Regarding the Value Obtained from the ADC

[kkuma256]

conducted an ADC experiment using the nRF5 DK (PCA10056).

The power supply for the nRF5 was provided via USB.

Based on the SAADC sample, I coded the following:

[main.c]
  void saadc_init(void)
  {
         :
      nrf_saadc_channel_config_t channel_config =
          NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_SE(NRF_SAADC_INPUT_AIN1);
+     channel_config.gain = NRF_SAADC_GAIN1_6;

      err_code = nrf_drv_saadc_init(NULL, saadc_callback);
                     :
  }

[sdk_config.h]
  // <o> NRFX_SAADC_CONFIG_RESOLUTION  - Resolution
  // <0=> 8 bit 
  // <1=> 10 bit 
  // <2=> 12 bit 
  // <3=> 14 bit 
  #ifndef NRFX_SAADC_CONFIG_RESOLUTION
- #define NRFX_SAADC_CONFIG_RESOLUTION 1
+ #define NRFX_SAADC_CONFIG_RESOLUTION 2
  #endif

While running, I checked the value of "channel_config," and it was as follows.

channel_config
  .resistor_p : NRF_SAADC_RESISTOR_DISA3LED ( 0 )
  .resistor_n : NRF_SAADC_RESISTOR_DISA3LED ( 0 )
  .gain       : NRF_SAADC_GAIN1_6 ( 0 )
  .reference  : NRF_SAADC_REFEREUCE_INTERNAL ( 0 )
  .acq_time   : NRF_SAADC_ACQTIME_10US ( 2 )
  .mode       : NRF_SAADC_MODE_SINGLE_ENDED ( 0 )
  .burst      : NRF_SAADC_BURST_DESABLED ( 0 )
  .pin_p      : NRF_SAADC_INPUT_AIN1 ( 2 )
  .pin_n      : NRF_SAADC_INPUT_DISABLED ( 0 )

I applied 3.0V to P0.03 and checked the value at the following line (the line marked with "!").

  void saadc_callback(nrf_drv_saadc_evt_t const * p_event)
  {
      if (p_event->type == NRF_DRV_SAADC_EVT_DONE)
      {
                :
          err_code = nrf_drv_saadc_buffer_convert(p_event->data.done.p_buffer, SAMPLES_IN_BUFFER);
!         APP_ERROR_CHECK(err_code);
                :
      }
  }

The expected value was:

3413.3333 = ((3.0 - 0.0)*(1/6))/0.6 * 2^12

However, the actual value was 860.

I verified the value with a tester and confirmed that 3.0V was properly applied.

Why is there such a discrepancy?

I haven’t performed calibration, but I don't think that would result in a 4x difference.

I believe there is a misunderstanding somewhere, but what could it be?