ADC offset

[PMichael]

Dear Sirs!

On one from 25 boards there is offset in ADC input about 30 ADC units. Vref = 0.6 V, Gain=6, 12 Bit ADC, oversampling 256 sample.

In other devices I do not see this problem.

My questions:

1. If it is a possible offset or there is a hardware problem?

2. If there is no hardware problem, can nrf_drv_saadc_calibrate_offset solve this problem?

3. How much time maximum can take running nrf_drv_saadc_calibrate_offset?

Thank you

[haakonsh]

Hey Michael,

1. This sounds a lot like an offset, but we can not rule out a HW issue either. 
2. 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. 
3. 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.

[PMichael]

Thank you

[PMichael]

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);

[PMichael]

Problem solved. Thank you

[PMichael]

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;
  }
}