ESP8266 Voltage Divider for ADC

Introduction

When a voltage is input to the Analog-to-Digital Converter of the ESP8266 or ESP32, care must be taken that the max voltage spec of the ADC is not exceeded.

This page provides a voltage divider calculator for the ADC. This configuration of resistors reduces the input voltage to a range that doesn’t exceed the ADC spec.

Calculator

This tool provides resistor values that reduce the max input voltage to the input voltage range of the ADC.

Enter:

  • Input voltage (Vin)
  • Desired output voltage Vout (this is the maximum allowed voltage into the ADC)
  • Either R1 or R2. Use the drop down menu to select the units

Example Calculation

The ESP8266 has a 12-bit SAR ADC with an effective range of 0 Volt to 3100 mV. This means that the max input voltage is 3.1 Volt (although it can be extended to a full scale value of 3.3 V)

If the input analog signal varies between 0V and 5V, using the calculator above, Vin is 5V, Vout is 3.1V. Choose R1 = 10 kohm. R2 is calculated to be 16.3 kohm.

In the case of the ESP chip, any input voltage between 0 V and 3.1 V can be accurately digitized. If a voltage of 5V is applied, the ADC’s output will be clipped at its maximum value (e.g., the output will be all ones for a binary ADC), and information about the actual input voltage will be lost.

Convert the analog input to digital output when it’s within the operating range of the ADC.

Max Source Impedance

Referring to the documentation for the ESP32 it states the following:

ADC is capacitive and can be considered as a large resistance.

Normally ADC datasheets will usually specify a maximum source impedance. However the Espressif team has not provided a number. We can safely assume that it is 10 kΩ.

Higher values of input impedance will increase the sampling time and result in errors as the input signal won’t settle to a stable value fast enough. See this post to understand the impact of impedance on settling time.

In this case with R1 = 10 kΩ and R2 = 16.3 kΩ. The effective impedance is 6.2 kΩ (using this calculator) which is less than 10 kΩ so it should be fine.

Related Posts and Calculators

References

[1] ESP32 ADC Range