Checking the ADC value range with SAADC

RE: Checking the ADC value range with SAADC

ovrebekk — Tue, 20 Aug 2019 12:08:28 GMT

It appears you are mostly using default SAADC settings, which means you are using the internal 0.6V reference, and a gain setting of 1/6.

You can calculate the voltage of the measured signal like this: Voltage = ADC_reading / 2^12 * INT_REF / GAIN

In your case this equals 3.003V, which seems pretty accurate if your voltage is supposed to be 3V.

In other words I think the ADC is working as intended 🙂

If you want the ADC scale to be 0-0xFFF for 0-VDD then you shoult set the internal reference to VDD1_4 and the gain setting to GAIN1_4, as described in the product specification.

Best regards
Torbjørn

RE: Checking the ADC value range with SAADC

yokokawa — Tue, 20 Aug 2019 02:47:15 GMT

void saadc_init(void)
{
ret_code_t err_code;
/*-< SAADC Register initial parameter >-*/
nrf_drv_saadc_config_t saadc_config = NRF_DRV_SAADC_DEFAULT_CONFIG; /* SET to SAADC Data area or initial data */

/*-< SAADC Channel Setting parameter >-*/
nrf_saadc_channel_config_t channel0_config =
NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_SE(NRF_SAADC_INPUT_AIN2); /* Channel 2 [Fingertip pulse wave] data */
nrf_saadc_channel_config_t channel2_config =
NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_SE(NRF_SAADC_INPUT_AIN0); /* Channel 0 [pulse wave] data */

/*-< SAADC initial >-*/
err_code = nrf_drv_saadc_init(&saadc_config, saadc_callback);
APP_ERROR_CHECK(err_code);

/*-<ADC offset calibration >-*/
err_code = nrfx_saadc_calibrate_offset();
APP_ERROR_CHECK(err_code);
while(nrfx_saadc_is_busy()) // Wait for calibration to complete
{
__WFE(); //
__SEV(); //
__WFE(); // This sequence puts the system to sleep (SystemON) while waiting
}

/*-< SAADC Channel Setting >-*/
err_code = nrf_drv_saadc_channel_init(0, &channel0_config); /*Channel 0 setting */
APP_ERROR_CHECK(err_code);
err_code = nrf_drv_saadc_channel_init(2, &channel2_config); /*Channel 2 setting */
APP_ERROR_CHECK(err_code);

/*-<SAADC double baffer Setting >-*/
err_code = nrf_drv_saadc_buffer_convert(m_buffer_pool[0], SAADC_SAMPLES_IN_BUFFER); /* 1st buffer setting */
APP_ERROR_CHECK(err_code);
err_code = nrf_drv_saadc_buffer_convert(m_buffer_pool[1], SAADC_SAMPLES_IN_BUFFER); /* 2nd buffer setting */
APP_ERROR_CHECK(err_code);
}

RE: Checking the ADC value range with SAADC

ovrebekk — Mon, 19 Aug 2019 13:38:45 GMT

Could you send me the SAADC configuration parameters that you use when you initialize it?

Depending on how you have configured the internal reference, which gain settings you are using etc, VDD might not correspond to 0xFFF.

Best regards
Torbjørn