When should a calibration be performed on the AD779x family of Σ-Δ ADCs ?

A calibration must be performed when there is a change in
• gain
• polarity
• temperature (when switching between channels that share coefficients registers but have different operating conditions)

With any gain change, there will be a matching error between gain ranges, and this error needs to be calibrated out.

Drift errors are due to changes in temperature. Calibration can be used effectively to remove any errors associated with temperature drift. Self-calibration will remove the effects of temperature drift within the ADC itself. System calibration can be used to remove the drift errors in the ADC itself and also the drift errors associated with the front-end signal conditioning circuitry. Chopping continuously removes offset and offset drift; thus, with chopping enabled, internal zero-scale calibrations are not required. If chopping is disabled, then zero-scale calibrations are required.

Therefore, it is important that a calibration be performed when switching between these channels if the operating conditions (gain and polarity) between the two differ. If the two channels are configured identically and self-calibration is used, there is no need to perform a calibration when switching channels. However, if system calibration is being used, a calibration should be performed when switching between channels, as the applied analog input levels may be different.