ESP32 DAC 真模拟输出入门：dacWrite 与「假 PWM」的区别

前置知识：PWM 完全入门。PWM 调光的本质是「快速开关骗过眼睛」，输出引脚上其实只有 0V 或 3.3V 两种电平。本篇讲的是 DAC——货真价实的连续电压输出。

一、DAC vs PWM：一字之差，本质不同

维度	PWM（占空比）	DAC（真模拟）
输出电平	只有 0V / 3.3V（高速跳变）	任意中间电压（连续）
原理	时间平均「骗」负载	电阻网络直接分压
可用引脚	任意 GPIO	仅 GPIO25/26（经典 ESP32）
分辨率	理论可到 16 位	8 位（256 级）
需要滤波吗	视负载而定	不需要
典型用途	LED 亮度、电机调速	波形发生、偏置电压、音频

一句话：PWM 是「占空比艺术」，DAC 是「电压直出」。驱动 LED 用 PWM 就够；要生成正弦波或给运放提供偏置，必须 DAC。

二、芯片可用性速查（重要）

芯片	DAC	说明
经典 ESP32	✅ 2 路（GPIO25/26，通道 1/2）	8 位
ESP32-S2	✅ 2 路（GPIO17/18）	8 位
ESP32-S3	❌ 无 DAC	用外置 DAC（如 MCP4725，走 I2C）
ESP32-C3/C6	❌ 无 DAC	同上

新项目若必须真模拟输出且用 S3/C3：MCP4725 模组（12 位、I2C、约 ¥3）是标准答案——I2C 用法见总线篇。

三、dacWrite 五分钟上手

#define DAC_PIN 25   // 只能是 25 或 26！

void setup() { /* DAC 无需 pinMode 初始化 */ }

void loop() {
  dacWrite(DAC_PIN, 0);     // 输出 0V
  delay(1000);
  dacWrite(DAC_PIN, 128);   // 输出约 1.65V
  delay(1000);
  dacWrite(DAC_PIN, 255);   // 输出约 3.3V
  delay(1000);
}

8 位 = 0~255，每级约 12.9mV
输出阻抗较高（不适合直接驱动低阻负载），带运放缓冲才稳
输出电压是「约」——实测有偏差，精确场合需要标定

四、实战：生成正弦波

#define DAC_PIN 25

void setup() {
  Serial.begin(115200);
}

void loop() {
  // 生成 100Hz 正弦：每周期 100 点，每点 100µs
  for (int i = 0; i < 100; i++) {
    float v = (sinf(i * 2 * PI / 100) + 1) / 2 * 255;
    dacWrite(DAC_PIN, (int)v);
    delayMicroseconds(100);
  }
}

用硬件定时器替换 delayMicroseconds 可获得稳定的频率输出——定时器篇的四层选型在此直接适用。

性能天花板：DAC 转换速率约 8µs/次 → 理论最高方波 ~1MHz、实用正弦几十 kHz。做音频（语音级）可行，HiFi 不现实。

五、进阶：音频入门（0.5 秒了解）

DAC + 功放（MAX98357A）就是最简 ESP32 音频链路。但现代做法是 I2S 数字音频（精度高、不占 DAC）——属于 L3 知识点，本站后续专文。DAC 音频的价值在于理解原理：把 PCM 采样值按采样率循环 dacWrite 就是裸音频输出。

六、避坑速查表

场景	结论
用 S3/C3 调 `dacWrite`	芯片无 DAC，编译可能过但无输出；改用 MCP4725
想在 GPIO25/26 用 ADC	GPIO25/26 复用 ADC2 通道——开 Wi-Fi 后冲突，别混用
DAC 输出带不动负载	输出阻抗高，加运放电压跟随器
波形频率不稳	`delay` 抖动，改硬件定时器驱动
要 12 位精度	外置 MCP4725（I2C），内置只有 8 位
LED 调光该用 DAC 吗	不，PWM 足够且任意引脚可用

七、延伸阅读

🧭 前作：PWM 完全入门——占空比调光
⏭️ 下一篇：UART/I2C/SPI 三大总线——外设通信（含外置 DAC 接法）

ESP32 DAC True Analog Output Guide: dacWrite vs “Fake PWM”

Prerequisite: PWM Complete Guide. PWM dimming is really “switching fast enough to fool the load” — the pin only ever sits at 0V or 3.3V. This article covers the DAC: genuine continuous voltage output.

