Ok. Suppose u want a 4 bit ADC o/p(binary), make a 2 bit up/down counter. (This method I believe is the simplest)
Suppose your signal is in the 50% range, so you get 0101010...
start with acc=counter=0
clock 1 : count=0 acc=0(here SD o/p=0)
clock 2 : count=1 acc=1(because SD o/p=1)
clock 3: count=2 acc=1(sd=0 here)
clock 4: count=3 acc=4(sd=1 here)
clock 5: count=2 acc=4
clock 6: count=1 acc=5
clock 7:count=0 acc=5 : 0101
If your SD mod gives all ones your acc o/p would be 9
And all zeros would give zero
So your full scale value is 9 for a 2-bit counter
Your acc value will be relative to this full scale value of 9.
If you have a large counter the 1 lsb error we get here will vanish.
I hope it is much clearer now
Giri