Measure analog voltage on ESP32 with ADC

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

There are 2 12-bit ADCs on ESP32, ADC1 with 8 channels and ADC2 with 10 channels. Each ADC channel allows a measurement to be done on a pin.

Pinout of the uPesy ESP32 Wroom Devkit board

ADC limitation on the ESP32

ADC is not a strong point of the ESP32 because it has many flaws. Prefer that the Arduino or use an external ADC if you want to make reliable measurements.

Warning

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

The ESP32 ADC 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.

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 response curve of the ADC is not a linear straight line), especially at the ends of its use range (towards 0V and 3.3V)

ESP32 ADC non-linearity

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, for small voltages, the ESP32 will not differentiate between a 0V and 0.2V signal.

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:

ESP32 ADC electrical noise

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

Correction of electrical noise from the ESP32 ADC

Usage

The basic 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); //Il faut déclarer le pin en entrée
analogRead(36);

Note

There are also more advanced functions.

To change the ADC resolution:

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

           

Project

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

Schematic

Try to write the program by yourself!

Solution

            // Le potentiomètre est connecté au GPIO 36 (Pin VP)
const int potPin = 36;

// Valeur du potentiomètre
int potValue = 0;

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

void loop() {
// Mesure la valeur du potentiomètre
potValue = analogRead(potPin);
Serial.println(potValue);
delay(250);
}

           

When we turn the potentiometer, we get:

Measure analog voltage on ESP32 with ADC

ADC limitation on the ESP32

Usage

Project

Schematic

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