Reading analog sensors on the ESP32 without fooling yourself
The ESP32's ADC is good enough for most sensors, if you respect which pins, which range, calibration and impedance.
Firmware, ESP32, Sensors
The ESP32's ADC is good enough for most sensor work. Most bad readings we see come from four mistakes, and all four are cheap to avoid.
1. Use ADC1 when Wi-Fi is on
The ESP32 has two ADCs, and ADC2 is shared with the Wi-Fi driver. On the original ESP32, ADC2 reads fail while Wi-Fi is running. On the ESP32-C3, one-shot reads on ADC2 are not supported at all.
Put analog sensors on ADC1 pins: GPIO32 to GPIO39 on the ESP32, GPIO0 to GPIO4 on the ESP32-C3.
2. Know the range of each attenuation setting
Attenuation trades range for resolution. The ESP-IDF documentation gives the range in which readings are accurate:
| Attenuation | ESP32 | ESP32-S3 | ESP32-C3 |
|---|---|---|---|
| 0 dB | 100 to 950 mV | 0 to 950 mV | 0 to 750 mV |
| 2.5 dB | 100 to 1250 mV | 0 to 1250 mV | 0 to 1050 mV |
| 6 dB | 150 to 1750 mV | 0 to 1750 mV | 0 to 1300 mV |
| 12 dB | 150 to 2450 mV | 0 to 3100 mV | 0 to 2500 mV |
Two traps sit in this table. On the original ESP32, readings below 100 to 150 mV are not reliable, so a sensor that rests near 0 V reads worst exactly where you care most. And a 5 V sensor output needs a divider to fit; the divider tool does the arithmetic and checks the range for you.
3. Calibrate in software
Raw counts are not millivolts. Every chip's ADC has its own offset and gain, and ESP-IDF's calibration schemes use values burned in at the factory to convert one to the other. Create a calibration handle (line fitting or curve fitting, depending on the chip) and convert with adc_cali_raw_to_voltage() instead of scaling raw counts yourself.
4. Give the ADC a quiet, low-impedance signal
The ADC samples by charging a small internal capacitor. A divider made of hundreds of kilohms can't charge it fully between samples, and readings sag.
- Put a 100 nF capacitor from the ADC pin to ground, close to the pin.
- Average 8 to 16 samples per reading. It costs microseconds and removes most of the noise.
- If readings jump while Wi-Fi transmits, take them before the radio starts.
What we do now
ADC1 pins only. A divider sized to the recommended range. A capacitor at the pin. The calibration API, an averaged reading, and on battery nodes, a reading taken before the radio wakes.
References
- ESP-IDF Programming Guide, Analog to Digital Converter: attenuation ranges, ADC2 limitations and the calibration API, for the ESP32, ESP32-S3 and ESP32-C3.