Voltage Divider for ADC Input | StrokeForge Electronics

Calculator

Platform / ADC reference:

Input voltage to measure (Vin)

V

Max voltage on the divider input

ADC reference voltage (Vref)

V

ADC full-scale input voltage

ADC resolution

R1 (top, series)

Leave blank to auto-calculate

R2 (bottom, to GND)

Leave blank to auto-calculate

Results

Vout (at Vin max)

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ADC input voltage

Calculated resistor

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R1 (series, top)

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R2 (shunt, to GND)

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ADC count (at Vin max)

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Resolution (mV/count)

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Current through divider

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ADC utilisation

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ADC range usage

ADC reference — common platforms

Platform	ADC ref	Resolution	Max counts	Notes
Arduino Uno / Nano	5.0 V	10-bit	1023	analogRead() returns 0–1023
Arduino Mega	5.0 V	10-bit	1023	Same as Uno
Arduino Due	3.3 V	12-bit	4095	analogReadResolution(12) needed
ESP32	3.3 V	12-bit	4095	Effective ~11-bit due to non-linearity. Max safe input: 3.3 V
ESP8266	1.0 V	10-bit	1023	NodeMCU has on-board 220k/100k divider → max 3.3 V
Raspberry Pi Pico	3.3 V	12-bit	4095	ADC pins: GP26, GP27, GP28
STM32 (typical)	3.3 V	12-bit	4095	Configurable 8/10/12-bit

Design guide

Formula

Vout = Vin × R2/(R1+R2)
R1/R2 = Vin/Vout − 1

ADC = Vout / Vref × (2ⁿ−1)
Vin = ADC × Vref / (2ⁿ−1)
× (R1+R2)/R2

Choosing R values

Too low (≪1 kΩ) — high current, heats up, loads source
Too high (≫100 kΩ) — ADC input impedance causes error
Sweet spot: 10–100 kΩ for most MCU ADCs
ESP32 ADC input impedance ~80 kΩ — keep R2 ≤ 10 kΩ
Add small cap (100 nF) across R2 to reduce noise

Protection

Always ensure Vout ≤ Vref under all conditions
Add a Zener diode across R2 for overvoltage protection
Add series resistor (1 kΩ) between divider and ADC pin to limit fault current
For automotive (up to 40V): use 100 kΩ / 10 kΩ divider

Reading in code

// Arduino (10-bit, 5V)
int raw = analogRead(A0);
float v = raw * (5.0/1023.0);
float vin = v * (R1+R2)/R2;

// ESP32 (12-bit, 3.3V)
int raw = analogRead(34);
float v = raw * (3.3/4095.0);
float vin = v * (R1+R2)/R2;