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How I leared what a decoupling capacitor is for, the hard way

nbelakovski.substack.com

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Re: How I leared what a decoupling capacitor is for, the hard way

#71

Post by sdenton4 »

vintermann wrote: post #60
madaxe_again wrote: post #27

Earlier quoted context omitted.

I feel it’s a function of abstraction. You learned when analogue circuitry was the norm. I learned when digital circuitry was simple enough that you could readily take something apart and understand it. Now, EE courses often start with cad, simulations, digital electronics, and you end up with people building ziggurats atop an ocean of incomprehension. It’s exactly the same thing with software. I don’t scorn people f…

> you end up with people building ziggurats atop an ocean of incomprehension. Everyone does. There's probably a layer below for everyone but the most theoretical physicists. I don't know where the leaks in electronics engineering's abstractions are, but I'm pretty sure they exist.

Well... https://xkcd.com/435/

Re: How I leared what a decoupling capacitor is for, the hard way

#72

Post by fuzzfactor »

raverbashing wrote: post #49
fps-hero wrote: post #32

Earlier quoted context omitted.

Loop inductance is what really matters with decoupling. Once you understand that, it becomes really easy to make good decisions. This article explains how you can approximate the inductance for a given layout, so it makes evaluating layouts much simpler. It actually used the data from the paper you referenced in example 3! https://learnemc.com/estimating-connection-inductance You can even use mutual inductance of via…

Yeah The ELI5 for decoupling capacitors is "imagine an energy storage for quick usage" The ELI(tired EE student) is more like the explanation above And this concept is ok for most of the 'low speed' circuits in RF ranges, everything is a capacitor (except when you need one), everything is an inductor (except when you need one) and the intuitive explanations break down and everything looks like dark magic

And every wire or PCB trace is an antenna, broadcasting and/or receiving whatever it has access to, at its own particular frequencies.

Across distances according to the power available, where ariel orientation makes a big difference, "as expected".

Re: How I leared what a decoupling capacitor is for, the hard way

#73

Post by ErroneousBosh »

dragontamer wrote: post #41
ErroneousBosh wrote: post #36

Earlier quoted context omitted.

> If it's really 20MHz++ noise that's screwing him, you need something faster than a through hole capacitor IMO to deal with it. That's always worked well enough in the past.

That's because you weren't dealing with 20MHz noise. Hobbyists are not dealing with 20MHz noise issues. Period. And if you are actually crazy enough to deal with high frequency circuits like that, you would well know that the land of through hole designs is simply insufficient, and that you are probably somewhere with some 0402 capacitors and some tweezers right now.

> Hobbyists are not dealing with 20MHz noise issues.

What makes you think that?

Edit: FWIW I consider 20MHz to be basically audio.

Re: How I leared what a decoupling capacitor is for, the hard way

#74

Post by lefra »

fps-hero wrote: post #63
raverbashing wrote: post #49

Earlier quoted context omitted.

Yeah The ELI5 for decoupling capacitors is "imagine an energy storage for quick usage" The ELI(tired EE student) is more like the explanation above And this concept is ok for most of the 'low speed' circuits in RF ranges, everything is a capacitor (except when you need one), everything is an inductor (except when you need one) and the intuitive explanations break down and everything looks like dark magic

I love your reverse psychology analogy. That does make me wonder, if a cap past its SRF is an inductor, and and inductor past its SRF is a cap, why not swap caps for inductors and vice versa, put an amplifier on the end and call it a day!

You can't, because all amplifiers oscillate (except when you need an oscillator).

Re: How I leared what a decoupling capacitor is for, the hard way

#75

Post by jacquesm »

dgroshev wrote: post #68
ceejayoz wrote: post #65

Earlier quoted context omitted.

I've struggled to find a proper introductory guide to stuff like this. Moving from pre-made Adafruit boards to my own PCBs was very tough to navigate; every guide I came across assumed you knew all sorts of stuff that the EEs writing them probably committed to deep memory decades earlier.

I found Phil's lab content [1] [2] indispensable for just this. Phil is a great communicator and gives in-depth explanations, so I didn't just watch most of his youtube, but also bought his mixed signals course and was very happy with it. Phil also recommends this lecture in one of his videos [3], which is still one of my all time favourite lectures ever. [1] https://www.youtube.com/@PhilsLab [2] https://www.phils-la…

Fantastic stuff, esp. #3 which I had not seen before. Thank you.

Re: How I leared what a decoupling capacitor is for, the hard way

#76

Post by kazinator »

I think the author's analysis of the problem is off. He writes about ripple from the regulator:

> This switching causes ripples in the voltage line,

But what is on the scope is not that ripple:

> Take another look at the pictures of the ripples above and notice the “M: 20ns” in the top left corner. This signifies that each vertical dotted line is 20ns apart, so the ripple you see has a frequency of something like 50MHz.

