ADC values ​​are changing each time you assemble or disassemble or tighten the screws of a mechanical assembly

[Laerte Junior]

Hello,

I'm developing a torque meter using nrf82832 I fixed complete wheatsotne bridge in my metal part that need measure deformation, but each time that I assemble or disassemple or thighten the screws, the value of adc is changing..

Below, the saadc configuration that I'm using:

void saadc_init(void) {
    //  SAADC config & calibrate
    nrfx_saadc_config_t saadc_config = NRFX_SAADC_DEFAULT_CONFIG;
    APP_ERROR_CHECK(nrfx_saadc_init(&saadc_config, saadc_event_handler));
    while (nrfx_saadc_is_busy())
        sd_app_evt_wait();

    // Channels config
    //Bateria
    nrf_saadc_channel_config_t battery_config = NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_SE(NRF_SAADC_INPUT_AIN0);//NRFX_SAADC_DEFAULT_CHANNEL_CONFIG_SE(NRF_SAADC_INPUT_VDD);
    APP_ERROR_CHECK(nrf_drv_saadc_channel_init( 0, &battery_config));

    //Celulas de carga
    nrf_saadc_channel_config_t cell_config = NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_DIFFERENTIAL(NRF_SAADC_INPUT_AIN4, NRF_SAADC_INPUT_AIN7);
    cell_config.reference = NRF_SAADC_INPUT_AIN5;
    cell_config.gain = NRF_SAADC_GAIN1_4;
    APP_ERROR_CHECK(nrf_drv_saadc_channel_init( 1, &cell_config));
    
    // double buffering
    APP_ERROR_CHECK(nrfx_saadc_buffer_convert(buffer[0], INPUTS));
    APP_ERROR_CHECK(nrfx_saadc_buffer_convert(buffer[1], INPUTS));
}

The values ​​read from the adc each time I modify the mechanical part

Just electronic boad
<info> app:  -1924.00 <CR><LF>
<info> app:  -1936.00 <CR><LF>
<info> app:  -1930.00 <CR><LF>
<info> app:  -1914.00 <CR><LF>
<info> app:  -1926.00 <CR><LF>
<info> app:  -1920.00 <CR><LF>
<info> app:  -1918.00 <CR><LF>
<info> app:  -1928.00 <CR><LF>
<info> app:  -1886.00 <CR><LF>
<info> app:  -1930.00 <CR><LF>
<info> app:  -1924.00 <CR><LF>
<info> app:  -1930.00 <CR><LF>
<info> app:  -1920.00 <CR><LF>
<info> app:  -1936.00 <CR><LF>
<info> app:  -1920.00 <CR><LF>
<info> app:  -1932.00 <CR><LF>
<info> app:  -1932.00 <CR><LF>
<info> app:  -1924.00 <CR><LF>
<info> app:  -1928.00 <CR><LF>
<info> app:  -1912.00 <CR><LF>
<info> app:  -1930.00 <CR><LF>


Just first metal borad part with the screws
<info> app:  -1850.00 <CR><LF>
<info> app:  -1842.00 <CR><LF>
<info> app:  -1844.00 <CR><LF>
<info> app:  -1828.00 <CR><LF>
<info> app:  -1852.00 <CR><LF>
<info> app:  -1838.00 <CR><LF>
<info> app:  -1846.00 <CR><LF>
<info> app:  -1846.00 <CR><LF>
<info> app:  -1838.00 <CR><LF>
<info> app:  -1850.00 <CR><LF>
<info> app:  -1840.00 <CR><LF>
<info> app:  -1848.00 <CR><LF>
<info> app:  -1838.00 <CR><LF>
<info> app:  -1840.00 <CR><LF>
<info> app:  -1852.00 <CR><LF>
<info> app:  -1842.00 <CR><LF>
<info> app:  -1842.00 <CR><LF>
<info> app:  -1844.00 <CR><LF>
<info> app:  -1834.00 <CR><LF>
<info> app:  -1850.00 <CR><LF>

Just first metal borad part with the screws again
<info> app:  -520.00 <CR><LF>
<info> app:  -492.00 <CR><LF>
<info> app:  -514.00 <CR><LF>
<info> app:  -528.00 <CR><LF>
<info> app:  -504.00 <CR><LF>
<info> app:  -512.00 <CR><LF>
<info> app:  -522.00 <CR><LF>
<info> app:  -514.00 <CR><LF>
<info> app:  -512.00 <CR><LF>
<info> app:  -510.00 <CR><LF>
<info> app:  -510.00 <CR><LF>
<info> app:  -500.00 <CR><LF>
<info> app:  -510.00 <CR><LF>
<info> app:  -500.00 <CR><LF>
<info> app:  -504.00 <CR><LF>
<info> app:  -514.00 <CR><LF>
<info> app:  -512.00 <CR><LF>
<info> app:  -512.00 <CR><LF>
<info> app:  -514.00 <CR><LF>
<info> app:  -504.00 <CR><LF>

