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Old 25th February 2005, 12:03 PM   #1
Mr Evil is offline Mr Evil  United Kingdom
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Default Your thoughts on simulation compared to reality

Reading a few opinions on the usefullness of simulation results in another thread, I wondered what everyone else thinks and if there are any tips for getting the most out of sims.

Personally I simulate a lot. It's very rare for me to build a circuit without having simulated it first, unless it's very simple. I usually find the measured performance of real circuits matches the simulations with uncanny accuracy. There are a few caveats though:
  • Simulator must be good. I'm sure most people use some form of Spice, so that's not a problem.
  • Models must be good. This can be a big problem. Some manufacturers don't provide models and even where they are available they aren't always accurate (op-amps with infinite bandwidth are a classic example!). Creating a good model from datasheets/measurements isn't exactly easy either.
  • Perfect component matching. Real components vary, whereas in simulations they are all identical. This can result in much higher DC offset and distortion in reality than sims and even circuits that don't work in reality at all (like complementary differential input stages with current mirrors). It's a good idea to swap out e.g. one of a pair of transistors in a sim with one of a different but similar model temporarily to simulate the effect of component mismatches. If performance changes a lot then the circuit is probably too sensitive to component variation.
  • High frequency stability. I sometimes find that circuits sims predict will be stable are unstable in reality and vice versa. Thus things like Miller capacitor values can only be accurately selected from real circuits. It also means you shouldn't necessarily be put off just because you can't get a circuit stable in sims.
  • Thermal effects. Although it is possile to simulate the effects of heat in Spice, it's not perfect.
  • Simulated devices never break. You can exceeed their ratings in sims and it won't tell you. If you don't take care to check carefully then you could end up liberating some magic smoke.

That's everything I can think of right now.
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Old 25th February 2005, 12:09 PM   #2
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I think you're quite complete. On the stability issue, I have found that sometimes adding a few pF to the inputs of opamps (and output) helps to get closer to the real thing: it will often oscillate as in the real world, unless you take measures.

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Old 25th February 2005, 12:22 PM   #3
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Actually, i have more problems with oscilation during simulation then in the real world. ( using multisim )

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Old 25th February 2005, 12:53 PM   #4
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i think simulation is very usefull for digital circuits
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Old 26th February 2005, 05:30 AM   #5
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When people get to talking about simulation, there's always some excuse as to why it doesn't match reality. Be it the code, the models, the user who doesn't know what he's doing...
Reminds me of religion. When something goes awry, you're supposed to pray. Prayer didn't work? Well, you didn't pray hard enough, or god had other things on her mind right then, or you'll get your wish in some other unexpected way, or...

Grey
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Old 26th February 2005, 11:33 AM   #6
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Default Re: Your thoughts on simulation compared to reality

Quote:
Originally posted by Mr Evil
[*]Simulated devices never break. You can exceeed their ratings in sims and it won't tell you. If you don't take care to check carefully then you could end up liberating some magic smoke.[/list]
That's everything I can think of right now.
I hear that the programmers at cadence (Orcad P-spice) are working on smoke parameters, where you can specify max parameters for your components, and let out virtual magic smoke if you exceed these parameters.

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Old 26th February 2005, 04:02 PM   #7
Mr Evil is offline Mr Evil  United Kingdom
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Quote:
Originally posted by GRollins
When people get to talking about simulation, there's always some excuse as to why it doesn't match reality...
You may scoff, but simulations are a valuable tool. Like any tool they are imperfect and not suitable for all jobs, but it's a hell of a lot quicker than doing all the calculations by hand.

Most of the time the difference between sims and reality is negligible. Even when there is a difference, this can in itself be informative: By adding stray impedances until the sims resemble real results (like the caps across op-amp inputs that janneman mentioned), it's often possible to determine why a circuit is not functioning as expected. Many times this has shown me that a particular track is too inductive, allowing me to fix it much more quickly than using measurements alone.
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Old 28th February 2005, 08:44 PM   #8
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Simulations are good, and useful, so long as you bear in mind that they are just simulations and inherently imperfect. Which statement I shall backup by pointing out that any transient sim is a discretized version of a continuous system. Hence you can get instability in the sim thats not there in the real thing, and vice versa.

I use sims a lot, to get me in the ballpark, then I build the thing on the bench.
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Old 1st March 2005, 12:49 AM   #9
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Yes, but the question is...how many potentially useful designs get abandoned because the simulation claims it won't work? You don't know that it won't work (or, conversely, that it will) until you build it. Case in point--the Aleph-X. Had I been "designing" it in a simulator, I would have abandoned the concept entirely because the simulator would have told me that it wouldn't work. Only, it does work, in spite of a gaggle of people with simulators telling me it doesn't.
Save time? Hardly. I can pull a resistor and solder it in as fast or faster than you can simulate it and have 100% confidence in the result--after all, it's reality. You may think you have 100% confidence in your result, but...
To me, this is all part of a disturbing trend wherein people insulate themselves from reality. They tell themselves that they're gaining a better understanding of things, but bit by bit they are actually disengaging from the real world. There's a lot of it going on in politics, religion, and the everyday things that we do. It's gotten to the point where at least some of the people, some of the time, are unable to judge reality accurately when they are forcibly confronted with it. They retreat to their sanitized, simplified versions in order to cope.
I think I need to write a story about this...

Grey
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Old 1st March 2005, 01:08 AM   #10
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Quote:
Originally posted by GRollins
Yes, but the question is...how many potentially useful designs get abandoned because the simulation claims it won't work? You don't know that it won't work (or, conversely, that it will) until you build it. Case in point--the Aleph-X. Had I been "designing" it in a simulator, I would have abandoned the concept entirely because the simulator would have told me that it wouldn't work.
Grey
Unfortunately, the reverse is too often the case in reality. A modern suite of simulation tools coupled with a properly equiped characterization group can achieve simulated results of IC's that are so close to reality it's scary. The problem is when you actually go to use the thing in a real board with different brands of external components you get stuck.
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