which ESP32 microcontroller board are you using?
remember the maximum input to a ESP32 ADC is 0 to +3.3Volts
what is the voltage you are attempting to read?
is it AC ? if so you have to offset it to the 0 to 3.3volt range
upload a schematic showing your wiring?
also have a look at fixing-the-non-linear-adc-of-an-esp32