I want to built a 2 or 3 bit ADC.

 

I want to built a 2 or 3 bit ADC.

An LM393 dual comparator would work for two bits.
An LM339 quad comparator would work for three or four bits.

 

As Papabravo suggest, I try to search for LM393 and LM339:

http://www.national.com/pf/LM/LM393.html
http://www.national.com/pf/LM/LM339.html

 

An LM393 dual comparator would work for two bits.
An LM339 quad comparator would work for three or four bits.

A flash A/D requires (2^n - 1) comparators, where n is the number of bits. For 2 bits, 3 comparators are required, because 4 states have to be discerned. For 3 bits, you need 7 comparators, etc.
Is there a way to use fewer comparators without including a D/A?

 

A flash A/D requires (2^n - 1) comparators, where n is the number of bits. For 2 bits, 3 comparators are required, because 4 states have to be discerned. For 3 bits, you need 7 comparators, etc.
Is there a way to use fewer comparators without including a D/A?

Right. My idea was to pick thresholds in such a way that not all codes are valid. I did not elaborate very well and my math was certainly off the mark.

So 4 comparators can distinguish 5 states and log2(5) is just a bit bigger than two. With 8 comparators you get 9 states for which log2(9) is just over three.