In adc.h file there is such notes ;
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The default ADC full-scale voltage is 1.1V. To read higher voltages (up to the pin maximum voltage, * usually 3.3V) requires setting >0dB signal attenuation for that ADC channel. * * When VDD_A is 3.3V: * * - 0dB attenuaton (ADC_ATTEN_DB_0) gives full-scale voltage 1.1V * - 2.5dB attenuation (ADC_ATTEN_DB_2_5) gives full-scale voltage 1.5V * - 6dB attenuation (ADC_ATTEN_DB_6) gives full-scale voltage 2.2V * - 11dB attenuation (ADC_ATTEN_DB_11) gives full-scale voltage 3.9V (see note below) * * @note The full-scale voltage is the voltage corresponding to a maximum reading (depending on ADC1 configured * bit width, this value is: 4095 for 12-bits, 2047 for 11-bits, 1023 for 10-bits, 511 for 9 bits.) * * @note At 11dB attenuation the maximum voltage is limited by VDD_A, not the full scale voltage. * * Due to ADC characteristics, most accurate results are obtained within the following approximate voltage ranges: * * - 0dB attenuaton (ADC_ATTEN_DB_0) between 100 and 950mV * - 2.5dB attenuation (ADC_ATTEN_DB_2_5) between 100 and 1250mV * - 6dB attenuation (ADC_ATTEN_DB_6) between 150 to 1750mV * - 11dB attenuation (ADC_ATTEN_DB_11) between 150 to 2450mV * * For maximum accuracy, use the ADC calibration APIs and measure voltages within these recommended ranges.