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Measure analog voltage on ESP32 with ADC

As its name suggests, the ADC (Analog to Digital Converter) allows it to convert an analog voltage into a binary value.

There are 2x12 bits ADCs on the ESP32, the ADC1 with 8 channels and the ADC2 with 10 channels. Each channel of the ADC allows measuring a pin.

complete ESP32 pin diagram

Pinout of the uPesy ESP32 Wroom Devkit board

ADC limitation on the ESP32

The ADC is not a strong point of the ESP32 because it has many flaws. Use the Arduino one or an external ADC if you want to make accurate measurements.

Warning

Although it sounds strange, the Arduino’s 10-bit ADC (1024 values) is more accurate and reliable than the ESP32’s 12-bit (4096 values).

The ADC of the ESP32 has several flaws:

  • ADC2 cannot be used with enabled WiFi since it is used internally by the WiFi driver. Since there is a good chance of using WiFi on a microcontroller designed to use it, only the ADC1 and its 8 channels can be used.

  • The ADC can only measure a voltage between 0 and 3.3V. You cannot directly measure analog voltages between 0 and 5V.

    Note

    A voltage divider bridge can reduce a voltage between 0 and 5V to a voltage between 0 and 3.3V.

    adc sensor voltage divider wiring

    Voltage divider bridge to switch from a voltage between 0-5V to 0-3.3V

  • Non-linearity

    The ADC of the ESP32 is not very linear (the ADC response curve is not a linear line), especially at the ends of its operating range (around 0V and 3.3V)

    ESP32 ADC measurement curve

    Non-linearity of the ESP32 ADC

    Basically, this means that the ESP32 cannot distinguish a signal of 3.2V and 3.3V: the measured value will be the same (4095). Likewise, the ESP32 will not differentiate between a 0V and 0.2V signal for small voltages.

    Note

    It is possible to calibrate the ADC to reduce this linearity flaw. An example is available here .

  • The electrical noise of the ADC implies a slight fluctuation of the measurements:

    Electric noise adc ESP32

    Electrical noise of the ESP32 ADC

    Here too, it is possible to try to “correct” this defect by adding a capacitor at the output and with oversampling :

    ESP32 adc noise correction comparison

    Correction of the electrical noise of the ADC of the ESP32

Usage

The primary use of the ESP32 ADC is the same as on the Arduino with the function analogRead() .

  • To read the voltage of the VP pin (GPIO36) of the ESP32:

    pinMode(36, INPUT); //It is necessary to declare the input pin
    analogRead(36);
    

Note

There are also more advanced functions.

  • To change the ADC resolution:

    analogReadResolution(resolution) // Resolution between 9-12 bits
    

Mini-Project

We will test the ADC using a potentiometer (variable resistor).

Electrical schematic for the potentiometer

Wiring diagram ESP32 adc and potentiometer

Electrical Circuit

Wiring diagram ESP32 adc and potentiometer

Electrical circuit of the potentiometer on a breadboard

Code for reading potentiometer values

Solution

// The potentiometer is connected to GPIO36 (Pin VP)
const int potPin = 36;

// Potentiometer value
int potValue = 0;

void setup() {
Serial.begin(115200);
delay(1000);
pinMode(potPin,INPUT_PULLUP);
}

void loop() {
// Measures the value of the potentiometer
potValue = analogRead(potPin);
Serial.println(potValue);
delay(250);
}

Final results

When you turn the potentiometer, you get :

Wiring on adc prototyping plate and potentiometer

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