When should a calibration be performed on the AD719x family of Sigma-Delta ADCs?
A calibration must be performed when there is a change in:
Gain, polarity, Temperature or 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. Internal-calibration (also known as 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 so, with chopping enabled, internal zero-scale calibrations are not required. If chopping is disabled, then zero-scale calibrations are required.
Some of the AD719x channels share offset/gain registers, for example, AIN5, to AIN8 on the AD7193 in pseudo-differential mode. Therefore, it is important that a calibration be performed when switching between these channels if the operating conditions (gain, 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.
You don't suggest doing a calibration if the update rate is changed. My understanding is that when a calibration is performed it is done at the current update rate and the offset and gain are at the current noise levels. So if I change to a slower update rate shouldn't a calibration be performed to improve the noise levels of the gain and offset?
A calibration is required if the gain is changed, if the polarity is changed or the channel is changed. However, the offset and gain errors of the ADC are not dependent on the output data rate. Therefore, it is not necessary to perform calibrations when the output data rate is changed.
A calibration is really a conversion with a known input. The ADC has lower noise at lower output data rates. Therefore, performing the calibrations at lower output data rates improves the accuracy of the calibrations – the offset and FS error removal is more accurate.
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