nRF52832 SAADC error versus temperature

RE: nRF52832 SAADC error versus temperature

Jørgen Holmefjord — Thu, 27 Oct 2016 07:10:14 GMT

Hi Paul, the INL and DNL descriptions found in the SAADC Electrical Specification should describe the non-linear properties of the SAADC over the voltage range. You need to periodically run the offset calibration task whenever there is major temperature change. This can either be done periodically, e.g. every few seconds, or you can measure the temperature on the die with the TEMP peripheral sensor and calibrate only if there is major temperature change. Gain error as result of temperature change is given in the above linked electrical specificaiton as parameter C_EG. If you make sure you enable the external 16MHz crystal before sampling, this should not vary with time.

RE: nRF52832 SAADC error versus temperature

Paul Dunn — Tue, 25 Oct 2016 14:14:25 GMT

Jorgen - Thanks for the response. Yes, my method is similar to the link you provided. I am already using an external capacitor and running the offset calibration. And yes, I understand I need to perform both the offset calibration periodically and my 2 point calibration for every part.

My question is this: If I do this 2 point calibration on every part, how stable will it be over my voltage range, temperature, and time? Will the error be a function of component lot? I'm going to characterize this error over my voltage and temperature range for a small number of parts, but I am concerned that all parts are from the same lot and this error might be a function of component lot. I don't want to "get lucky" with a good lot of parts, and later find out that even the calibrated error can change, more than I expect, over temperature or time.

RE: nRF52832 SAADC error versus temperature

Jørgen Holmefjord — Tue, 25 Oct 2016 12:16:32 GMT

Hi,

It looks like your calibration method is similar to the one described in this answer. There will be some deviation from device to device, so you will have to calibrate the SAADC for every device to get the reduced error you want.

If you want to make the SAADC output more stable then you could enable oversampling, or connect a capacitor to reduce noise, description on this blog. Offset calibration is also recommended. Oversampling and offset calibration is coded here.

Best regards,

Jørgen