Hi,
This is my approach to making an really simple ADC.
It uses the Capacitor discharge formulae. Since we know the RC time constant is 1(by multiplying R and C). We can measure the time it need for the voltage between the cap's terminal to reach 0.001v by using a OpAmp as a comparator.
Those mosfets(yes i know they are overkill) were used as a switch to discharge the capacitor through the resistor.
That's basically how the circuit works. Let me know if you want the code of the arduino.