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Design >> Analog Design >> A question about delta sigma modulators stability
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Message started by aidnaal on Jan 3^rd, 2008, 3:17am

Title: A question about delta sigma modulators stability
Post by aidnaal on Jan 3^rd, 2008, 3:17am

Hi,
In delta sigma modulators the maximum SNDR is given by the famous equation of Shreier book;

But in most cases this is not valid and the measured or simulated SNDR is below this value. In Shreier book it's said that in an unstable DS modulator the SNDR is below this value, and the unstability causes the degradation in SNDR. Could you please explain that what is the criteria for stability and how we can recognize if a modulator is stable or unstable?
Thanks a lot.

Title: Re: A question about delta sigma modulators stabil
Post by Berti on Jan 6^th, 2008, 10:16pm

It is not possible to give an accurate analytical expression for the overload level (stability) of a DS modulator.
However, check CLANS (NTF optimization tool), which is part of R. Schreiers DS-toolbox for matlab.
There it is described how stability is determined without simulation.

Regards

Title: Re: A question about delta sigma modulators stabil
Post by thechopper on Jan 20^th, 2008, 5:05pm

Another extra piece of info.
In that book you´ll find that stability can be predicted based on the value of the NTF as frequency approaches infinite. The criteria change depending on the modulator being single or multi bit.

Regards
Tosei