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

[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.

[Nakul Nair]

No I am not using different settings and I set my gain as 1_5 and in the battery_lvl_indicator func I saw that the voltage range is from 3v is 2.1v, so that is why all the time I get 100%.because mine is from 3v to 2.7v?

What should I change to get appropriate battery level percentage? 

[Jørgen Holmefjord]

You should set to your 1/gain setting, i.e., if you use 1_5 gain, you set this to 5.

You need to create your own function converting the voltage in millivolts to percentage battery. This is the function used in SDK 15.0.0:

/** @brief Function for converting the input voltage (in milli volts) into percentage of 3.0 Volts.
 *
 *  @details The calculation is based on a linearized version of the battery's discharge
 *           curve. 3.0V returns 100% battery level. The limit for power failure is 2.1V and
 *           is considered to be the lower boundary.
 *
 *           The discharge curve for CR2032 is non-linear. In this model it is split into
 *           4 linear sections:
 *           - Section 1: 3.0V - 2.9V = 100% - 42% (58% drop on 100 mV)
 *           - Section 2: 2.9V - 2.74V = 42% - 18% (24% drop on 160 mV)
 *           - Section 3: 2.74V - 2.44V = 18% - 6% (12% drop on 300 mV)
 *           - Section 4: 2.44V - 2.1V = 6% - 0% (6% drop on 340 mV)
 *
 *           These numbers are by no means accurate. Temperature and
 *           load in the actual application is not accounted for!
 *
 *  @param[in] mvolts The voltage in mV
 *
 *  @return    Battery level in percent.
*/
static __INLINE uint8_t battery_level_in_percent(const uint16_t mvolts)
{
    uint8_t battery_level;

    if (mvolts >= 3000)
    {
        battery_level = 100;
    }
    else if (mvolts > 2900)
    {
        battery_level = 100 - ((3000 - mvolts) * 58) / 100;
    }
    else if (mvolts > 2740)
    {
        battery_level = 42 - ((2900 - mvolts) * 24) / 160;
    }
    else if (mvolts > 2440)
    {
        battery_level = 18 - ((2740 - mvolts) * 12) / 300;
    }
    else if (mvolts > 2100)
    {
        battery_level = 6 - ((2440 - mvolts) * 6) / 340;
    }
    else
    {
        battery_level = 0;
    }

    return battery_level;
}

[Nakul Nair]

Jørgen Holmefjord said:

You should set to your 1/gain setting, i.e., if you use 1_5 gain, you set this to 5.

Yes I have did that.

how do I set the percentage drops?

[Jørgen Holmefjord]

You need to look on the discharge curve of your battery to figure that out.