Why evryone prefer to design 6-bit flash ADC than 12 bit..Many applications requires 6 bit??..or we can design 12 bit using 6 bit flash ADC's.

Vinay,

  I agree with Raj, your question is hard to understand. Some things
to consider with flash ADCs
1) Adding a bit doubles complexity
2) Adding a bit doubles power
3) Adding a bit doubles the load the input buffer has to drive
   --> In addition, to flash ADC having high non-linear input cap
4) Adding a bit requires improving the matching by a factor of two.
   Practically, the yield for a 12 bit ADC would be very low.    

  --> Most literature puts the limit of matching at 10 bits, 1 part
        in 1000 for device characteristics that match well, for
        example, resistance

5) I have not worked through the numbers, but settling of the
   reference voltage would also be a big challenge.

   --> The reference resistor/input comparator is a highly non-linear
         transmission line. As the number of comparators increases
         settling time increases. The reference resistor can be tapped
         but then accurate tap voltages need to be generated.

6) The clock generator needs to trigger all the comparators at the
   same time or accuracy is degraded. The physically larger the
   ADC the more difficult this is to do.

   There techniques to reduce the impact of these effects. However,
the better solution is to adopt an  architecture more suited for the
application. Building a 12 bit flash ADC really is not even an
interesting science experiment. It would be slow, hard to use,
consume large amounts of power, ...    

                                                             Best Regards,

                                                                Sheldon

Thank you..
I mean to say in literature everyone preferred to design 6 bit ADC rather than 12 bit .So i thought of 2 reason for this..
Either 12 bit can be made rfom 6 bit ADC or practically 12 bit ADC's are not required..

ya that is true,i didnt think in that way..