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); 
}

circuit diagram

• Mats Karlsson: Please add the schema.
• Marr Madden: My multimeter reads the correct Vout...
• Marr Madden: this gets me closer, but still low... float voltage = (sensorValue * 3.3) / (4095)
• tavis: ESP32 chips have a problem with ADC and WIFI at the same time. Maybe disable WIFI ?
• jsotola: both the Huzzah32 and circuit are grounded ... are the grounds joined? ... your schematic does not show a common ground

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.