Spice as a design tool..the kosher list

SPICE can also be a design tool in the discrete world. Often I do the more complicate loop optimizations on it. Yeah, you can calculate everything until the cows come how, or be done with SPICE in 20mins. Same with unorthodox RF stages. All you need is good models.

Nearly everything in life can be a design tool. Think about the guys doing ice sculpturing. One of their design tools is ... ... a chain saw.

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I don't think they make chain saws (yet) :-)

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I think he was referring to the Texas Instruments Masacre. ;-)

I don't know what that means. Are you saying that you can't use Spice to find the component values that work? If so, why not?

I sometimes use Spice to design voltage dividers.

I designed a fairly hairy 3-opamp instrumentation amplifier, with gain switching mux's, and I wanted to use Susumu thinfilm resistors in the gain-set string. They are only available in weird values. So I just Spiced it, plugged in various available resistor values, and got the gain steps I needed. I could have done it with a calculator, in 40x the time, so why not use Spice?

And what's wrong with using Spice to tune a nonlinear control loop, or to tweak a filter design?

I sometimes just fiddle with parts in Spice, topologies as well as values, until I get something that works. Instinct + Simulation.

John

If you are tweaking filters by hand, the design is probably not optimal. There are plenty of filter optimizers around, or you can hack with matlab, octave, or set it up on excel if you plug in the solver. I haven't done the excel trick in a while, so I don't know if the optimizer is still supplied.

Parts have tolerances, right? You want the design to be centered to get the highest yield and least susceptibility to drift.

I would hate to fly in an airplane where the control system was just hacked a bit in spice.

The calculator method would have allowed you to do it at Zeitgeist. Of course, 40x can mean a lot of Russian Ale so you'd have to walk :-)

That's called cheating. But yes, I do that as well (with SPICE, not the other kind ...).

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Yep, done it yesterday. Then you end up with inductor values such as

937.54nH which you can't buy. So you have to send it through SPICE anyhow or bench-optimize it.

Many people would hate to fly in one where the control system was not run through SPICE. I fly with everything that holds together and somehow gets into the air, and not too much stuff falls off while doing so.

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Obviously.

There are plenty of filter optimizers around, or you can hack

So matlab, octave, and excel are design tools, but Spice isn't?

Why?

I'm not designing airplane control systems; they get checked and crosschecked a zillion ways. And tested really hard. I do design gear that tests airplane control systems.

But what's different about a design that was evolved using Spice from any other design? You couldn't tell by looking at the hardware.

I'll use anything that works, including writing code to randomly permutate parts values. Not often, but now and then.

Design starts with architectures and circuit topologies, and there's no systematic way to do those; they just pop into your head. So the most critical part of electronics design has *no* analytical approach. You can hardly complain that the downstream stuff lacks intellectual rigor.

John

Spice is great for evaluating nonlinear corner cases, with real device models. But I suppose you could call that "analysis" and not "design."

I don't make a hard distinction between those.

John

Hardly. I'm an easy drunk, and they have 40 beers on tap.

One sometimes comes up with circuits that you actually don't understand. Nothing wrong with that.

John

And when did I say not to simulate the circuit. Uh, that would be never. My point is you do not design in spice. It is far better to know what you are doing than just to hack.

Have you ever run an optimizer? You need to seed it with a starting point, and that takes some design skill. You need to create the error function, which means you have a specification that you are trying to meet. Reducing the error function centers the design. Thus Matlab, Octave or an excel roll your own optimizer is design.

If you don't understand why a circuit works, you will not be able to understand why it doesn't work. Processes change, parts have tolerance, etc.

I'm really amazed I have to state such obvious facts.

I also design in SPICE :-)

Not always, but sometimes. Like yesterday, doing some filter work.

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[snip]

One does not "design in Spice". You may use the Spice schematic capture to lay down _your_ ideas... where do they come from other than in your head.

Spice should only be a fine-tuning and verification tool. ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.

I guess it's all a matter of semantics. In your definition almost nothing would be a design tool. Not vellum, not the pencil, not even your hand. Only your brain would be.

To me a design tool is anything that helps me achieve a design, including stuff that greatly speeds up the process. SPICE is really high up on that list. I frequently doodle around in it, see "what happens if I use this RF transistor to do something really unorthodox".

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Who is it that is playing semantics? Spice isn't an entry tool... it runs _after_you_enter_ the circuit information.

"design in Spice" and "design tool" aren't the same thing ;-) ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.

Not quite. Often I enter, sim, don't like it, revise the entry, sim, still don't like it, revise again, like it more. How is that not a design tool? It is almost the same as pencil, vellum, eraser and HP11C. Except that SPICE is usually faster these days.

To me they are. Even a hammer can be a design tool. Said my former arts teacher who was an internationally recognized sculpturist :-)

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Regards, Joerg

http://www.analogconsultants.com/

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