Adc consumption

[dblasco]

Hello all,

I am trying to activate ADC to measure a voltage every few seconds using nrf51822. When it is done, I supposedly deactivate ADC so it does not consumes while I don't need it. I find that the consumption increases when I activate the ADC, but it does not decreases when it is finished, so I suppose I am not deactivating well.

Code:

Every few seconds, I run this function:

void measure_voltage(void)
{
	uint32_t err_code;
    // Configure ADC
    NRF_ADC->INTENSET   = ADC_INTENSET_END_Msk;
    NRF_ADC->CONFIG     = (ADC_CONFIG_RES_8bit                        << ADC_CONFIG_RES_Pos)     |
                          (ADC_CONFIG_INPSEL_SupplyOneThirdPrescaling << ADC_CONFIG_INPSEL_Pos)  |
                          (ADC_CONFIG_REFSEL_VBG                      << ADC_CONFIG_REFSEL_Pos)  |
                          (ADC_CONFIG_PSEL_Disabled                   << ADC_CONFIG_PSEL_Pos)    |
                          (ADC_CONFIG_EXTREFSEL_None                  << ADC_CONFIG_EXTREFSEL_Pos);
    NRF_ADC->EVENTS_END = 0;
    NRF_ADC->ENABLE     = ADC_ENABLE_ENABLE_Enabled;

    // Enable ADC interrupt
    err_code = sd_nvic_ClearPendingIRQ(ADC_IRQn);
    APP_ERROR_CHECK(err_code);

    err_code = sd_nvic_SetPriority(ADC_IRQn, NRF_APP_PRIORITY_LOW);
    APP_ERROR_CHECK(err_code);

    err_code = sd_nvic_EnableIRQ(ADC_IRQn);
    APP_ERROR_CHECK(err_code);

    NRF_ADC->EVENTS_END  = 0;    // Stop any running conversions.
    NRF_ADC->TASKS_START = 1;
}

Then, I wait for the handler to come up:

void ADC_IRQHandler(void) 
{
    if (NRF_ADC->EVENTS_END != 0)
    {
        uint8_t     adc_result;

        NRF_ADC->EVENTS_END     = 0;
        adc_result              = NRF_ADC->RESULT;
        NRF_ADC->TASKS_STOP     = 1;
	    NRF_ADC->ENABLE     = ADC_ENABLE_ENABLE_Disabled;
			
	    paquet_advertising[0]=adc_result;
	}
}

Am I missing something?

Thank you very much

[zhwzh308]

First, I would assume you have calculated a range of voltage you want to measure. Looking at your configuration for ADC_CONFIG_INPSEL_SupplyOneThirdPrescaling you will get

1.2 VBG * 3 = 3.6V.

If your input is exceeding this range you will get extra current draw. For more information I recommend reading nRF51 Reference Manual v 3.0, page 165 onward.

Second, assume your input voltage is in range, you should consider using a voltage divider to lower the input voltage. It has an added benefit of lowering current draw because you are adding resistance to the ADC pin. This way you need less prescaling, a lower input can lead to much lower current draw during measurement.

Third, you should be aware of the input impedance on these GPIOs when you design the voltage divider, I would assume you have the knowledge to solve RC circuits, so I will only tell you the values are on Table 313 of Page 167 in the reference manual. Just make sure the impedance is appropriate otherwise you will get very unstable read.

I hope these points will be helpful to your circuit design.

[dblasco]

Thank you for your response. I understand I can get an extra current draw in these conditions. But I assume this current draw should appear only when ADC is enabled, right? This does not explain why I don't get the same current draw I had before enabling ADC when I deactivate it.