ESP32: How to use DAC to generate true analog signals

ESP32 Tutorial: DAC and Audio | Generating True Analog Signals

Abstract

The Digital-to-Analog Converter (DAC) is the inverse of the ADC. It takes a digital value (a number) from the microcontroller and instantly converts it into a corresponding, precise analog voltage level. Unlike PWM, which only simulates an analog voltage by varying the duty cycle of a square wave, the DAC produces a true, steady analog DC voltage. This tutorial details the ESP32’s dual 8-bit DAC channels and demonstrates their use for generating simple voltage outputs and basic audio signals.

1. Introduction: DAC vs. PWM

While PWM is excellent for non-critical tasks like LED dimming or motor speed control, it’s unsuitable for applications requiring a smooth, stable DC voltage or high-quality audio reproduction. The DAC is designed for these tasks.

Feature	DAC (Digital-to-Analog Converter)	PWM (Pulse Width Modulation)
Output Type	True, stable DC voltage (Analog)	Pulsed square wave (Digital Simulation)
Resolution	8 bits (0 to 255)	Configurable up to 16 bits (e.g., 0 to 4095)
Fidelity	High fidelity (for audio, waveforms)	Low fidelity (requires filtering for stability)
Purpose	Generating audio, reference voltages, smooth control	Dimming LEDs, controlling motors, generating heat

ESP32 DAC Features

The ESP32 includes two independent, built-in 8-bit DAC channels:

DAC Channel 1: Mapped to GPIO 25.
DAC Channel 2: Mapped to GPIO 26.

Since the DAC is 8-bit, the output voltage is mapped to a value range of 0 to 255:

Digital Value 0 = Analog Output 0V.
Digital Value 255 = Analog Output 3V (VCC).

The voltage increment (resolution step) is approximately 3.3V/255≈12.94mV.

2. Core Function ESP32's DAC Control

Controlling the DAC is simple, requiring only the standard Arduino function dacWrite().

2.1 Function for DAC Output: dacWrite(pin, value)

This function outputs a stable voltage to the specified DAC pin.

pin: Must be either GPIO 25 or GPIO 26.
value: An integer between 0 and 255.

2.2 DAC Output Code Example (Generating a Steady Voltage)

This sketch continuously outputs a steady voltage of approximately 1.65V (half of 3.3V) on GPIO 25.

				
					#define DAC_PIN 25
#define HALF_VOLTAGE_VALUE 128 // 255 / 2 = 127.5, use 128

void setup() {
  Serial.begin(115200);
  Serial.println("Setting DAC to 1.65V (Value 128)");
 
  // No pinMode() needed; the DAC peripheral controls the pin directly.
 
  // Set the DAC output to the middle value (1.65V)
  dacWrite(DAC_PIN, HALF_VOLTAGE_VALUE);
}

void loop() {
  // Since the voltage is steady, the loop can be used for other tasks.
  delay(1000);
  Serial.println("DAC is stable at 1.65V...");
}

3. Generating Simple Audio (Sine Wave)

Audio is essentially a rapid sequence of voltage changes (an oscillating waveform). To generate sound, we must continuously write new DAC values to simulate a waveform like a sine wave.

3.1 Concept: Waveform Generation

A standard sine wave can be generated by looping through a pre-calculated array of 8-bit values (0-255). By writing these values to the DAC pin rapidly, the output voltage mimics the wave’s smooth oscillation.

3.2 Audio Generation Code Example (Sine Wave Tone)

This sketch generates a basic tone by cycling through a small sine wave lookup table.

				
					#define AUDIO_PIN 25
#define FREQUENCY 440 // A4 note (440 Hz)

// Small sine wave lookup table (20 points for fast cycling)
// Values are biased up to center around 128 (1.65V)
const int SINE_LUT[] = {
  128, 169, 203, 227, 240, 240, 227, 203, 169, 128,
  87, 53, 29, 16, 16, 29, 53, 87
};
const int LUT_SIZE = 18; // The number of points in the array

void setup() {
  // No setup needed for DAC pin other than the implicit setup via dacWrite
}

void loop() {
  // Speed of the loop dictates the audio frequency
  // Time = 1 / (Frequency * LUT_SIZE)

  for (int i = 0; i < LUT_SIZE; i++) {
    dacWrite(AUDIO_PIN, SINE_LUT[i]);
   
    // Calculate the required delay in microseconds per sample
    // For 440 Hz, Period = 1/440 Hz ≈ 2272 us.
    // Delay per sample = Period / LUT_SIZE = 2272 us / 18 ≈ 126 us.
    delayMicroseconds(126);
  }
}

// NOTE: For practical audio, the I2S peripheral (not covered here) is preferred

// due to its ability to handle much higher sample rates. The DAC is best for

// simple tones or reference voltages.

4. Lab Recap

You’ve learned the basics of generating true analog output on the ESP32:

The ESP32 features two 8-bit DAC channels on GPIO 25 and GPIO 26.
The DAC generates a stable, true analog voltage, offering higher fidelity than PWM for audio and voltage referencing.
The voltage range is 0V to 3.3V, mapped to the digital value range of 0 to 255.
The output voltage is set using the function dacWrite(pin, value).
Simple audio generation involves using the DAC to rapidly cycle through a waveform lookup table, with the speed of cycling determining the frequency of the generated tone.