"Unfortunately, it is the most component-intensive for any given number of output bits. This three-bit flash ADC requires eight comparators. A four-bit version would require 16 comparators. With each additional output bit, the number of required comparators doubles. Considering that eight bits is generally considered the minimum necessary for any practical ADC (256 comparators needed!), the flash methodology quickly shows its weakness. "

http://www.allaboutcircuits.com/vol_4/chpt_13/4.html

I thought that 2n-1 comparators were needed, so shouldnt a 4 bit require 15 comparators (not 16) and a 8 bit - 255 comparators? - and a 3 bit should require 7?

Thanks for any help.