What's wrong with that value/those values? Theyre very slightly negative, depending on what resolution you have set, seems likely within error tolerance.
Try to periodically run offset calibration task as done in the saadc_low_power example on Github. But as RK mentions, the error you are seeing is most likely within tolerance. There is always some noise present. You can however minimize the noise by oversampling, which is also shown in the saadc_low_power example.
Stefan, could you tell me if it is specified somewhere how long that calibration takes? I cannot find it anywhere in specification. Is it fixed amount of time, or variable depending of environment (temperature)?