The switching regulator does not operate anywhere near 50 Mhz. Those voltage fluctuations are caused by the magnetometer itself: its own internal switching causing rapid current demand fluctuations. Or, possibly, it could be some other nearby device, in which case that device needs the decoupler (also).

This is why the decoupling capacitor addresses the problem. The purpose of the decoupling capacitor isn't to filter power supply ripple, but to provide a local, low-impedance current source that can swallow changes in current demand. That's why it's placed close to the device. It not only ensures that the device has smooth power, but also reduces the noise that it generates, protecting other devices.

Re: How I leared what a decoupling capacitor is for, the hard way

#77

Post by nippoo »

PunchyHamster wrote: post #54
nippoo wrote: post #19

This is probably a good place to debunk the usual wisdom that "decoupling capacitors must be placed very close to the IC pins". If you're using a solid power plane, rather than routing power through traces (and honestly 4/6 layer boards are cheap enough these days) it really doesn't matter where you place decoupling capacitors for most uses - keep the via traces short or ideally in the pad, and you can put all your d…

Well, till it does. Paper talks about frequencies in 200MHz range, not every project can afford solid power planes and putting it next to a chip costs literally nothing. It's like safety helmet, 99.9% of the time it's not needed

Above 400MHz or so, the on-die/on-chip capacitance starts being the most important thing (you're going to have inductance through the legs or balls of the chip).

Putting your decoupling capacitors next to the power pins _does_ cost. Not just in board space, but I've seen and reviewed layouts where the signal traces had to snake around decoupling caps or in some cases through vias because the designer believes that putting the caps close to the pins was the most important thing...

Re: How I leared what a decoupling capacitor is for, the hard way

#78

Post by derefr »

st_goliath wrote: post #35
michaelt wrote: post #30

Earlier quoted context omitted.

> How such critical knowledge can get lost in university training these days just amazes me. It will probably have been taught.... but very briefly. Before going go back to analysing circuit schematics, where connections between components don't show resistance or inductance, and the capacitance of two parallel capacitors sums.

This is why lab exercises are important. I remember first building some actual TTL circuits on bread board, I learned very quickly that this whole digital stuff is a lot uglier and messier than on paper or in the simulator. With sharp rise times, synced up to a common clock, even after soldering in a whole bunch of capacitors, you can still stick a probe pretty much anywhere and see switching spikes all over the plac…

> or in the simulator

Inadequate simulator, then, no?

(I imagine analogue RF board-level simulation is a lot more expensive than digital-logic board-level simulation. Might have been impractical way back when, such that we only used to have the digital-logic kind. But we certainly have both kinds today.)

Re: How I leared what a decoupling capacitor is for, the hard way

#79

Post by dragontamer »

WarmWash wrote: post #70
dragontamer wrote: post #23

Earlier quoted context omitted.

Look up parasitic inductance. Through hole parts cap out at maybe low MHz. Many electrolytic caps frankly cannot effectively decouple signals above 100s of kHz even. Above that value, capacitors become inductors due to lead lengths, parasitic resistance, and other details. To make capacitors work faster, we make them smaller and smaller. Surface Mount Caps are the only way to reach 20MHz++ decoupling speeds, and you…

Yes, but we are splitting hairs at that point. The transient spike is a high impedance voltage that is tripping the high impedance internal protection circuitry of the magnetometer. So whether we have 20mOhms of capacitive decoupling or 500mOhms of inductive decoupling, both are better than the infinite impedance of nothing there. We're not building a precision filter, were cutting the paws off of a paper tiger. No n…

This is a circuit with a switching regulator that is, presumably, stabilized with something on the order of a 10uH inductor + 22uF capacitor.

So from my perspective, increasing the capacitance from 22uF on that output line to 22.1uF with a 100nF cap will likely do jack diddly shit.

It is far more likely that, ex, the author of this post screwed up the regulator design. Ex: did the author mistakenly think that more capacitance is better-er and stick a 100uF cap there, blowing out the phase margin of the feedback of the switching regulator?

Was the inductor properly sized? Not just inductance but also saturation current and internal resistance?

Re: How I leared what a decoupling capacitor is for, the hard way

#80

Post by Neywiny »

kazinator wrote: post #76

I think the author's analysis of the problem is off. He writes about ripple from the regulator: > This switching causes ripples in the voltage line, But what is on the scope is not that ripple: > Take another look at the pictures of the ripples above and notice the “M: 20ns” in the top left corner. This signifies that each vertical dotted line is 20ns apart, so the ripple you see has a frequency of something like 50M…

I don't think you're entirely correct here. The regulator will create noise that has a fundamental frequency of 500 kHz. That we agree on. However, this 50 MHz noise can most certainly be caused by the regulator. It's critical to note that it's not a constant 50 MHz noise. It's clearly attenuating quickly and occurring sharply. If the author had zoomed out to the 500 kHz time scale, I'd bet we'd see this noise every time the switches change.
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