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Design >> Mixed-Signal Design >> Between 6bit-12bit ADC:why 12 bit flash ADC is unpopular than 6 bit:?
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Message started by VINAY RAO on Oct 10^th, 2009, 4:33am

Title: Between 6bit-12bit ADC:why 12 bit flash ADC is unpopular than 6 bit:?
Post by VINAY RAO on Oct 10^th, 2009, 4:33am

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.

Title: Re: Between 6bit-12bit ADC:why 12 bit flash ADC is unpopular than 6 bit:?
Post by raja.cedt on Oct 10^th, 2009, 8:57am

hi,
i didn,t understand your question properly..but normally it's no use to do such a huge no of comparator ..where you can do 6 bit flash and use over sample technique to get 12 bit accurassy

Thanks
Rajasekhar.

Title: Re: Between 6bit-12bit ADC:why 12 bit flash ADC is unpopular than 6 bit:?
Post by sheldon on Oct 11^th, 2009, 4:22am

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

Title: Re: Between 6bit-12bit ADC:why 12 bit flash ADC is unpopular than 6 bit:?
Post by raja.cedt on Oct 11^th, 2009, 6:07am

hi,
i didn't find any 12 bit flash converter in literature. you can see all ADC state of art at a glance here
http://www.stanford.edu/~murmann/adcsurvey.html

Thanks,
rajasekhar.

Title: Re: Between 6bit-12bit ADC:why 12 bit flash ADC is unpopular than 6 bit:?
Post by VINAY RAO on Oct 12^th, 2009, 8:06pm

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..

Title: Re: Between 6bit-12bit ADC:why 12 bit flash ADC is unpopular than 6 bit:?
Post by vivkr on Oct 12^th, 2009, 11:03pm

VINAY RAO wrote on Oct 12^th, 2009, 8:06pm:

1. How would you make 12 bit flash ADCs practically? I cannot imagine any easy way that one could build these with or without an existing 6-bit flash ADC.

2. Flash ADCs are large, power hungry and hence very expensive when you start to crank up the resolution. They are hence limited to < 10 bits, mostly < 8 bits. Once you go higher, there are other better alternatives to flash.

3. Based on the above, is it a surprise that all publications list 6-8 bit flash ADCs. You could probably build a 12-bit flash if you were really desparate but that would simply be a waste of time and money and you wouldn't manage to get it published anywhere.

Regards,

Vivek

Title: Re: Between 6bit-12bit ADC:why 12 bit flash ADC is unpopular than 6 bit:?
Post by VINAY RAO on Oct 13^th, 2009, 6:23am

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

Title: Re: Between 6bit-12bit ADC:why 12 bit flash ADC is unpopular than 6 bit:?
Post by loose-electron on Oct 23^rd, 2009, 5:59pm

an 8 bit flash converter defines 255 states and requires 255 comparators. 10 bits, are 1023 states and 1023 comparators

12 bits is 4095 states and a whole heap of comparators.

Net result:
1. Big
2. Glows in the dark (lots of power)
3. Lots of capacitance on the input due to all the comparators connected.
4. Not practical - see 1,2,3