On the ESP32 the voltage measured with the ADC caps out at ~1.1V by default as the sensing range (attenuation of the ADC) is set to ``0db`` by default.
There's more information `at the manufacturer's website <https://docs.espressif.com/projects/esp-idf/en/v4.4.7/esp32/api-reference/peripherals/adc.html#_CPPv425adc1_config_channel_atten14adc1_channel_t11adc_atten_t>`__.
To simplify this, we provide the setting ``attenuation: auto`` for an automatic/seamless transition among scales. `Our implementation
<https://github.com/esphome/esphome/blob/dev/esphome/components/adc/adc_sensor.cpp>`__ combines all available ranges to allow the best resolution without having to compromise on a specific attenuation.
In our tests, the usable ADC range was from ~0.075V to ~3.12V (with the ``attenuation: auto`` setting), and anything outside that range capped out at either end.
Even though the measurements are calibrated, the range *limits* are variable among chips due to differences in the internal voltage reference.
The ADC output reads voltage very accurately since 2021.11 where manufacturer calibration was incorporated. Before this every ESP32 would read different voltages and be largely inaccurate/nonlinear. Users with a manually calibrated setup are encouraged to check their installations to ensure proper output.
For users that don't need a precise voltage reading, the "raw" output option allows to have the raw ADC values (0-4095 for ESP32) prior to manufacturer calibration. It is possible to get the old uncalibrated measurements with a filter multiplier:
..code-block:: yaml
# To replicate old uncalibrated output, set raw:true and keep only one of the multiplier lines.
raw: true
filters:
- multiply: 0.00026862 # 1.1/4095, for attenuation 0db
- multiply: 0.00036630 # 1.5/4095, for attenuation 2.5db
- multiply: 0.00053724 # 2.2/4095, for attenuation 6db
Note we don't recommend this method as it will change between chips, and newer ESP32 modules have different ranges (i.e. 0-8191); it is better to use the new calibrated voltages and update any existing filters accordingly.
On the Raspberry Pi Pico and Pico W the ADC can measure VSYS voltage.
Depending on how VSYS is powered the readings will have different meanings - either power supply voltage when it is connected to VSYS pin directly, or USB voltage (VBUS) minus some drop on the Schottky diode the Raspberry Pi Pico has between those pins.
Our experiments indicate the drop being ~0.1V for Pico and ~0.25V for Pico W; you can use sensor filters to adjust the final value.
..note::
On Raspberry Pi Pico W the ADC GPIO29 pin for VSYS is shared with WiFi chip, so attempting to use it explicitly will likely hang the WiFi connection.
It is recommended to use ``VCC`` as ADC pin in that case.
The RP2040 has an internal temperature sensor that can be used to measure the core temperature. This sensor is not available on the GPIO pins, but is available on the internal ADC.
The below code is how you can access the temperature and expose as a sensor. The filter values are taken from the RP2040 datasheet to calculate Voltage to Celcius.
You can only use as many ADC sensors as your device can support. The ESP8266 only has one ADC and can only handle one sensor at a time. For example, on the ESP8266, you can measure the value of an analog pin (A0 on ESP8266) or VCC (see above) but NOT both simultaneously. Using both at the same time will result in incorrect sensor values.
