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Message started by langvelt on Dec 1st, 2006, 3:31am

Title: montecarlo analysis of an opamp
Post by langvelt on Dec 1st, 2006, 3:31am

Hello all!
I am doing montecarlo analysis of an opamp and finding some strange results.
In normal condtions (without any mismatch in the circuit elements) gain of this opamp
is 60db, but with montecarlo( with mismatch in the circuit elements), total range of  
gain of this opamp is coming 50db to 120db. Is it possible to get such results (120db) with mismatch in the
circuit components.
If this is possible can't we bias this opamp in such condition(putting mismatch into the circuit elements) so that it gives 120db in normal condition.

thanks in advance for answers......

Title: Re: montecarlo analysis of an opamp
Post by Croaker on Dec 1st, 2006, 8:08am

Well, Monte Carlo simulation gives you a statistical result, and in some cases you are going to get better performing, more ideal circuits and the opposite is true as well.

Title: Re: montecarlo analysis of an opamp
Post by sheldon on Dec 3rd, 2006, 6:14am

Langvelt,

  Do your models only contain mismatch information? Or do the models
also include process variation information? If you run corner analysis,
what is the range in open-loop gain? In the ADE environment, what is the
open-loop gain with just process variations? What is the open-loop gain
variation when just mismatch is considered?

How reliable are

                                                              Best Regards,

                                                                 Sheldon

Title: Re: montecarlo analysis of an opamp
Post by sheldon on Dec 3rd, 2006, 6:19am

Langvelt,

  Other questions to consider:
1) How reliable are the Monte Carlo models you are using?
2) What are the bias current variations?
3) Are the bias current variations strongly correlated with open-loop gain?
4) Is 60dB consistent with hand calculated values of open-loop gain?
   Or, is it possible that there is a saturated gain stage in the current
   design and under process conditions the stage leaves saturation so there
   is a big increase in gain?

Title: Re: montecarlo analysis of an opamp
Post by bharat on Dec 3rd, 2006, 8:22am

I think, its better to know your Monte Carlo model takes care of.
It should be Variation in Vth, Channel length, Width (W). these parameters can be tamed to an extent by choosing larger device dimension.
For other variations i.e. overlap cap, doping concentration, die to die variation, one has to live with it.


Title: Re: montecarlo analysis of an opamp
Post by langvelt on Dec 4th, 2006, 10:26pm

Hi sheldon, croaker, bharat
I'm using models which have the mismatch information at particular
process corner. These results i am getting at typical conditions.
Range of the open loop gain with all process variation, +/-20% bias current
variation, +/-20% supply variation and -40 to 125 temp variation(without montecarlo), is 60-80db.
for ur another question about saturated gain stage, every transistor has a good
satuartion margin.
This 60db is appx. similar with the hand calculated gain.

What i am worrying is that wherever i bias this opamp i cannot get 150db kind of gain with this structure. it's
a simple single stage gain boosted folded cascode amplifier, means every transistor has a good saturation margin.

What the results generally you people get, when you do montecarlo analysis in terms of gain of the amp.



Title: Re: montecarlo analysis of an opamp
Post by loose-electron on Dec 5th, 2006, 2:32pm

suggest -

Find the specific net and conditions that give the issue, run some transient and DC op point sims on that one and see what they look like.

Also, what foundry and model set are these from???
There are a lot of trash models out there. GIGO

Jerry

Title: Re: montecarlo analysis of an opamp
Post by bharat on Dec 6th, 2006, 6:44am

I am throwing the question:
Whatever the spread I am seeing in MC simulation, the designer owe a part of it and major part goes to process inaccuracy. If all the transistors are in saturation, what else a designer can do. I am also seeing similar results in my op-amp while calculating voltage offset. The ratio of mean value to 3 sigma value is close to 150. Designer's task is to meet the specs for mean value and some percantage in the neighborhood of it.
If the model is in a way that variation in L, W, Vth, overlapping cap, doping concentration is so much, how can really design can contain these variations and go for 'robust design'. If the expectation is that all those variations should be taken as design challenge, I am afraid if I am agree.
Also, for example, if the model considers that there may be mismatch in the device dimension, shouldn't they account for that all the devices are laid out in common centroid fashion for perfect matching with dummy devices to avoid any distortion in the devices while polishing in the process. (because I ran pre-layout MC sims)
In addition, is it worth to burn the night oil for a designer to design for such a device which has worst L, W, Vth, overlap cap, doping conc mismtach in one device which may be one in millions. Isn't it overly pessimistic and far from real life scenario.
Also, what do you say for die to die variation. How can one contain it?
Please comment if I shouldn't think this way as a Designer.

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