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vivkr

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adding redundancy to successive-approximation ADCs
Mar 28^th, 2006, 1:24am

Hi,

I am wondering if one could add some sort of redundancy to conventional hybrid successive-approximation
ADCs (using R-ladder + binary-weighted cap array) as shown in Allen & Holberg/Johns&Martin.

The offset of the comparator is very critical here. Although one may use offset cancellation by closing the
loop around the comparator, this necessarily reduces the speed of the circuit, especially if one wants to sample
a fast input signal.

Any references or suggestions would be very welcome.

Regards
Vivek

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sheldon

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Re: adding redundancy to successive-approximation
Reply #1 - Mar 28^th, 2006, 8:14am

Vivkr,

� Can't you auto-zero the comparator while sampling the input signal?
You don't need to auto-zero the entire comparator, you only need to
auto-zero the first few stages. Or you can ping-pong multiple ADCs,
this is approach is used to build high speed pipeline ADC. �BTW, here
are a few papers on sub-radix 2 successive approximation ADC design.

Design and Modeling of a 16-bit 1.5MSPS Successive Approximation
ADC with Non-binary Capacitor Array, GLSVLSI �03, April 28-29, 2003,

A 12-bit ADC Successive-Approximation-Type with Digital Error Correction,
Kanti Bacrania, JSSC, Dec. 1986

Best Regards,

Art Schaldenbrand

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vivkr

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Re: adding redundancy to successive-approximation
Reply #2 - Mar 29^th, 2006, 11:00pm

Hi Sheldon,

Thanks for the tip. I can use this. However, I considered another possibility that would involve
measuring the offset and storing it digitally on-chip. This would then allow self correction.
However, resetting the first stage as you suggest sounds simpler to me.

However, I would really be interested in knowing if one can add redundancy to the SAR ADC
in a similar fashion as one does for pipelined of cyclic ADCs where the 1.5-bit stage relaxes the
performance constraints and accuracy requirements of the comparator itself.

In the absence of any gain in conventional SAR ADCs, I don't see offhand how one could go about
adding redundancy of this kind, but perhaps someone has an idea.

Regards
Vivek

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RobG

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Bozeman, MT

Re: adding redundancy to successive-approximation
Reply #3 - Mar 30^th, 2006, 9:26am

Vivkr -
I don't know if this adds anything to what Sheldon said, but I believe redundancy is the intent of the SAR presented at ISSCC 2006 by Chen ("A 6b 600MS/S 5.3mW..."). It was paper 31.5 on pages 574-575. From the paper:
A non-binary capacitor array is used that requires 7 comparison cycles from 6b resolution, which allows extra tolerance for dynamic error through overlapped searching range.

The author credits the idea to: Kuttner, "A 1.2V 10b...," ISSCC Dig tech papers, pp 176-177, Feb 2002.

If you sub out the offset digitally, be aware that it will change over temp. A rough number would be 5-10uV/C, but you can make that better (or worse!) if you know what you are doing.

Hope this helps...

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Andrew Beckett

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me about Life...

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Bracknell, UK

Re: adding redundancy to successive-approximation
Reply #4 - Mar 31^st, 2006, 2:35am

I am joint author of a patent (US 5870052) for a "redundant" SAR method.

It's discussed in the following paper:

http://ieeexplore.ieee.org/iel5/9916/31525/01470867.pdf?isnumber=31525&prod=CNF&...

which was written by some of my former colleagues after I'd moved from Fujitsu to Cadence.

Regards,

Andrew.

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vivkr Community Fellow Offline Posts: 780	Re: adding redundancy to successive-approximation Reply #5 - Mar 31^st, 2006, 3:34am Hi RobG, Thanks for the links. I ran through Kuttner's paper and it appears to be exactly what I was looking for. Now, I should try to look at it in more detail to see if I understand the correction scheme. Regards Vivek
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vivkr

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Re: adding redundancy to successive-approximation
Reply #6 - Mar 31^st, 2006, 3:44am

Hi Andrew,

I guess you posted a few seconds before me. Thanks for the link. Indeed, this approach looks quite
interesting as well. I think with the links sent by you and Rob, I will be able to figure out a way
of getting redundancy into my scheme. However, I don't think I will be able to use either
your scheme or Kuttner's as yours is patented, and the same would probably hold for Kuttner's.

Regards
Vivek

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