ESP32 ADC Pins: Complete Analog Input Guide

The ESP32 has 18 Analog-to-Digital Converter (ADC) channels spread across two independent ADC blocks: ADC1 and ADC2. Understanding the difference between them — especially the Wi-Fi conflict on ADC2 — is critical for reliable analog sensor readings in ESP32 projects.

This guide covers every ESP32 ADC pin, explains the ADC1 vs ADC2 conflict, shows you how to read analog values in Arduino, and helps you choose the right pins for your sensors.

ESP32 ADC Pin Table

Here is the complete list of all 18 ADC channels on the classic ESP32 (ESP-WROOM-32):

ADC Block	Channel	GPIO Pin	Notes
ADC1	CH0	GPIO36	Input-only (SENSOR_VP)
ADC1	CH3	GPIO39	Input-only (SENSOR_VN)
ADC1	CH6	GPIO34	Input-only
ADC1	CH7	GPIO35	Input-only
ADC1	CH4	GPIO32	Also TOUCH9, RTC
ADC1	CH5	GPIO33	Also TOUCH8, RTC
ADC1	CH0	GPIO36	Also SENSOR_VP
ADC1	CH1	GPIO37	Not broken out on most dev boards
ADC1	CH2	GPIO38	Not broken out on most dev boards
ADC2	CH0	GPIO4	Also TOUCH0
ADC2	CH1	GPIO0	Strapping pin
ADC2	CH2	GPIO2	Strapping pin
ADC2	CH3	GPIO15	Strapping pin
ADC2	CH4	GPIO13	Also TOUCH4
ADC2	CH5	GPIO12	Strapping pin (MTDI)
ADC2	CH6	GPIO14	Also TOUCH6, HSPI_CLK
ADC2	CH7	GPIO27	Also TOUCH7
ADC2	CH8	GPIO25	Also DAC_1
ADC2	CH9	GPIO26	Also DAC_2

ADC1 vs ADC2: The Wi-Fi Conflict

Both ADC1 and ADC2 are 12-bit successive-approximation ADCs with the same resolution (0–4095). However, there is one critical difference:

• ADC1 is independent and works reliably whether Wi-Fi is on or off.
• ADC2 is shared with the Wi-Fi subsystem. When the Wi-Fi driver is active, ADC2 measurements become unreliable or fail entirely.

This is a hardware limitation documented by Espressif. The Wi-Fi driver uses the ADC2 hardware for its own signal detection. If your project uses any Wi-Fi functionality (HTTP, MQTT, WebSockets, OTA, etc.), you must avoid ADC2 pins for analog sensors.

Input-Only ADC Pins

Four ADC1 pins — GPIO34, GPIO35, GPIO36, and GPIO39 — are input-only. They lack output drivers and cannot drive LEDs, relays, or other output loads. They can only read analog or digital input signals.

These pins are excellent choices for dedicated analog sensors, as there is no risk of accidentally configuring them as outputs.

Arduino Code: Reading ESP32 ADC

Here is a simple Arduino sketch to read analog values from an ADC1 pin:

// ESP32 ADC1 Example - Read analog from GPIO34
const int adcPin = 34;  // GPIO34 is ADC1_CH6

void setup() {
    Serial.begin(115200);
    // Optional: set ADC resolution (default is 12-bit)
    analogReadResolution(12);
    // Optional: set ADC attenuation for higher voltage range
    analogSetAttenuation(ADC_11db);  // 0-3.3V range
}

void loop() {
    int adcValue = analogRead(adcPin);
    float voltage = adcValue * (3.3 / 4095.0);
    
    Serial.print("ADC Value: ");
    Serial.print(adcValue);
    Serial.print("  Voltage: ");
    Serial.println(voltage);
    
    delay(500);
}

ADC Attenuation Settings

The ESP32 ADC has configurable attenuation that changes the measurable voltage range:

Attenuation	Measurable Range	Typical Use
ADC_0db	0–1.1V	Low-voltage sensors
ADC_2_5db	0–1.5V	Medium range
ADC_6db	0–2.2V	General purpose
ADC_11db	0–3.3V	Most common (default)

Reading Multiple ADC Channels

Here is an example that reads all available ADC1 pins:

// ESP32 Read All ADC1 Channels
const int adcPins[] = {32, 33, 34, 35, 36, 39};
const int numPins = 6;

void setup() {
    Serial.begin(115200);
    analogReadResolution(12);
    analogSetAttenuation(ADC_11db);
}

void loop() {
    for (int i = 0; i < numPins; i++) {
        int val = analogRead(adcPins[i]);
        Serial.print("GPIO");
        Serial.print(adcPins[i]);
        Serial.print(": ");
        Serial.print(val);
        Serial.print("  ");
    }
    Serial.println();
    delay(1000);
}

ESP32 ADC Accuracy and Calibration

The ESP32 ADC is known for non-linearities across its range. For precise measurements, Espressif provides an ADC calibration API (esp_adc_cal) that uses factory-programmed eFuse values to improve accuracy.

Key accuracy facts:

• Typical uncalibrated error: ±2% to ±6% depending on voltage range
• With eFuse calibration: ±1% to ±2%
• For precise measurements, use an external ADC (e.g., ADS1115) or the calibration API

Best ADC Pins for Common Sensor Types

Sensor Type	Recommended Pins	Notes
Potentiometer / Voltage Divider	GPIO34 GPIO35	Input-only, safe, no conflicts
Temperature Sensor (LM35, TMP36)	GPIO32 GPIO33	Also usable as GPIO
Light Sensor (LDR + resistor)	GPIO36 GPIO39	Input-only but detectable via ADC
Soil Moisture Sensor	GPIO34	Input-only, dedicated analog
Hall Effect Sensor	GPIO32	Also supports touch sensing

Frequently Asked Questions

Can I use ADC2 pins if I don't use Wi-Fi?

Yes. If your project does not enable Wi-Fi or Bluetooth, ADC2 pins work identically to ADC1. This is common in battery-powered deep-sleep sensor nodes that wake, read sensors, process data, and sleep without connecting to Wi-Fi.

What is the maximum ADC sampling rate?

The ESP32 ADC can sample at up to 6 kHz (6,000 samples/second) with 12-bit resolution in continuous mode. For most sensor applications, this is more than sufficient.

Why are my ADC readings noisy?

ESP32 ADC noise is common due to the chip's internal switching. Mitigation strategies include: using the ADC calibration API, averaging multiple readings, adding a 100nF capacitor between the analog pin and ground, and using shielded wires for sensitive measurements.

Related Guides

• ESP32 PWM Pins Guide
• ESP32 Safe GPIO Pins
• ESP32 Strapping Pins Explained
• ESP32 UART Pins Guide