Why bypass caps make a difference - Part 1: How a regulator and its output capacitor can interact

-April 11, 2013

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 reposting 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's Postscript
Regular readers of my columns will know that I’m as appreciative of Linear Technology’s simulation platform today as I was five years back. This series was the beginning of my love affair with the best free electronic engineering tool on the web. The name has changed, and the product has become faster and more stable. But even five years ago it did a great job on the simulations I needed.

I must admit that I do a lot more simulation and a lot less construction these days, compared to say fifteen years ago. Some old-school engineers have wondered whether that’s too much like going over to the dark side. Tailors say “measure twice, cut once”. For me, it’s “simulate thousands of times, build once”. These days I would not dream of building something with matter before exhaustively probing it with mind.

The main comments I received about this first part centered around the transmission line modeling approximations that I had used for printed circuit board traces. Cheap circuit board materials can introduce rather more loss than represented by these simple models. When pressed, my correspondents admitted that they probably relied a little too much on “getting away with it.” As better materials and components are chosen to suit today’s higher speed digital signals, much of this loss goes away. As designs get ever more compact and lower in cost, physically large lossy chemical capacitors are frequently replaced with small low-loss ceramics.

In the last five years I’ve encountered several cases, and heard of more, where removing chemical caps had some unexpected consequences. The moral: always have one component per rail on your board whose main job is to introduce loss on each power rail. A small, low-value resistor in series with one of the high-value ceramics will do the job. A supply without designed-in losses is like a car suspension without shock absorbers – you could get a rough ride!

Every generation of audio equipment designers rediscovers that you can change how audio systems sound by changing component types and values. Several audio friends of mine reported interesting results from ‘tuning’ the resonance between the LDO and the main output capacitor, as I suggested here. Douglas Self’s books, particularly his recent one on audio filter design, have made a big contribution to the canon since I first wrote these articles, showing that component choices can have surprisingly large measurable effects in audio equipment.

In "Know the sometimes-surprising interactions in modelling a capacitor-bypass network" (abbreviated to "Know the..." when referred to here), Tamara Schmitz of Intersil and I provided some simulation background behind the interaction between multiple decoupling capacitors used in parallel. The series inductances inherent in the capacitors cause several resonant peaks and dips in the impedance response, sometimes at frequencies that might be critical for circuit operation.

We advised that, if you intend to use decoupling capacitors of different values in parallel (there's almost always more than one capacitor attached to the power rail), you'd better be sure that the location of the resonant peak won't cause trouble in your circuit. But we didn't say what kind of trouble that might be.

So this series of articles is an attempt to quantify what's happening on the supply line of a representative analog circuit with regulated supply rails and decoupling capacitors. We'll do this entirely in simulation - mainly to show that this is both possible and revealing - and the path to interesting results will prove a somewhat bumpy one. Over the course of the six articles we will:

  • look at the overall supply impedance seen by a component when we include the decoupling caps, the voltage regulator and the board traces providing the component's power. In passing, we'll discover just how supply voltage affects the value of some ceramic capacitors;

  • see how such an impedance responds with a characteristic voltage transient when you 'ping' it with a small test current step such as an IC might demand from the power rail. Choice of capacitor dielectric turns out to have a significant effect;

  • see how these supply variations punch through to the output of an op-amp running on these supplies;

  • discover that many op-amp simulation models, used for this purpose, are so inaccurate that they can produce seriously misleading and even physically impossible results;

  • run a signal through the attached op-amp, drive a load, and see how the actual load current interacts with the modeled supply, and how this affects the amplifier output;

  • see that simulations provide an objective approach to selecting decoupling capacitors in order to alleviate a previously poorly documented effect on the precision of op-amp circuits. And discover another way in which many op amp simulation models are so inaccurate they are useless for this work!

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