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.
Measuring higher voltages requires setting ``attenuation`` to one of the following values: ``0db``, ``2.5db``, ``6db``, ``11db``.
There's more information `at the manufacturer's website <https://docs.espressif.com/projects/esp-idf/en/latest/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 innacurate/nonlinear. Users with a manually calibrated setup are encouraged to check their the 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
- multiply: 0.00095238 # 3.9/4095, for attenuation 11db
# your existing filters would go here
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.
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.
