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Design >> Mixed-Signal Design >> optimum gray coding for pipelined ADCs
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Message started by vivkr on Jul 19^th, 2006, 6:54am

Title: optimum gray coding for pipelined ADCs
Post by vivkr on Jul 19^th, 2006, 6:54am

Hi,

I would like to use some gray coding in each of my pipeline stages in the event of sparkles in the
thermometer code although these are not very likely owing to a reduced stage resolution.

I designed a simple gray code keeping in mind not to map any thermometer line to more than one
output gray code. However, testing it with a single sparkle gives me a code which is away from the
corresponding error-free code by 2 steps instead of just 1. I would like to get an opinion on the optimal code for such a scenario.

I have 4 thermometer-coded lines T4-T1, and I am mapping these to 3 gray coded lines G3-G1,
where the respective MSBs are T4 and G3.

I use the following logic:

G3 = T4;
G2 = T2;
G1 = T1 (and) (not T3);

Thanks
Vivek

Title: Re: optimum gray coding for pipelined ADCs
Post by ywguo on Jul 20^th, 2006, 2:34am

Vivek,

I don't understand your method clearly. And I think a pipelined ADC doesn't need such a conversion.

Usually the sub-ADC in a pipelined ADC doesn't have sparkles because there is a front-end SHA. Even for a pipelined ADC which hasn't a front-end SHA, the sparkle is not a concern because of the reduced stage resolution.

Best regards,
Yawei

Title: Re: optimum gray coding for pipelined ADCs
Post by vivkr on Jul 20^th, 2006, 8:35am

Hi Yawei,

I agree that normally it is not needed to perform such a conversion. However, if you are clocking at a very high rate, and latching the comparators well before the MDAC output is settled, then there is some scope for such errors, when taking the comparator offsets into account, at low supply levels.

Nonetheless, how would you select an appropriate Gray code assignment out of all the possible ones for say, a flash ADC.

Regards
Vivek