nRF51 and nRF52 ADC calibration information

[Masahiro Kondo]

Hello. I have read a topic about ADC calibration (gain and offset error in 0x10000024) of nRF51822 here but I can't find the document describing it. Could anyone point me where the description of this register is (is it part of FICR?)? And also, does nRF52832 have the same register that I can use to adjust gain and offset inside firmware?

Thank you for the help.

[Stefan Birnir Sverrisson]

Hi Masahiro

The calibration values at address 0x10000024 only apply to nRF51 third revision chips. For nRF52, there are no calibration values that I know of. You need instead to periodically calibrate the SAADC by calling the TASKS_CALIBRATEOFFSET task.

[Masahiro Kondo]

Thank you for your help. About nRF51 third revision chips, how much offset error and gain error should I expect after calculation with calibration values? Are they still +-2%?

Regards.

[Stefan Birnir Sverrisson]

You are correct, there is maximum +-2% offset error and +-2% gain error for the nRF51 ADC. This is worst case error for both temperature variations and production variations. The calibration method should mostly remove the offset and gain errors for production variations, but the calibration numbers are given for room temperature (~25 deg C). For different temperatures within the operating temperature range there might still be some offset and gain error.

The given offset error and gain error specified in the product specification are correlated, meaning the total worst case error is less than 2%+2%=4%.

[Masahiro Kondo]

I am not sure if I understood you correctly. Do you mean +-2% "after" calibration method applied? I guess +-2% is "before" calibration otherwise it doesn't make sense that Rev2 chips which don't have calibration coefficients also has +-2% as ADC error. It sounds like it will be much better for 25C (my application is pretty close to 25C) after calibration done but you don't have any spec like it will be +-0.5% or something like that? Thank you for your help.

[Stefan Birnir Sverrisson]

Sorry for late reply. Yes, +-2% is worst case before any calibration is applied. So if you apply the calibration numbers applied for third revision hardware at address 0x10000024, then you will be having considerably lower error than +-2%. How much lower, I am not sure, the spec does not state how much of the error is a result of temperature changes, but I know that majority of the temperature error is close to the edges of the temperature range of the nRF51 chip, i.e. close to -25 deg C and +75 deg C. So if your device is generally close to room temperature and you apply the calibration numbers, then you should have relatively accurate ADC readings.