Comparator/NAND/NOR ADC not working properly
You cannot make an ADC with logic gates. Your approach to use op-amps was the correct one. If it didn't work surely there must be some mistake there. Try to test your comparators one by one, to see that they work OK, each one for its intended range.

You could even start with a '1 bit converter' where a comparator divides the analog input range to > Vcc/2 and &lt;=Vcc/2. Then move on to the 2 bit ADC.