Lecture

ESP32 DAC

16 Mar 2022 • Richard Kuo

Introduction to DAC

DAC (Digital to Analog Converter)

Introduction to Digital-Analog Conversion

Binary-Weighted Resistor DAC

Sigma-Delta Digital Audio Converters (DAC)

Ref. 淺談 Delta-Sigma 之工作原理

Digital To Analog Converter (DAC)

ESP32 has two 8-bit DAC (digital to analog converter) channels, connected to GPIO25 (Channel 1) and GPIO26 (Channel 2).
The DAC channels can also be driven with DMA-style written sample data by the digital controller, via the I2S driver when using the “built-in DAC mode”.

For other analog output options, see the Sigma-delta Modulation module and the LED Control module. Both these modules produce high frequency PWM output, which can be hardware low-pass filtered in order to generate a lower frequency analog output.

Sigma-delta Modulation

ESP32 has a second-order sigma-delta modulation module. This driver configures the channels of the sigma-delta module.
There are 8 independent sigma-delta modulation channels identified with sigmadelta_channel_t. Each channel is capable to output the binary, hardware generated signal with the sigma-delta modulation.

Prescaler of the sigma-delta generator - sigmadelta_set_prescale()
Duty of the output signal - sigmadelta_set_duty()
GPIO pin to output modulated signal - sigmadelta_set_pin()

The range of the ‘duty’ input parameter of sigmadelta_set_duty() is from -128 to 127 (eight bit signed integer). If zero value is set, then the output signal’s duty will be about 50%, see description of sigmadelta_set_duty().
Also, it is recommended to use an active filter instead of a passive filter to gain better isolation and not lose too much voltage. For example, you can take the following Sallen-Key topology Low Pass Filter as a reference.

Application Example:
Setting DAC channel 1 (GPIO25) voltage to approx 0.78 of VDD_A voltage (VDD * 200 / 255). For VDD_A 3.3V, this is 2.59V:

#include <driver/dac.h>

...

    dac_output_enable(DAC_CHANNEL_1);
    dac_output_voltage(DAC_CHANNEL_1, 200);

Examples>ESP32>AnalogOut>SigmaDelta

NodeMCU-32S pinout

Audio Amplifier: PAM8403

The PAM8403 is a 3W, class-D audio amplifier.

[Homework]: ESP32-DAC-HAL

Create ESP32-DAC-HAL.ino

#include <Arduino.h>
#include <esp32-hal-dac.h>//DAC功能引用該庫，可以不進行#include<>引用

//查看源碼後得到，DAC引腳只能是這兩個引腳
#define LED1 25
#define LED2 26
 
void setup(){
  //長時候發現，DAC的IO口也可以不進行初始化
  pinMode(LED1,OUTPUT);
  pinMode(LED2,OUTPUT);
}
 
void loop()
{
  // 逐漸變亮
  for (int dutyCycle = 0; dutyCycle <= 255; dutyCycle = dutyCycle + 1)
  {
    dacWrite(LED1, dutyCycle);  // 輸出DAC
    dacWrite(LED2, 255 - dutyCycle);  // 輸出DAC
    delay(5);
  }
 
  // 逐漸變暗
  for (int dutyCycle = 255; dutyCycle >= 0; dutyCycle = dutyCycle - 1)
  {
    dacWrite(LED1, dutyCycle);  // 輸出DAC
    dacWrite(LED2, 255 - dutyCycle);  // 輸出DAC
    delay(5);
  }
}