1. DAC vs PWM: one letter apart, fundamentally different

Aspect	PWM (duty cycle)	DAC (true analog)
Output levels	Only 0V / 3.3V (fast switching)	Any intermediate voltage (continuous)
Principle	Time-averaging “fools” the load	Resistor ladder divides directly
Pins	Any GPIO	GPIO25/26 only (classic ESP32)
Resolution	Up to 16 bits theoretically	8 bits (256 steps)
Filtering needed	Depends on the load	No
Typical uses	LED brightness, motor speed	Waveform generation, bias voltage, audio

One-liner: PWM is duty-cycle art; DAC is voltage direct out. LEDs want PWM; sine waves and op-amp bias voltages want a DAC.

2. Per-chip availability (important)

Chip	DAC	Notes
Classic ESP32	✅ 2 channels (GPIO25/26)	8-bit
ESP32-S2	✅ 2 channels (GPIO17/18)	8-bit
ESP32-S3	❌ no DAC	Use an external DAC (e.g. MCP4725 over I2C)
ESP32-C3/C6	❌ no DAC	Same

New project on S3/C3 that truly needs analog out: the MCP4725 module (12-bit, I2C, ~¥3) is the standard answer — wiring covered in the buses guide.

3. dacWrite in five minutes

#define DAC_PIN 25   // must be 25 or 26!

void setup() { /* DAC needs no pinMode setup */ }

void loop() {
  dacWrite(DAC_PIN, 0);     // 0V
  delay(1000);
  dacWrite(DAC_PIN, 128);   // ~1.65V
  delay(1000);
  dacWrite(DAC_PIN, 255);   // ~3.3V
  delay(1000);
}

8 bits = 0–255, ~12.9mV per step
The output impedance is high (don’t drive low-impedance loads directly); buffer with an op-amp follower
Voltages are “approximately” — calibrate if you need precision

4. Project: generating a sine wave

#define DAC_PIN 25

void setup() {
  Serial.begin(115200);
}

void loop() {
  // 100Hz sine: 100 points per cycle, 100µs each
  for (int i = 0; i < 100; i++) {
    float v = (sinf(i * 2 * PI / 100) + 1) / 2 * 255;
    dacWrite(DAC_PIN, (int)v);
    delayMicroseconds(100);
  }
}

Replacing delayMicroseconds with a hardware timer yields a rock-stable frequency — the four-layer selection from the timers guide applies directly.

Ceiling: DAC conversion takes ~8µs → theoretical max square wave ~1MHz, practical sines in the tens of kHz. Voice-grade audio: fine. HiFi: no.

5. Primer: audio in half a page

DAC + amp (MAX98357A) is the simplest ESP32 audio chain. The modern approach is I2S digital audio (higher precision, doesn’t consume the DAC) — an L3 topic with its own article. DAC audio’s value is understanding the principle: looping dacWrite over PCM samples at the sample rate is bare-metal audio output.

6. Pitfall cheat sheet

Scenario	Verdict
Calling `dacWrite` on S3/C3	No DAC — may compile but outputs nothing; use MCP4725
Wanting ADC on GPIO25/26	These are also ADC2 channels — conflict once Wi-Fi starts
DAC can’t drive the load	High output impedance — add an op-amp follower
Wobbly waveform frequency	`delay` jitter — drive from a hardware timer
Need 12-bit precision	External MCP4725 (I2C); the built-in one is 8-bit
Should LEDs use the DAC?	No — PWM is enough and works on any pin

7. Further reading

🧭 Previous: PWM Complete Guide — duty-cycle dimming
⏭️ Next: UART/I2C/SPI — the three buses (includes wiring an external DAC)

License: CC BY-SA 4.0. Repost & remix with attribution: ESPDIY Maker Lab - espdiy.com

ESP32 DAC 真模拟输出入门：dacWrite、GPIO25/26 与「假 PWM」的区别ESP32 DAC True Analog Output Guide: dacWrite, GPIO25/26 and the Difference from 'Fake PWM'