This ADC uses the ramp counter design. It's the easier to make, but the slowest too.
An 8-bits flash ADC is impossible to make here as it needs 255 op amps.
You can get more infos here (well explained) : http://www.hardwaresecrets.com/how-analog-to-digital-converter-adc-works/
The voltage source on the left is Vref,
The right one is the input you want to compare.
The button in the middle is the counter reset.
The output is in Hexa, and has to be read from bottom to top (LSB is the top resistor, MSB is the lowest resistor)
