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https://designers-guide.org/forum/YaBB.pl Design >> Mixed-Signal Design >> Basic ADC query https://designers-guide.org/forum/YaBB.pl?num=1177655417 Message started by adesign on Apr 26th, 2007, 11:30pm |
Title: Basic ADC query Post by adesign on Apr 26th, 2007, 11:30pm Dear all, Why can't we get 20-bit resolution with pipeline or SAR ADCs. What would be the issues if we increase the bumber of bits in Nyquist ADCs? Regards, |
Title: Re: Basic ADC query Post by ywguo on Apr 28th, 2007, 3:38am 1. Device mismatch. 2. thermal noise and flicker knoise. I think those are two basic limits. |
Title: Re: Basic ADC query Post by sheldon on Apr 28th, 2007, 6:20am Adesign, Just to expand on Yawei's comments about device matching, typically mismatch is on the order of 1:1000. So to achieve 20 bits of accuracy would require 10 bits of segmentation in the first stage. This level of segmentation limits performance and yield. In addition at this level of resolution there are capacitor effects that need to be accounted for including, capacitor non-linearity and memory effects. The effect of capacitor non-linearity is discussed in the following paper, "Fully Differential ADC with Common-Mode Range and Rail-to-Rail Nonlinear Capacitor Compensation", Hester, et al., JSSC, Feb. 1990 Designing for 20 bits of resolution is difficult because accuracy is sensitive to higher order effects including material properties. Best Regards, Sheldon |
Title: Re: Basic ADC query Post by adesign on Apr 29th, 2007, 8:35pm Thanks all for your valuable comments.... |
Title: Re: Basic ADC query Post by krishnap on Apr 30th, 2007, 11:40pm Speed of operation will also be limited, with the High resolution. |
Title: Re: Basic ADC query Post by loose-electron on May 23rd, 2007, 6:15pm Noise will become the one that will bite you. I have done 14/16 bit conversion, but you need to get very fussy about thermal noise, linearity and offsets, and due to sampling accuracy your sampling rate has to go down. There are some low convert rate devices out there at 20 bits I believe, but they are in KHz convert rates. After you get around the problems with thermal/flicker noise, you can do some digital averaging to reduce the noise some more, but again, your conversion rates have gone down some more. The prior comments by others are very valid also - some can be solved (the non-linear capacitor can be corrected for with digital curve compensation for example) but there are a large number of subtle things that become an issue. An antenna, with a 28 bit converter connected to it, converting at 6GHz is not going to happen in the forseeable future. Never say never! ;) |
Title: Re: Basic ADC query Post by RobG on May 25th, 2007, 9:44am adesign wrote on Apr 26th, 2007, 11:30pm:
Dr. Temes asked me this the other day. Not because I knew, but because he was grilling me for an exam :-). Like LE alluded to, mismatch and other issues (finite opamp gain, offset, etc) can be calibrated out, but thermal noise cannot. It is for this reason that oversampling (noise averaging) is more practical for higher resolution. rg |
Title: Re: Basic ADC query Post by mg777 on May 25th, 2007, 10:26am I was intrigued enough by this question to go and re-read (perhaps for the twentieth time) Feynman's timeless chapter on the Brownian ratchet. Afterwards I was doodling and came up with an interesting observation: kT/q is a voltage & so is sqrt(kT/C). So I idly equate the two at room temp and get C = 6.4 aF (a=atto) which is interestingly close to the parallel plate Cox of a W=1u MOS transistor. I wonder what that means (there's a resistor in there somewhere). On a more airy note: there's much physical richness in the behavior of atoms, and oxides are already a few atoms thick - so I'd say that the analog designer of 10-15 years from now (I hope I'm not completely senile by then) will be working with completely different paradigms. A celebrated analog design text would have expanded to 16 authors (8 of whom are from the physics department) and an SC filter would refer to Schrodinger's Cat. M.G.Rajan www.eecalc.com |
Title: Re: Basic ADC query Post by loose-electron on May 25th, 2007, 6:46pm Quote:
Hm... Well, the methods used have not really changed in the last 27 years that I have been involved. Smaller sizes, smaller voltages, higher frequencies, lower currents, more transistors, but the methods are still pretty close to being the same. More dependency on EDA is the big one I have noticed. Jerry |
Title: Re: Basic ADC query Post by mg777 on May 26th, 2007, 8:14am Here are some interesting references - I realize I'm getting off-topic, so I'll leave it here. 1. An Ising model description of sigma-delta data converters http://adsabs.harvard.edu/abs/2000APS..MARY25008M 2. Novel Analog-to-Digital Conversion Architecture Using Electron Waveguides http://www-mtl.mit.edu/~alamo/pdf/1993/RJ-59.pdf 3. From analog to digital: exploring cell dynamics with single quantum dots. http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=16284775&dopt=Abstract |
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