LTSpice UI

You can run the tolerances looser if you like. Occasionally it saves time. A custom dialog box giving some guidance on how to do that would be a nice feature.

I've occasionally needed to do that, e.g. when doing photoreceiver sims at the 1-nA photocurrent level, you sort of need to tighten the abstol settings. I'm no SPICE guru, though.

Cheers

Phil Hobbs

.opt abstol=2nA .opt chgtol=10pC .opt trtol=5 .opt reltol=0.002 .opt method=trap

seems to help slow ones. I just slop things up until the sim doesn't look believable, and back off some.

I've done sims that take an hour to run a few milliseconds. Feedback that slow doesn't train your instincts.

A bunch of my stuff wouldn't budge off the peg with those settings. If you already know what it should look like, why simulate?

Using universal opamps and VCVSes helps sometimes. Cheers

Phil Hobbs

To play with ideas fast day or night, and to get the parts values right. It's often easier to sim, say, a voltage divider than to do a lot of math.

I've invented some cool but improbable circuits by fiddling in Spice. My ac/dc programmable dummy load was an unexpected accident and is now a product.

Doing the .opt tricks is safe if you are careful, which isn't hard. Run a slow sim, note a few points, booger the sim until it changes much, back off. I suppose that the options affect the internals of imported models, so all you can do is try.

I most always use one of the universal opamps, because LT Spice seldom models mine, and it's too hard to research their list to find one that's close to what I intend to use. The bv block can really simplify and speed up a sim, and is an easy way to compute things to plot.

I've been known to dork frequency compensation that way too, and to look at tolerances for filters.

The last thing I designed by poking at in SPICE was a collapsible BJT cascode, where the top transistor had to take a bunch of voltage. When the bottom transistor turned on, the top one saturated, pulling down its relatively high-Z base bias string, so the low level was just the sum of the V_CE(sat)s.

I knew that part would work, but needed to figure out what the turn-off transient would look like. The answer was, "Nothing too pretty, but good enough for government work." It didn't look like the top transistor would reliably come out of saturation fast enough to avoid blowing up the bottom one, but a zener on its base solved that.

Our ideas of "a lot of math" are a bit different, but doing that is no worse than using a calculator. ;) (I try to keep in practice doing mental arithmetic, because it makes me a much better troubleshooter.)

I've done that in a pinch. I normally don't simulate big wodges of circuitry, so it doesn't come up much.

Once in a great while I've needed to simulate the start-up or ring-down of an oscillator, which can take forever, but you do more of those, I expect.

Op amp models are such random-number generators anyway.... Once in awhile you get one that models asymmetrical slewing, but not very often.

A depressing number even get the supply current all cattywampus. Even UniversalOpamp2 gets that right!

Yup. I usually compute efficiencies that way, for instance.

Cheers

Phil Hobbs

Right. Any interesting control loop is nonlinear.

A non-trivial voltage divider, with more than two resistors, can be a pain to work out with a calculator, especially using the parts you already have in stock.

And Spice sims, with comments, are easy to save in a design notes folder.

I do a lot of mental arithmetic, but it's analog computing, guesswork and not manipulating digits in my head. I usually come in around 5 or

10% accuracy, which is handy enough in most cases. Doing this at a whiteboard, in a meeting, seems to annoy some people. This used to be called "Lightning Empiricism"; see Williams 1991.

Thank goodness for that! I sometimes use the supply rails as the outputs of an opamp, to drive big power booster fets for example, and I need that to be right, statically and dynamically. As I noted recently, I found one ADI opamp that pulls 1e17 amps on V+.

Some will *output* free kilowatts from their power rails.

Yup. They go in our git repos too (one repo per project).

I like to make it a game. It's memory more than anything--in 1 Hz, 3 uA of photocurrent has 1 pA shot noise; for Johnson, 60.4 ohms has 1 nV and 16k has 1 pA.

Going up and down decibels is fun too: 1 dB is 12% voltage and 25% power, pi = 10 dB (voltage), and so 2 pi = 16 dB. A noise contribution that's 6 dB down raises the floor by 1 dB, and it goes down linearly (in dB) for lower noise.

Whiteboards are great fun, when combined with an interesting problem and one-to-three smart and good-natured colleagues. (And possibly a pitcher of beer.)

It's a good trick, for sure, but not all amps like it. Old-timey 'single supply' amps often have very very poor PSR on V-, because of course the designer expects you to ground it, duh.

Since CMR is input-referred, you can get _voltage gain_ between V- and the output, which makes life very exciting. UniversalOpamp2 will _not_ model that correctly. ;)

Very non-green, for sure.

Still need a lot to make up for that 12V * 1e17 A.

Cheers

Phil Hobbs

I'm not pressing you for any information. What's wrong with your reading comprehension? I'm simply pointing out, very clearly with a quote, that you stated the UI changed between IV and XVII. If you don't know anything about it, how could you know that it changed? If you do know something that changed, you should be able to talk about it, instead of insisting you know nothing.

I don't get you. Which of the words I'm using, do you not understand?

Do you understand now? You said one thing, now you are saying something different. Can you at least acknowledge that?

I don't expect anything, other than a civil tone in a discussion. You seem to be very frustrated that I'm not conforming to your ideas of appreciation of a tool. I don't believe that we should never speak ill of free tools. If the world were mute about crap software, it would never get any better.

Sorry if I offend your fascist vies of software.

I see you ignored this. But that's expected in an ad hominem attack. I'm sorry you aren't mature enough to have a reasonable discussion of a tool. I guess some people get too emotional about inanimate things and software.

Heresy. Whiteboards run on chocolate.

What, because I tried it superficially and didn't like it so I never went further? All I knew is that the version and owner changed. You are one stupid ass.

How can anyone understand you with your flawed ability to communicate? I acknowledge your lack of logic. Feel better?

Go to hell, asshole.

Well, here we agree. You are sorry.

If you are sorry, which was addressed above, then why carry on with the conversation? I don't get emotional about tools. I get emotional about people who bait others to get their jollies, like yourself. That is all you're doing in this conversation. Trolling. Get lost, D.H.