SAADC wrong readings & impossible to calibrate

RE: SAADC wrong readings & impossible to calibrate

fab — Tue, 02 May 2017 07:05:06 GMT

The offset is not constant but 0.015 provides pretty good readings at least rounding to 2 decimal places. Since I have to read voltages in range 0-5v, I added a voltage divider and this probably increased the noise, the offset increased and is not more constant across the range. To overcome this issue, I measured the offset in 15 points across the range 0-5, I have interpolated the deltas with a 4 grade polynomial and now I get pretty good readings. Last issue I have is that when the voltage is 0 (pin grounded), the readings are completely off value. Probably because the device is measuring only the noise. In my device, voltages never gets close to 0 but I have not figured how to overcome this issue. I should try a active low-pass filter at ADC pin, but I am not sure that it could help. Any suggestion will be appreciated. Thank you for your help.

RE: SAADC wrong readings & impossible to calibrate

Jørgen Holmefjord — Thu, 27 Apr 2017 12:11:39 GMT

If oversampling and offset calibration does not help, I don't have any good advice to come with. You will allways have some noise in such applications, as described in multiple cases here on DevZone. Are you seeing a constant offset of 0.015 V?

RE: SAADC wrong readings & impossible to calibrate

fab — Sun, 23 Apr 2017 14:09:32 GMT

I played around with the SAADC parameters (resolution, oversample, acq_time) without much success. I still have an offset of about 0.015 v. A took a look with my scope and apparently there is a lot of noise at the SAADC pin (see the update at my question). Shown measurements are using the DK-52 powered via USB but I have almost the same situation with a third party module powered by battery. Have you any suggestion on how to reduce the noise?

RE: SAADC wrong readings & impossible to calibrate

Jørgen Holmefjord — Wed, 19 Apr 2017 11:08:13 GMT

Hi,

Your results does not look too far off to me. See this and this thread about expected deviation.

The reason why you get NRF_ERROR_BUSY when trying to calibrate is most likely that you have a buffer setup for sampling. The SAADC is considered sampling when nrf_drv_saadc_buffer_convert have been called. You will have to abort any ongoing buffer conversions using nrf_drv_saadc_abort, or wait to the operation is finished before starting calibration (when you get the NRF_DRV_SAADC_EVT_DONE event). Note that if you use double buffering, you need to call nrf_drv_saadc_abort or wait for two NRF_DRV_SAADC_EVT_DONE events without setting up a new buffer for sampling, before calling the calibration task.

Best regards,

Jørgen