Thanks guys for all the good information. I understand the big job you are attempting and wish I had some funds to share, but this is just a personal project right now. I'm happy help out in other ways as time permits.
Last night I ended up playing with LTSpice (very simple install under Ubuntu 8.04- just click on the link!) and learned a lot about my circuit (different from the simple example I provided). My goal was to select, and verify, an op-amp that would work in the circuit. I found that I needed a part with a larger GB than my initial selection. There is probably a way that I could have calculated that, but my analog design skills are very rusty. The other thing I found was that I had given up a bit early on gnucap. When I didn't get anything close to the results I expected, I thought it might be a compatibility problem between the LT model and gnucap. However, it was simply a problem with the performance of the initially selected op-amp. For the few op-amps I tried, the gnucap accepted and seemed to work well with the LT models. In wrap-up, I guess Microchip has missed out on my design because of the spice models they provided. LT gets the design win! Thanks for all your help, ...jerry On Fri, May 30, 2008 at 10:06 AM, al davis <[EMAIL PROTECTED]> wrote: > On Thursday 29 May 2008, Dan McMahill wrote: > > al davis wrote: > > > Those fancy models that you get by downloading or from > > > databases are usually overkill. If you don't understand > > > them, use a simpler model. > > > > I think there can be some use for those vendor models, but > > you really have to watch out because it is not always clear > > what is modeled and what isn't. The result is you end up > > having to trace out the model or build up a model > > characterization test bench that compares some simulated > > values to the datasheet or real measured values. > > That implies you know how to do that. Most people who can > really use the detailed models also know enough to make minor > hacks to them, like changing "table" to "pwl". > > The problem is that lots of people just need a simple model, and > are misled into believing that the need something like that, > when in fact the detail could even be misleading. > > If you are designing high performance analog IC's, you probably > need the detail. If you are using stock op-amps for audio > filters, I can almost guarantee you don't. > > That is one of the principles of the "fast-spice" simulators. > They get their speed partly by using simple models, and partly > by algorithm tricks. They get away with the simple models > because they know the extra accuracy is not needed. > > That is one of the advantages of gnucap. If you use the simple > models, and set the tolerances for the accuracy you actually > get with the "fast" simulators, it should run as fast as they > do. > > On the other hand, if you use the detailed models and set the > tolerance tight, gnucap can be more accurate than Spice. > Unfortunately, with "more accurate" comes "slower". > > > And I agree with Al's comment that they're often times > > overkill. In one case I can think of, all you really needed > > was the dominant pole to be modeled and you instantly could > > predict some extremely important behaviour. I can see > > wanting to look at distortion in an audio filter or something > > but then I have doubts about the models. > > The time-step control in Spice (Berkeley Spice or NGspice) is > not accurate enough for meaningful distortion measurements > regardless of the models you use or tolerance settings. Gnucap > is if you tighten the tolerances and use good models. The > default tolerances are not tight enough for that. > > Same goes for transient noise analysis. > > > _______________________________________________ > geda-user mailing list > [email protected] > http://www.seul.org/cgi-bin/mailman/listinfo/geda-user >
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