Use AREF1(pin0.06) as analog reference voltage for ADC instead of internal VBG

[yunlong.wang]

Dear Sir: Could I use the external reference AREF1(pin0.06) instead of internal VBG 1.2V for ADC operation ? I do the following settings to config using external reference

   nrf_adc_config.resolution = NRF_ADC_CONFIG_RES_10BIT;
   nrf_adc_config.scaling = NRF_ADC_CONFIG_SCALING_INPUT_FULL_SCALE;
   nrf_adc_config.reference = NRF_ADC_CONFIG_REF_EXT_REF1;

But it doesn't work, is there any other settings before using external reference for ADC.

Thanks a lot

[yunlong.wang]

Hi: I use the following setting as you recommanded, the voltage value I read is 1.013V, But the real value is 0.9V. And when I use internal 1.2V as reference voltage ,the value I get is close to 0.9V.

NRF_ADC->CONFIG = (ADC_CONFIG_EXTREFSEL_AnalogReference1 << ADC_CONFIG_EXTREFSEL_Pos) /* Bits 17..16 : ADC external reference pin selection. */
                    | (ADC_CONFIG_PSEL_AnalogInput2 << ADC_CONFIG_PSEL_Pos)                      /*!< Use analog input 2 as analog input. */
                    | (ADC_CONFIG_REFSEL_External << ADC_CONFIG_REFSEL_Pos)                      /*!< Use external voltage as reference for conversion. */
                    | (ADC_CONFIG_INPSEL_AnalogInputOneThirdPrescaling << ADC_CONFIG_INPSEL_Pos) /*!< Analog input specified by PSEL with no prescaling used as input for the conversion. */
                    | (ADC_CONFIG_RES_10bit << ADC_CONFIG_RES_Pos);

[Øyvind Karlsen]

Were you able to get any readings with the previous code?

[Øyvind Karlsen]

By the way, be sure to adjust the scaling, in the code I provided I had set ADC_CONFIG_INPSEL_AnalogInputOneThirdPrescaling, but this might not be right for your applciation, you need to tweak around with this.

[yunlong.wang]

Yes, I try to change ADC_CONFIG_INPSEL_AnalogInputOneThirdPrescaling to ADC_CONFIG_INPSEL_AnalogInputNoPrescaling, but the voltage I read is also larger than 1.0V. The "previous code "you mentioned is my own code ? Yes, I can get value by the following code

void mp_adc_config(void)
{    
    nrf_adc_config_t nrf_adc_config = NRF_ADC_CONFIG_DEFAULT;

		nrf_adc_config.resolution = NRF_ADC_CONFIG_RES_10BIT;
		nrf_adc_config.scaling = NRF_ADC_CONFIG_SCALING_INPUT_FULL_SCALE;
		nrf_adc_config.reference = NRF_ADC_CONFIG_REF_EXT_REF1;

    nrf_adc_configure( (nrf_adc_config_t *)&nrf_adc_config);

    nrf_adc_int_enable(ADC_INTENSET_END_Enabled << ADC_INTENSET_END_Pos);  /*!< Interrupt enabled. */
    NVIC_SetPriority(ADC_IRQn, NRF_APP_PRIORITY_HIGH);   //Configure the priority
    NVIC_EnableIRQ(ADC_IRQn);
	
		mp_adc_init();

		nrf_adc_input_select(g_mpADC[MP_ADC_INDEX_TEMP].channel);
}

[Øyvind Karlsen]

Ah, then I understand. I thought you were not getting any output, my bad.

Referring to the product specification the ADC will get less accurate for lower voltages, with specificed 3 LSB error at 1V8, I think this may increase if the voltage decreases further. The 1.013V you see is likely within specifications of the ADC device.

As you suggested earlier you can try to add a reservoir capacitor to the AREF port and see if this improves things.

Best regards,

Øyvind