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

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

Øyvind Karlsen — Tue, 19 Jul 2016 05:58:46 GMT

I imagine the RC circuit was some kind of filter? It depends on your usecase, if you know you have noisy power lines then it might be a good idea. If you do have noisy power lines it might not be the best idea to use an external reference to start with.

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

yunlong.wang — Tue, 19 Jul 2016 05:56:44 GMT

Hi:

One more question, I search for the relation between ADC and capacitor yesterday , I found some suggestion that insert an RC circuit before ADC input. So, is it neccessary to put a RC circuit before ADC input pin, or a reservoir capacitor is just enough ?

Thanks a lot

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

Øyvind Karlsen — Tue, 19 Jul 2016 05:42:02 GMT

It has to be "large enough" so that if the power lines are pulled down the capacitor will still provide stable reference, somewhere in the µF range probably around 5-10 should be fine.

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

yunlong.wang — Tue, 19 Jul 2016 02:39:49 GMT

Hi： I will try to add a capacitor to the AREF port, but how to define the capacitor value? Is there a formula to caculate the value ? Thanks a lot

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

Øyvind Karlsen — Mon, 18 Jul 2016 13:04:51 GMT

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

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

yunlong.wang — Mon, 18 Jul 2016 12:49:17 GMT

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);
}

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

Øyvind Karlsen — Mon, 18 Jul 2016 12:45:13 GMT

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.

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

Øyvind Karlsen — Mon, 18 Jul 2016 12:43:47 GMT

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

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

yunlong.wang — Mon, 18 Jul 2016 12:35:56 GMT

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);

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

Øyvind Karlsen — Mon, 18 Jul 2016 11:36:33 GMT

Instead of nrf_adc_configure() try calling

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);                              /*!< 8bit ADC resolution. */

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

yunlong.wang — Mon, 18 Jul 2016 10:35:24 GMT

Hi Yvind： The VREF is 1.24V， and the following is ADC related Code

void mp_adc_config(void)
{    

    //1.10 BIT ADC    2.SCALING_INPUT_FULL_SCALE     3.Vref = 1.24V  
    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;

    // Initialize and configure ADC
    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);

		g_currAdcChannelIndex = 0;
}

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

Øyvind Karlsen — Mon, 18 Jul 2016 10:25:07 GMT

What voltage are you applying to VREF? It should be between 0.83V - 1.3V. A capacitor on the pin helps keep constant voltage, however I still think you should see something, this leads me to think that there is something wrong with your code.

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

yunlong.wang — Mon, 18 Jul 2016 10:18:58 GMT

Hi Yvind： It seems no work, the following is the register setting of ADC on the product, Is there something wrong ? 1. ADC->CONFIG = 0x00020822 2. ADC->INTENSET = 0x01 3. ADC->INTENCLR = 0x01 4. ADC->POWER = 0x01 If we want to use external reference voltage, should I pay attention to the hardware desgin ? Do I need to connect a capacitor from the AIN7( gpio0.06) pin to ground ?

Thanks a lot

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

Øyvind Karlsen — Mon, 18 Jul 2016 08:40:00 GMT

Hi,

I will assume you are using nRF51. Essentially you will need to configure EXTREFSEL with AREF1 and REFSEL with External. Something like this should work (I use hardware registers by preference).

static void adc_init(void)
{	
	/* Enable interrupt on ADC sample ready event*/		
	NRF_ADC->INTENSET = ADC_INTENSET_END_Msk;   
	sd_nvic_SetPriority(ADC_IRQn, NRF_APP_PRIORITY_LOW);  
	sd_nvic_EnableIRQ(ADC_IRQn);
	
	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);							     /*!< 8bit ADC resolution. */ 
	
	/* Enable ADC*/
	NRF_ADC->ENABLE = ADC_ENABLE_ENABLE_Enabled;
}

This answer also contains some useful information.

Best regards,

Øyvind