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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, 5:33am

Title: Between 6bit-12bit ADC:why 12 bit flash ADC is unpopular than 6 bit:?
Post by VINAY RAO on Oct 10^th, 2009, 5:33am

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, 9:57am

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, 5:22am

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, 7:07am

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, 9:06pm

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 13^th, 2009, 12:03am

VINAY RAO wrote on Oct 12^th, 2009, 9:06pm:

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, 7:23am

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, 6:59pm

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