ADC offset

RE: ADC offset

PMichael — Wed, 05 Sep 2018 20:41:50 GMT

Addtional question.

Can I perform calibration each time after I init SAADC device before channel initialization?

Can it cause any problem except issue that calibration takes time?

nrf_drv_saadc_init(NULL, AD_saadc_callback);

nrf_drv_saadc_calibrate_offset ();

    while(!AD_CalibrationDone);
    nrf_saadc_event_clear(NRF_SAADC_EVENT_CALIBRATEDONE);
    nrf_saadc_int_disable(NRF_SAADC_EVENT_CALIBRATEDONE);

nrf_saadc_channel_config_t channel_config_bat = NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_SE(NRF_SAADC_INPUT_AIN0);
nrf_drv_saadc_channel_init(&channel_config_bat);

void AD_saadc_callback(nrf_drv_saadc_evt_t const * p_event)
{
// set calibration flag if is calibration event
if(p_event->type==NRF_DRV_SAADC_EVT_CALIBRATEDONE)
{
AD_CalibrationDone = true;
}
}

RE: ADC offset

PMichael — Wed, 28 Feb 2018 12:53:20 GMT

Problem solved. Thank you

RE: ADC offset

PMichael — Wed, 28 Feb 2018 08:48:45 GMT

Additional question. If I want to perform calibration after power up, will it be correct sequence?

It is before any ADC initialization.

volatile unsigned long i;
err_code=nrf_drv_saadc_calibrate_offset ();
for(i=0;i<256000; i++); wait for finish calibration

nrf_drv_saadc_init(NULL, AD_saadc_callback);

RE: ADC offset

PMichael — Mon, 26 Feb 2018 13:40:24 GMT

Thank you

RE: ADC offset

haakonsh — Mon, 26 Feb 2018 10:05:00 GMT

Hey Michael,

This sounds a lot like an offset, but we can not rule out a HW issue either.
If it's an offset issue then calibration should be the solution. The SAADC is temperature dependent and you should calibrate the offset every few centigrade. Using differential inputs with offset calibration will yield the best results.
Max time for calibration is determined by the acquisition time of channel 0. At 3µs the calibration time will be ~102µS. See this post for more information. https://devzone.nordicsemi.com/f/nordic-q-a/30042/saadc-offset-calibration-for-each-input/119389#119389

Cheers,

Håkon.