The ADC0804 is a commonly used ADC module, for
projects were an external ADC is required. It is a 20-pin Single channel
8-bit ADC module. Meaning it can measure one ADC value from 0V to 5V
and the precision when voltage reference (Vref –pin 9) is +5V is 19.53mV
(Step size). That is for every increase of 19.53mV on input side there will be an increase of 1 bit at the output side.