analogRead Values Fluctuating? Fix ADC Noise on Arduino & ESP32

Is this your problem?

If two or more of these sound familiar, the reference or supply is your prime suspect:

• Readings jump by several counts with nothing changing
• It’s worse on a phone charger, or when the laptop is charging
• It gets worse when Wi-Fi, a servo or an LED strip turns on
• Temperature from a TMP36 / LM35 wobbles by a degree or more
• pH, TDS or soil readings drift and never settle
• ESP32 raw values scatter by tens of counts

Why it happens, in one picture

An ADC doesn’t measure volts directly. It measures your input as a fraction of a reference voltage: reading = Vin ÷ Vref × 1023. On an Uno or Nano the default reference is the board’s own 5 V. If that 5 V wobbles by 1%, a mid-scale reading wobbles by about 5 counts, even though your sensor didn’t change at all.

Does it affect your sensor? It depends on the type

Some sensors are dividers: their output is a fraction of their own supply (pots, LDRs, thermistors). If the sensor and the ADC reference share the same rail, supply noise mostly cancels. Others output a fixed voltage (TMP36, pH modules, a battery divider). For those, every bit of reference noise lands in the reading.

The 2-minute test: is it the ADC or the sensor?

Take the sensor out of the picture and give the ADC something perfectly steady to measure: a single AA cell. At about 1.5 V it’s inside the range of both Arduino and ESP32 inputs, and it’s electrically quiet.

// Print the spread of 100 readings
void loop() {
  int lo = 1023, hi = 0;
  for (int i = 0; i < 100; i++) {
    int v = analogRead(A0);
    lo = min(lo, v); hi = max(hi, v);
    delay(5);
  }
  Serial.print("min "); Serial.print(lo);
  Serial.print("  max "); Serial.println(hi);
}

The fix for Arduino Uno / Nano, in 3 steps

1. Make a clean 3.3 VFeed a dedicated low-noise 3.3 V regulator from the Arduino’s 5 V pin. Its output becomes your quiet analog rail.
2. Use it for AREF and the sensorConnect the clean 3.3 V to the AREF pin and to the sensor’s VCC. In setup(), call analogReference(EXTERNAL) before any analogRead(). Readings now run from 0 to 3.3 V, so keep the sensor output below 3.3 V.
3. Add 100 nF and averagePut a 100 nF ceramic capacitor from the analog pin to GND, close to the board. Then average 8 to 16 readings in code. Together these remove what little noise is left.

const float VREF = 3.300;   // measure VOUT with a multimeter and put the real value here

void setup() {
  analogReference(EXTERNAL); // FIRST line: before any analogRead()
  Serial.begin(115200);
}

float readVolts(int pin) {
  analogRead(pin);                       // discard first reading after switching pins
  long sum = 0;
  for (int i = 0; i < 16; i++) sum += analogRead(pin);
  return (sum / 16.0) * VREF / 1023.0;
}

void loop() {
  Serial.println(readVolts(A0), 4);
  delay(200);
}

Which board are you using?

Using an ESP32? Read this first

The ESP32’s built-in ADC has no external reference pin, so the AREF trick doesn’t apply. Its readings are also noisy and slightly non-linear even on perfect power. A clean supply still helps: it removes Wi-Fi burst noise from the sensor and the board’s ground. But it works best together with these settings:

Still jumping? The other common causes

If the AA-cell test was steady but your real sensor still jumps, the problem is between the sensor and the pin:

Frequently asked questions

© Ramanu Systems · Precision electronics modules, Made in India · ESP32 ADC guidance from the Espressif ESP-IDF ADC documentation. Arduino AREF guidance from the Arduino analogReference() reference. Regulator figures from the Texas Instruments TPS7A20 datasheet. Found an error? Tell us and we will fix it and credit you.