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Old 26th March 2005, 07:44 AM   #11
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Adding networks for CMRR and PSRR is a very simple method and doesn't alter anything as far as the model. Stop whining
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Old 26th March 2005, 01:35 PM   #12
ilimzn is offline ilimzn  Croatia
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To be honest such a dismissive post tends to undermine your credibility. Having re-read the thread, I believe the original poster was complaining about CMRR and PSRR modeling networks assuming ground referenced power supplies, which will not be accurate if the power supplies are not ground referenced (i.e. are floating). I think this would obviously influence the model accuracy...
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Old 28th March 2005, 03:55 PM   #13
jcx is online now jcx  United States
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I presume this 2 yr old thread has been bumped by Soufiane in response to a recent post of mine on Yahoo’s LtSpice usergroup:


an-138 shows a really good high frequency fit, including a peak
beyond the unity gain frequency

unfortunately even Analog Devices can't seem to manage to
institutionalize this level of excellence - frankly the spice macro
models coming out of the California group suck by any standard of
accuracy and the authors seem to be clueless about the issues
discussed in an-138

the ad8610 spice model fails to show ~20 degrees phase shift @~ 1MHz
from internal pole-zero compensation that is visible on the data
sheet gain curve – when questioned, the author added 3 poles @~ 2.5
GHz to trim the unity gain phase "accuracy" to < 1 degree - you can
see this pointless trick in the op177 [edit: op1177] listing in
http://www.edn.com/contents/images/260060.pdf ( the whole premise of
having to add psrr and cmrr "patches" comes from their ignorance of
how to properly use the an-138 macro modeling technique)


my motivation for exploring these modeling issues was an attempt to use the AD8610 spice model in a floating power supply circuit where somewhat realistic psrr/cmrr modeling is critical to stability – knowing that Boyle models totally fail in this application (and in the worst possible way – they show no sign of instability in sim when the physical circuit is hopelessly unstable) I now look inside at the spice model netlist to get an idea of the type of model I’m dealing with

http://www.analog.com/UploadedFiles/...01016AN138.pdf

"SPICE-Compatible Op Amp Macro-Model Application Note (AN-138)

having read AN-138 I was optimistic that a world class operation like Analog Devices must be doing a great job, presumably thousands of engineers today rely on spice simulation to validate designs and often go directly to pcb in their prototyping – with customers collectively investing thousands of hours in simulation based evaluation you might expect a commensurate effort by the manufacturer in making the models as accurate as practical – or at least as good as implied in their applications literature


Soufiane’s own article: “Spice macro models increase design accuracy and reduce time to market” has the heavily ironic header: “Their usefulness depends on the degree
of their accuracy.”

http://www.edn.com/contents/images/260060.pdf

(which further exasperates me by being about op amp spice circuit modeling but not showing the full spice model circuit diagram he uses – I don’t “read” spice netlists like a schematic, communicating with circuit designers about analog circuits really should be done with circuit diagrams)

After weeks of email customer service correspondence with Soufiane I gave up on trying to get enough of his attention to get a responsive reply addressing my questions about the AD8610 macromodel (for which he was unable to supply a circuit diagram to make discussion simpler)

Talking with “the big guys” at Analog Devices Op Amp Applications seminar in Andover months later in a side-conversation I was told the Ca group was using a simplified model and that they had been asked to change it, when another AD “heavyweight” joined the conversation he was clued in to the topic’s context with the phrase “its those California guys…” by the AD presenter I was speaking with



AN-138 is the only op amp spice modeling technique published by Analog Devices I’ve seen – as a customer I might expect them to use for all of their op amps

AC psrr modeling is “native” to the an-138 modeling technique – it even handles different +/- psrr transfer functions (best thought of as ps common and diff mode gains)

Cmrr is largely determined by the input stage – here macro models use diff pair input Q models, correct sizing of these Q parameters with the addition of a few parasitic tail current source and isolation junction components should capture most of the AC cmrr performance – some DC cmrr behavior is determined by diff input Q mismatches ( - talk to Scott Wurcer )

So we have a product manager encouraging us to use his product’s spice models – and Oh, by-the-way you’ll have to “patch” them yourself for the accuracy that we – the people with the characterization data and (hopefully?)the expertise can’t be bothered to – even when we as a company say we do know how to do it right

Yes Soufiane, I am trying to embarrass you since nothing else I can do from outside seems to likely to motivate you to improve the spice models you are expecting your customers to use – to toss off poor AC fits implemented in a simplified model that happens to save you some time putting together a release package is disrespectful of the time your customers are going to spend using the models and I’m sure a bad business practice in $ terms simply for the applications guys time supporting customers “surprised” by behavior that the model should be able to capture

I don't know that his patch is incorrect for my floating supply app - I don't intend to reverse engineer a schematic from the netlist, but if the spice models comming from AD's Ca group complied with AN-138 I could be confident they would work in my application
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Old 22nd March 2007, 02:04 PM   #14
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if i can help you let me know if all you want to do is yell then i can't help you, sorry.
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Old 22nd March 2007, 02:12 PM   #15
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Old 22nd March 2007, 02:36 PM   #16
Fanuc is offline Fanuc  United Kingdom
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Default RC Filters on Op Amp supply pins - PSRR

Hello,

I have a quick question for people in the know. Does adding an RC network on the supply pin of an Op Amp boost it's PSRR ?

I know it would attenuate the junk going into the pin, but does it actually improve it's PSRR ?

What if the op amp was bootstrapped also ? RC filter in a loop sort of thing...

I was thinking of an RC filter with a 3db point set at the point where the PSRR/loop gain falls with frequency.

Plus PSRR is reduced by gain also.

Advice welcome.

Kevin
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Old 24th March 2007, 09:39 PM   #17
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kevin, I don't think that would help your PSRR. Proper bypassing of the supplies is always helpful but as far as increasing the actual PSRR (DC), if anything an RC might screw up the resonnance of your C's
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Old 24th March 2007, 09:50 PM   #18
Nordic is offline Nordic  South Africa
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Two jobs to get done, keep PSU noise away from opamp... and keep opamp noise away from clean power rails....
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Old 24th March 2007, 10:02 PM   #19
Fanuc is offline Fanuc  United Kingdom
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Quote:
Originally posted by soufiane
kevin, I don't think that would help your PSRR. Proper bypassing of the supplies is always helpful but as far as increasing the actual PSRR (DC), if anything an RC might screw up the resonnance of your C's
Hello,

Thanks for the reply.

What if the op amp supply pins was bootstrapped to it's output with an RC filter in the loop sort of thing... (aka. Walt Jung reg.)

Increased loop gain is the another answer just wanted to see what could be "milked" out of an RC filter!

Yeah I agree the R component of RC filters makes the impedance worst (a) and with varying loads (twitchy high speed op amps) versus frequency. (b)

We could load it with a CCS or high impedance mosfet to try and circumvent this. Just some thoughts.....

Cheers

Kevin.

PS. I am racking my head over a Netware OES server with some beers and on a saturday night. Depressing...

I think because it stimulates me intellectually I quite enjoy it.... nerd speak I guess.
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Old 25th March 2007, 09:25 AM   #20
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... I may be wrong but what I see is that Uce of Q5 is equal to Ube of Q4 which would be something like 0.6-0.7V - I don't like that.
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