According to AN3949 ADC calibration results are best if VDDA > 3 V. We have a system running from battery (3.3 V lithium), where VDDA decreases over time. Now we are wondering, what is better, calibrate the system once in the beginning when the battery is fresh (> 3 V), store calibration values in flash and use this values for the rest of the life-time. Or do a calibration every time the system is switched on. In the second case VDDA will decrease over time, because battery voltage decreases. The calibration is then done under the same environmental conditions as the normal ADC measurements (which I consider as good?), but with VDDA < 3 V which is not optimal according to application note.
Which way is the better way? Do calibration once when VDDA > 3 V and re-use this values over the whole life-time, or do a re-calibration on every boot?