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nRF52832 SAADC configuration effects on calibration

Though it doesn't seem to be mentioned in the product specification, I've confirmed the hint from https://github.com/NordicPlayground/nRF52-ADC-examples/tree/master/saadc_low_power that SAADC calibration time is dependent on both the `OVERSAMPLE` value and the contents of `SAADC->CH[0].CONFIG`, in particular for acquisition time. Questions arise:

* Is calibration improved by setting OVERSAMPLING to a non-zero value?
* Is calibration improved by using TACQ_40us instead of the default TACQ_10us?
* Are there other changes to the ADC sampling configuration that would require calibration be re-run, e.g. changes to GAIN, MODE, or REFSEL?

I've noticed when measuring battery voltage on Thingy:52 that calibration does not always produce the same result (i.e. for a 3.9 V signal I might get values that range between 3.88 +/- 0.01 or 3.92 +/- 0.01: i.e. the samples are tightly grouped but are offset by a value that's presumably a result of calibration.

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