Why is 270 mV offset added to voltage read by ADC?

[jfathman]

In this example code:

nRF5_SDK_13.0.0_04a0bfd/examples/ble_peripheral/ble_app_proximity/main.c

Where the ADC is configured to read the battery voltage:

nrf_saadc_channel_config_t config =
    NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_SE(NRF_SAADC_INPUT_VDD);

Why is this 270 mV offset added to the voltage read by the ADC?

/**< Typical forward voltage drop of the diode . */
#define DIODE_FWD_VOLT_DROP_MILLIVOLTS  270

batt_lvl_in_milli_volts = ADC_RESULT_IN_MILLI_VOLTS(adc_result) +
                  DIODE_FWD_VOLT_DROP_MILLIVOLTS;

percentage_batt_lvl = battery_level_in_percent(batt_lvl_in_milli_volts);

Is that 270 mV offset only appropriate when running on the nRF52-DK, or also when running the nRF52832 powered from battery?

Thanks.

[Stian Røed Hafskjold]

Hi, this is just showing that if you have a reverse voltage protection diode in your design you need to take the offset into account. On the DK there are diodes like this, both for USB, external supply and the battery socket, so that's why they had to include it in the example. If you don't have diodes in your design there's no offset to add.