all the ADC1INTGN register is always 21846 which I guess is a default value.
My normal procedure with other ADCs is to do a zero and full-scale calibration at the intended gain before taking a sample. As the application involves periodic sampling over a wide temperature range this tends to remove any drift from the ADC. Any suggestions as to how to account for the gain drift without being able to do a full-scale calibration at the intended gain?