esp32 voltage divider readings off

Question

Trying to read from a voltage divider circuit using two 240 Ohm resistors. Getting a reading of 6.0, when it should be 1.65. Circuit is powered by a bench 3.3V and both the Huzzah32 and circuit are grounded. Using USB for the Huzzah32. Any thoughts?

  void setup() 
{
  Serial.begin(9600);
}
void loop() 
{
  int sensorValue = analogRead(A9);
  float voltage = sensorValue * (3.3 / 1023.0);
  Serial.println(voltage);
  delay(1000); 
}

Answer 1

First, ESP32 ADC is 12-bit, not 10-bit. therefore the formula should be:

float voltage = sensorValue * (3.3 / 4096.0);

Secondly, ESP32 ADC is not really linear, and tend to have lower reading than the actual value. You will need to do some calibration if you want to get a more accurate reading. You can take a look at my github on esp32-adc-calibrate.