Completely mechanical board
<info> app:  -496.00 <CR><LF>
<info> app:  -500.00 <CR><LF>
<info> app:  -500.00 <CR><LF>
<info> app:  -500.00 <CR><LF>
<info> app:  -512.00 <CR><LF>
<info> app:  -514.00 <CR><LF>
<info> app:  -514.00 <CR><LF>
<info> app:  -498.00 <CR><LF>
<info> app:  -488.00 <CR><LF>
<info> app:  -500.00 <CR><LF>
<info> app:  -506.00 <CR><LF>
<info> app:  -486.00 <CR><LF>
<info> app:  -532.00 <CR><LF>
<info> app:  -488.00 <CR><LF>
<info> app:  -496.00 <CR><LF>
<info> app:  -494.00 <CR><LF>
<info> app:  -484.00 <CR><LF>
<info> app:  -510.00 <CR><LF>
<info> app:  -494.00 <CR><LF>
<info> app:  -456.00 <CR><LF>

Dissasselmby all parts and assembly mechanical board again
<info> app:  -436.00 <CR><LF>
<info> app:  -442.00 <CR><LF>
<info> app:  -450.00 <CR><LF>
<info> app:  -434.00 <CR><LF>
<info> app:  -460.00 <CR><LF>
<info> app:  -440.00 <CR><LF>
<info> app:  -444.00 <CR><LF>
<info> app:  -440.00 <CR><LF>
<info> app:  -466.00 <CR><LF>
<info> app:  -458.00 <CR><LF>
<info> app:  -440.00 <CR><LF>
<info> app:  -448.00 <CR><LF>
<info> app:  -450.00 <CR><LF>
<info> app:  -444.00 <CR><LF>
<info> app:  -440.00 <CR><LF>
<info> app:  -440.00 <CR><LF>
<info> app:  -454.00 <CR><LF>
<info> app:  -448.00 <CR><LF>
<info> app:  -444.00 <CR><LF>
<info> app:  -446.00 <CR><LF>

Others values of my electronic circuit:

Brigde: 3V supply using Vref3v (ISL21080CIH330Z-TK)

Using INA333 with gain 1000 and reference with 1.5V supply with Vref1.5V (ISL21080CIH315Z-TK)

The wheatsone bridge using 350ohm strain gauges

Anyone help me if I need change any SAADC configuration?

Regards

Laerte

[Laerte Junior]

Hi,

hmolesworth said:

Yes, doing the SAADC internal calibration (once or periodically) would help. Couple of tickets which might help, hard to find the best as there are so many:

I tried to do it using the example code of the "continuous-adc-sampling-issue", where I inserted the calibration function in my saadc_init, but now, I have problems with my saadc_event_handler function.

below, my modified function:

void saadc_init(void) {
    //  SAADC config & calibrate
    nrfx_saadc_config_t saadc_config = NRFX_SAADC_DEFAULT_CONFIG;
    APP_ERROR_CHECK(nrfx_saadc_init(&saadc_config, saadc_event_handler));
    while (nrfx_saadc_is_busy())
        sd_app_evt_wait();

    // Channels config
    //Bateria
    nrf_saadc_channel_config_t battery_config = NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_SE(NRF_SAADC_INPUT_AIN0);//NRFX_SAADC_DEFAULT_CHANNEL_CONFIG_SE(NRF_SAADC_INPUT_VDD);
    APP_ERROR_CHECK(nrf_drv_saadc_channel_init( 0, &battery_config));

    //Celulas de carga
    nrf_saadc_channel_config_t cell_config = NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_DIFFERENTIAL(NRF_SAADC_INPUT_AIN4, NRF_SAADC_INPUT_AIN7);
    cell_config.reference = NRF_SAADC_INPUT_AIN5;
    cell_config.gain = NRF_SAADC_GAIN1_4;
    APP_ERROR_CHECK(nrf_drv_saadc_channel_init( 1, &cell_config));
    
    // double buffering
    APP_ERROR_CHECK(nrfx_saadc_buffer_convert(buffer[0], INPUTS));
    APP_ERROR_CHECK(nrfx_saadc_buffer_convert(buffer[1], INPUTS));

// Calibrate the SAADC by finding its offset
    NRF_SAADC->TASKS_CALIBRATEOFFSET = 1;
    while (NRF_SAADC->EVENTS_CALIBRATEDONE == 0);
    NRF_SAADC->EVENTS_CALIBRATEDONE = 0;
    while (NRF_SAADC->STATUS == (SAADC_STATUS_STATUS_Busy << SAADC_STATUS_STATUS_Pos));
}

This form is correct?

The returned error of the saadc_timeout_handler is:

<error> app: ERROR 8 [NRF_ERROR_INVALID_STATE]

static void saadc_timeout_handler(void *p_context) { // ADC for battery status
    UNUSED_PARAMETER(p_context);
    APP_ERROR_CHECK(nrfx_saadc_sample());
}

Regards

[Karl Ylvisaker]

Hello,

nrfx_saadc_sample() returning NRF_ERROR_INVALID_STATE means that the SAADC is in the IDLE state when you attempt to call nrfx_saadc_sample().
Please make sure to have started the SAADC before you attempt to sample, and see if this resolves your issue.
Please also make sure that your calibrateoffset procedures follows the approach described in the workaround for Errata 86.

Best regards,
Karl