Some experiments on how to deal with the ESP32's ADC in Arduino IDE code

esp32_adc_to_voltage.ino

float v_supply  = 3.29; // measured: 3.29
int   adc_pin  =  4;
int   adc_bits = 12;
  /* effective range of measurement:
   * ADC_0db:    100-950mV  (+-23mV)
   * ADC_2_5db: 100-1250mV (+-30mV)
   * ADC_6db:    150-1750mV (+-40mV)
   * ADC_11db:   150-2450mV (+-60mV) - default value
   */
adc_attenuation_t adc_attn = ADC_0db;

/* taken from
 *  https://www.espressif.com/sites/default/files/documentation/esp32_datasheet_en.pdf
 *  page 23
 */
float adc_max   = pow(2, adc_bits);
float v_ref_internal = 1.1; // should reroute to GPIO and also measure!

float v_adc_max() {
  float val;
  switch (adc_attn) {
    case ADC_0db:
      val = 0.95;
      break;
    case ADC_2_5db:
      val = 1.25;
      break;
    case ADC_6db:
      val = 1.75;
      break;
    default:
      val = 2.45;
      break;
  }
  return val;
}

float v_adc_min() {
  float val;
  switch (adc_attn) {
    case ADC_0db:
    case ADC_2_5db:
      val = 0.1;
      break;
    default:
      val = 0.15;
      break;
  }
  return val;
}

void setup() {
  Serial.begin(115200);
  
  analogReadResolution(adc_bits);
  analogSetAttenuation(adc_attn); // or analogSetPinAttenuation(pin, atten)
}

float adc_val = 0;
float V       = 0;

void loop() {
  adc_val = analogRead(adc_pin);

  // also check sources for https://docs.espressif.com/projects/esp-idf/en/latest/api-reference/peripherals/adc.html#_CPPv426esp_adc_cal_raw_to_voltage8uint32_tPK29esp_adc_cal_characteristics_t
  V = (adc_val / adc_max * v_adc_max()) * v_ref_internal;

  delay(100);
  Serial.print("Own code:\t\t\t");
  Serial.print(V); Serial.println("V");
  Serial.print("Radio amateur guy's code:\t");
  Serial.print(ReadVoltage(adc_pin)); Serial.println("V\n");
}

// from https://github.com/G6EJD/ESP32-ADC-Accuracy-Improvement-function/blob/master/ESP32_ADC_Read_Voltage_Accurate.ino
// doesn't handle ADC attenuation settings other than ADC_11db yet...
double ReadVoltage(byte pin){
  double reading = analogRead(pin);
  if(reading < 1 || reading > adc_max) return 0;

  // use either
  // return -0.000000000009824 * pow(reading, 3) + 0.000000016557283 * pow(reading, 2) + 0.000854596860691 * reading + 0.065440348345433;
  return -0.000000000000016 * pow(reading, 4) + 0.000000000118171 * pow(reading, 3)- 0.000000301211691 * pow(reading, 2)+ 0.001109019271794 * reading + 0.034143524634089;
}