UBM Tech
UBM Tech

Why bypass caps make a difference - Part 4: Op amp macromodels: A cautionary tale

-May 16, 2013

[Part 1 looks at the overall supply impedance seen by a component when the decoupling caps, the voltage regulator and the board traces are taken into account. Part 2 looks at how a real-world supply impedance - comprising decoupling capacitors, regulator output impedance, and PCB traces - responds to a small test current step. Part 3 examines how a varying power supply voltage effects the output of a basic op amp circuit.]

In 2008, Kendall Castor-Perry began an extended investigation into the effect that bypass capacitor selection has on analog circuit performance. Five years on, we’re re-posting this classic series, with crisper graphics, fixed typos and a new postscript from the author for each article. The insights and analysis are every bit as valid now as they were back then.

Author Postscript for part 4
The postscripts for the first three parts (really they are prescripts, because they are here presented before the original article) were really just anecdotal chatter about questions people have asked, and related experiences I've had, since these articles first came out in 2008.

Part 4 was essentially a six-page grumble at how hard it was to find op-amp macromodels that delivered realistic results for power supply gain (remember, that's essentially the inverse of power supply rejection). And now, writing in 2013, I can't get away without returning to the scene of the various manufacturers' crimes, and checking just how much or how little of what I said in 2008 is still true.

This involved some reconstruction work. Yes, you guessed it, backup failure. Not of hardware, but of the user - me. I used LTspice in the way that many people do, adding my own models in neatly organized subfolders in the LTspice library install folder, which, you guessed it, is in C:\Program Files\ and, sadly, was not in my backup script. I was able to locate all my simulation files, and the graphics for the original articles. But my library of actual LTspice parts for all those non-LT op-amps had to be rebuilt.

Fortunately the data was not that difficult to find, and I was able to essentially recreate the plots from the original article, by analyzing the models that I had used five years back. This was the first step - making sure that, doing now what I did five years ago, I'd get the same results. And almost every time, I got identical plots. The only differences were in places where improvements to numerical precision in the AC analysis have for instance made a physically irrelevant computational residual of -170 dB become an even more irrelevant -540 dB.

It's worth pausing to think just how impressive it is that I was able to find an LTspice schematic file (from a time when it wasn't even called LTspice) and run it instantly on a current install of that program.

So, the first step was to check through the LT parts that I had talked about. And I wish I had better news to report! The plot for the LT1355 has improved from its previous dismal level (figure 4.2) but it's still not physically tenable. Results from the previously plausible LT1723 (figure 4.3) have changed, and if anything look worse now at high frequencies, picking up about 15 dB of gain at 100 MHz. The LT1812 didn't work at all in the new sim, because they changed the size of the symbol to include a shutdown pin! I used the dual LT1813 instead, and that also showed behaviour that was perhaps believable at low frequencies, but physically unrealistic at high frequencies.

My ex-colleagues at TI saved me a lot of work, though. Having got all the old models working and checking that they matched what I published in 2008, I went to the website and downloaded their very latest files from the product pages of the TL071, the TLE2082, the OPA350 and the OPA627. All super op-amps still, in their product categories. And all still supplied with the very same models as I used five years ago, warts and all. So, I stand by the text of the original piece there.

Harry Holt of ADI had provided me with a revised model of their TL082 clone not long after I published this episode in the series, and it worked fine in the simulation. I didn't actually go to their website to see if it got changed there, though.

All in all, we're still pretty much in the same macromuddle that we were in five years ago. And it may get even worse; I haven't even looked at the issues raised in part 5 yet!

Previously, by the end of part 3 we had just got to the point of seeing that when we apply a current excitation to our modelled power supplies, the voltage variations we see on those supplies punch right through to the op-amp output in a quite predictable way. You want to see what that looks like in the time domain, don't you! So do I, but this is the middle third of the story and there has to be an encounter with a scary monster.

Yes, it's time to start using the models of real amplifiers (if that isn't a contradiction!) supplied with LTSpice, and also imported into it from other vendors. It would be disingenuous of me to pretend that I wasn't expecting trouble from these models - and my fears turned out to be justified.

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