The PGA gain 1 of each channel is trimmed independently in the factory. This value will be loaded in the registers CHx_GAIN_UPPER_BYTE, CHx_GAIN_MID_BYTE, and CHx_GAIN_LOWER_BYTE after a RESET.
The gain correction factor value is relative to 0x555555, which represents a gain factor of x1. If the PGA gain of any channel is changed, from PGAgain=1 to either 2, 4 or 8, then it might be necessary to adjust its gain and offset correction factors. Otherwise just overwrite to 0x555555 and 0x000000, gain and offset calibration register respectively.
The correct calibration process is as mentioned below:
With the default values, calibrate the offset
Once the offset is calibrated, select the 280mV supply and measure it with the default gain and the calibrated offset. This voltage supply is not very accurate so it can differ from the expected 280mV. The measured value will be used later as a reference.
Set the gain of the PGA to the desired value.
Set the gain to 0x555555.
Calibrate the new offset.
Calibrate the gain by measuring the 280mV and adjusting the registers until the same value as step 2 is measured.