📊 ESP32 ADC Inaccurate Reading - Complete Fix Guide

• 📌 Using ADC2 with WiFi – ADC2 pins are unreliable when WiFi is active
• 📌 No Attenuation Configuration – Default range is only 0-1.1V, not 0-3.3V
• 📌 Electrical Noise – No filtering capacitor on analog input
• 📌 Missing Calibration – ESP32 ADC is non-linear and needs calibration
• 📌 High Source Impedance – Sensor output impedance is too high
• 📌 Voltage Divider Issues – Incorrect resistor values for measuring higher voltages

❌ WRONG - Using ADC2 with WiFi:

// GPIO 4 is ADC2 - WILL FAIL when WiFi is on!
pinMode(4, INPUT);
int value = analogRead(4);  // Random values!

✅ CORRECT - Use ADC1 pins:

// GPIO 34 is ADC1 - Works perfectly with WiFi
pinMode(34, INPUT);
int value = analogRead(34);  // Stable readings!

By default, ESP32 ADC measures only 0-1.1V. You need attenuation to measure up to 3.3V:

#include <esp_adc_cal.h>

void setup() {
    Serial.begin(115200);
    
    // Configure ADC attenuation for 0-3.3V range
    analogReadResolution(12);  // 12-bit (0-4095)
    analogSetAttenuation(ADC_11db);  // 0-3.6V range
    
    // For specific pins:
    analogSetPinAttenuation(34, ADC_11db);
}

void loop() {
    int raw = analogRead(34);
    float voltage = raw * (3.3 / 4095.0);
    Serial.print("Raw: ");
    Serial.print(raw);
    Serial.print(" | Voltage: ");
    Serial.println(voltage);
    delay(100);
}

Electrical noise causes readings to jump. Add a capacitor to smooth the signal:

// Connect a 0.1µF to 1µF capacitor between:
// - ADC pin and GND (as close to the pin as possible)
// This filters out high-frequency noise

Average multiple readings to reduce noise:

// Method 1: Simple averaging
int readADC(int pin, int samples = 10) {
    long total = 0;
    for (int i = 0; i < samples; i++) {
        total += analogRead(pin);
        delay(1);  // Small delay between samples
    }
    return total / samples;
}

// Method 2: Exponential moving average (lighter on memory)
float filteredValue = 0;
float alpha = 0.1;  // Smoothing factor (0-1)

float readADC_Filtered(int pin) {
    float newValue = analogRead(pin);
    filteredValue = (alpha * newValue) + ((1 - alpha) * filteredValue);
    return filteredValue;
}

// Method 3: Remove outliers (median filter)
int readADC_Median(int pin, int samples = 10) {
    int values[samples];
    for (int i = 0; i < samples; i++) {
        values[i] = analogRead(pin);
        delay(1);
    }
    // Sort array
    for (int i = 0; i < samples - 1; i++) {
        for (int j = i + 1; j < samples; j++) {
            if (values[i] > values[j]) {
                int temp = values[i];
                values[i] = values[j];
                values[j] = temp;
            }
        }
    }
    // Return median (middle value)
    return values[samples / 2];
}

ESP32 ADC is non-linear. Use esp_adc_cal library for better accuracy:

#include <esp_adc_cal.h>

esp_adc_cal_characteristics_t adc_chars;

void setup() {
    Serial.begin(115200);
    
    // Configure ADC
    analogReadResolution(12);
    analogSetAttenuation(ADC_11db);
    
    // Calibrate ADC
    esp_adc_cal_characterize(
        ADC_UNIT_1,      // ADC unit
        ADC_ATTEN_DB_11, // Attenuation
        ADC_WIDTH_BIT_12,// Bit width
        1100,            // Default Vref (can measure actual)
        &adc_chars
    );
}

void loop() {
    int raw = analogRead(34);
    uint32_t voltage_mv = esp_adc_cal_raw_to_voltage(raw, &adc_chars);
    float voltage = voltage_mv / 1000.0;
    
    Serial.print("Raw: ");
    Serial.print(raw);
    Serial.print(" | Calibrated Voltage: ");
    Serial.print(voltage);
    Serial.println("V");
    delay(100);
}

To measure voltages above 3.3V, use a voltage divider:

// Measure up to 10V with 10kΩ and 3.3kΩ resistors
// 
// Vin ----/\/\/\----+----/\/\/\---- GND
//           R1       |       R2
//                    |
//                 ADC Pin
//
// Vout = Vin * (R2 / (R1 + R2))
// For Vin max 10V: 10 * (3.3 / (10 + 3.3)) = 2.48V (safe for ESP32)

float measureVoltage(int pin, float r1, float r2) {
    int raw = analogRead(pin);
    float vout = raw * (3.3 / 4095.0);
    float vin = vout * (r1 + r2) / r2;
    return vin;
}

// Example: Measure 12V battery
float batteryVoltage = measureVoltage(34, 10000, 3300);  // R1=10k, R2=3.3k

#include <esp_adc_cal.h>

#define ADC_PIN 34  // Use ADC1 pin!

esp_adc_cal_characteristics_t adc_chars;

void setup() {
    Serial.begin(115200);
    
    // Configure ADC
    analogReadResolution(12);
    analogSetAttenuation(ADC_11db);
    
    // Calibrate
    esp_adc_cal_characterize(ADC_UNIT_1, ADC_ATTEN_DB_11, 
                             ADC_WIDTH_BIT_12, 1100, &adc_chars);
    
    Serial.println("ADC Ready!");
}

int readADC_Filtered(int pin, int samples = 10) {
    long total = 0;
    for (int i = 0; i < samples; i++) {
        total += analogRead(pin);
        delay(5);
    }
    return total / samples;
}

void loop() {
    int raw = readADC_Filtered(ADC_PIN);
    uint32_t voltage_mv = esp_adc_cal_raw_to_voltage(raw, &adc_chars);
    float voltage = voltage_mv / 1000.0;
    
    Serial.print("Raw: ");
    Serial.print(raw);
    Serial.print(" (0-4095) | Voltage: ");
    Serial.print(voltage, 3);
    Serial.println("V");
    
    delay(500);
}

• Always use ADC1 pins (32-39) when WiFi is active
• Set attenuation to ADC_11db for 0-3.3V range
• Add a 0.1µF capacitor between ADC pin and GND
• Average 10-50 samples to reduce noise
• Use esp_adc_cal library for calibration
• Keep wires short to minimize noise pickup
• Avoid running analog wires near high-current or high-frequency signals

• ESP32 GPIO Not Working
• ESP32 Low Memory Crash
• ESP32 Not Connecting to WiFi
• NTC Thermistor Not Working

OceanRemote supports analog sensors with:

• ✓ Built-in filtering and averaging
• ✓ NTC thermistor support with Steinhart-Hart
• ✓ Automatic calibration
• ✓ No coding required