What is the right factor to convert voltate using SAADC?

[ndarkness]

Hello,

I am getting really confused when using te SAADC to convert the read values into voltages.

I have seen that the example ble_app_proximity() uses the following macro

#define ADC_REF_VOLTAGE_IN_MILLIVOLTS     600
#define ADC_RES_10BIT                     1024   
#define ADC_PRE_SCALING_COMPENSATION      6   
ADC_RESULT_IN_MILLI_VOLTS(ADC_VALUE)\
        ((((ADC_VALUE) * ADC_REF_VOLTAGE_IN_MILLIVOLTS) / ADC_RES_10BIT) * ADC_PRE_SCALING_COMPENSATION)

whereas the thingy example uses the following method

#define ADC_GAIN                    NRF_SAADC_GAIN1     // ADC gain.
#define ADC_REFERENCE_VOLTAGE       (0.6f)              // The standard internal ADC reference voltage.
#define ADC_RESOLUTION_BITS         (8 + (SAADC_CONFIG_RESOLUTION * 2)) //ADC resolution [bits].
static uint32_t adc_to_batt_voltage(uint32_t adc_val, uint16_t * const voltage)
{
    uint32_t err_code;
    float    adc_gain;
    uint16_t tmp_voltage;

    err_code = adc_gain_enum_to_real_gain(ADC_GAIN, &adc_gain);
    RETURN_IF_ERROR(err_code);

    float tmp = adc_val / ((adc_gain / ADC_REFERENCE_VOLTAGE) * pow(2, ADC_RESOLUTION_BITS));
    tmp_voltage =  (uint16_t) ((tmp / m_battery_divider_factor) * 1000);
    *voltage = ( (tmp_voltage + 5) / 10) * 10;  // Round the value.

    return M_BATT_STATUS_CODE_SUCCESS;
}

Which one has to be used and when?

Thanks in advance

[Jørgen Holmefjord]

Hi,

Both methods are similar and is based on this formula.

The Thingy method is more flexible, as it takes the ADC resolution in bits, ADC_RESOLUTION_BITS, instead of the calculated value of the resolution, ADC_RES_10BIT. It also use the gain settings from SAADC HAL header file and a function to convert this to actual gain value, while the ble_app_proximity method use a static value for gain compensation, ADC_PRE_SCALING_COMPENSATION.

With similar settings, both methods should give same result. Note that the proximity example gives the result as integer in millivolts, while the Thingy method gives the result in volts as a float.

Best regards,

Jørgen

[ndarkness]

Thanks Jorgen, thus if I would like more flexibillity I should use the thingy one right? And If I use as input fort the SAADC NRF_SAADC_INPUT_VDD should I set as adc_gain = NRF_SAADC_GAIN1_6 ?

[Jørgen Holmefjord]

Yes, If you want to change resoulution etc., this is a better way to go. The gain settings will depend on your input VDD. When using the internal reference, a gain setting of 1/6 will cover the whole input range with VDD=3.6V, while with VDD=1.8V, gain settings of 1/3 will give better resolution.

[Nakul Nair]

Hey Jørgen Holmefjord,

By default the ADC_REF_VOLTAGE_IN_MILLIVOLTS is 600 and ADC_PRE_SCALING_COMPENSATION is 6. Which values should I change this to. I am using CR2032 coin cell and when the battery gets at 2.7v the app stops.

[Jørgen Holmefjord]

Are you using a different reference voltage, or different gain setting? If not, you should not change this. If you are using different settings, you should set the defines to the values you are using. If your app stops when the voltage falls below some point, this should not be related to these parameters.