ESP8266 Community Forum

Just following the conversation, if its of any interest I only have one esp8266 so not a great sample, but I found the ADC pin to be very linear in its response along with an offset voltage. My sample follows this formula: adc = 0.973 * mV - 82, ie: its 0 until 82mv and then linear until 1024 at 1136mv upwards. I use SPI or I2C ADC chips usually. A document marked 'confidential' referenced by someone earlier states that the IO pins are protected against over-voltage but i've not tried increasing the voltage on ADC above 1.2v.

the secret document marked "confidential" probably is one of the early datasheets on the esp8266 and you can trust it just as much as the later public versions ...