nrf52840 saadc calibration error

[HJLee]

After starting the program, I do the first calibration and read the data 30 times.
The standard deviation is between 2.0 and 2.3.

Perform 30 "Calibration -> Read Data" each time
The standard deviation is 3.8 to 4.7 degrees.

The offset varies with each calibration.
How can I minimize this?

I use the internal reference(0.6V) and 1/6 gain

[ovrebekk]

Hi 

I will help Stian out with this case. 

Can you let me know how you calibrate the ADC and trigger the sample? 

Are you able to share a code sample, or a reference to the code used?

Best regards
Torbjørn

[HJLee]

void saadc_calibration(void){
    uint8_t busy_count = 0;

    nrfx_saadc_abort();
    (void)nrfx_saadc_calibrate_offset();

    while (nrfx_saadc_is_busy() && busy_count++ < 100U) {
        nrf_delay_us(1U);
    }

    if (busy_count < 100U) {
        s_saadc_cal_state = (uint8_t)SAADC_CAL_DONE;
        NRF_LOG_INFO("SAADC calibration complete, busy_count %d", busy_count );
    }
    else {
        s_saadc_cal_state = (uint8_t)SAADC_CAL_RETRY;
        NRF_LOG_INFO("SAADC calibration fail");
    }
}

#define ADC_VOLTS(ADC_VALUE) \
    (int16_t)(((((double)ADC_VALUE * ADC_REF_VOLTAGE_IN_MILLIVOLTS) / ADC_RES_10BIT) * \
    ADC_PRE_SCALING_COMPENSATION) + 0.5)
#define SAMPLES_COUNT 10
static nrf_saadc_value_t m_buffer[SAMPLES_COUNT];

void saadc_read_data(uint8_t channel){
    uint8_t i = 0;
    uint16_t min_index = 0;
    uint16_t max_index = SAMPLES_COUNT - 1U;
    int16_t min = INT16_MAX;
    int16_t max = INT16_MIN;
    int32_t avg, sum = 0;
    int16_t res = 0;

    for (i = 0 ; i < SAMPLES_COUNT ; i++) {
        (void)nrfx_saadc_sample_convert(channel,&m_buffer[i]);
    }

    //Searching a min or max value
    for (i = 0 ; i < SAMPLES_COUNT ; i++) {
        if (min > m_buffer[i]) {
            min_index = i;
            min = m_buffer[i];
        }

        if (max < m_buffer[i]) {
            max_index = i;
            max = m_buffer[i];
        }

        sum += (int32_t)m_buffer[i];
    }

    //Check index
    if (max_index != min_index) {
        //Get an average value without the max value and the min value
        sum -= (int32_t)m_buffer[max_index];
        sum -= (int32_t)m_buffer[min_index];
        avg = (int32_t)(((double)sum / (SAMPLES_COUNT - 2U)) + 0.5);
    }
    else {
        avg = (int32_t)(((double)sum / SAMPLES_COUNT) + 0.5);
    }

    return ADC_VOLTS(avg);
}

The busy_count used in saadc_calibration() is a time out function.
When busy_count exceeds 100 and s_saadc_cal_state becomes SAADC_CAL_RETRY, saadc_calibration() is called again from another place.
(However, I get a busy count of 43 in all calibrations, and calibration succeeds.)

Adc sampling actually reads 10 times and uses 8 averages excluding the minimum and maximum values.