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Old 22nd February 2015, 09:50 PM   #1
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Default Does simulation always match reality

I've been using Texas Instruments "TINA" simulator for a while now and it has been helpful. I must admit it has saved me a re-spin of circuit board design or two.
Assuming I have described the real world conditions accurately, is it always right?
I have made mistakes like not taking into account the signal source impedance and such but so far it seems to be always right once I've described the real conditions correctly.
Specifically I'm wondering if its' FFT simulation can be counted on. It outputs a THD, total harmonic distortion, figure.
I don't care to own a stand alone spectrum analyzer insturment- big $.
Maybe I should be considering a software FFT for my laptop or something to verify the real circuit matches simulation. Of course then my signal generator would need to put out a prefect sign wave - I don't think it does. Perhaps one can synthesize this pretty good with a computer/software too
What has been your experience?
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Old 22nd February 2015, 10:14 PM   #2
dmills is offline dmills  United Kingdom
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The sims are only ever as good as the models, including your models of the parasitic values in the layout, and some of the supplied models are rather lacking in terms of modelling things like CMRR.

Simulation is useful, but I would never consider it definitive for anything much, ya got to measure your real device to verify that the simulation is correct on this occasion.

A good soundcard and software can take you a long, long way in audio frequency measurement, not a good as a Prism or AP, but more then sufficient for many things.

Regards, Dan.
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Old 22nd February 2015, 10:15 PM   #3
rayma is offline rayma  United States
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Quote:
Originally Posted by campsquire View Post
I've been using Texas Instruments "TINA" simulator for a while now and it has been helpful. Assuming I have described the real world conditions accurately, is it always right?
It's just SPICE, and as such is best used as a design and development aid, to be confirmed by careful bench testing. It's not always right.
The device models are approximate and the pcb layout is critical, for example.
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Old 22nd February 2015, 10:19 PM   #4
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Quote:
Originally Posted by dmills View Post
The sims are only ever as good as the models, including your models of the parasitic values in the layout
This is what I was going to say.

Simulation assumes "perfect" grounds and zero parasitics. Poor grounding can introduce distortion and even instability.

You can add some inductance and resistance to your simulation to see how it affects your circuit.
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Old 22nd February 2015, 10:53 PM   #5
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OTOH it's more probably right than you are. That is to say, just because you don't get the result you want, don't rush to blame the simulator. It depends on the level of sophistication of what you're trying to achieve, most simulators do simple things faultlessly.
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Old 23rd February 2015, 02:57 PM   #6
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OK thanks fellas.
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Old 24th February 2015, 07:37 PM   #7
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I actually have the case whereby simulation is yielding incredibly low THD and I'm not sure weather to believe it or not. Although I can make it worse with design change.
I'll look for some spectrum analyzer in software/laptop solution to check reality.
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Old 24th February 2015, 08:05 PM   #8
sreten is offline sreten  United Kingdom
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Hi,

Simulation will never reveal real THD+N for good circuits.
Too many ideal assumptions, and not enough reality.

rgds, sreten.
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Old 25th February 2015, 02:55 AM   #9
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Don't make the simulation "ideal".
simulate a real 2 device CCS (instead of a spice current source).
Add some out of phase ripple - simulate an imperfect unregulated PS.

A "good circuit" should show minimal degradation .

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Old 25th February 2015, 10:16 AM   #10
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Quote:
Originally Posted by campsquire View Post
I actually have the case whereby simulation is yielding incredibly low THD and I'm not sure weather to believe it or not. Although I can make it worse with design change.
I'll look for some spectrum analyzer in software/laptop solution to check reality.
A sim THD calculation is normally small-signal, assuming that the signal variations in current and voltage in the circuit are very small. So this always is much more optimistic, except maybe if you also measure with very small signals.

OTOH 'real' distortion measurements are more useful using transient analysis and spectrum analysis. Using an FFT analyzer you can set the signal levels as you like and the sim will use that to calculate the spectral lines which gives you distortion info at any signal level you want.

Jan
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