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

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

#61

Post by frangonf »

To makers that want to play and learn with power converters I recommend you:

- Test the converter at various points of load (when prototiping keep some 0ohm resistor/jumper for attaching a resistor load or electronic load).

- When you have to measure things, look around app notes/white papers of manufacturers, you will usually find practical actionable info and some examples. Doing proper measurements is really a discipline of its own, but for low frequency you can get far with the basics of craftsman/rule of thumb engineering. [0] [1]

For example the author here in the videos is mostly measuring the inductance loop between the positive of the rail and wherever ground is (we cannot even see where the osc negative is??) and how this particular loop responds to a cap, not the real bus.

[0] https://www.analog.com/en/resources/app-notes/an-1144.html

[1] https://www.richtek.com/Design%20Support/Technical%20Documen...

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

#62

Post by hilbert42 »

lefra wrote: post #34
hilbert42 wrote: post #18

Ah how things have changed. When I was learning electronics we mainly dealt with radio and TV circuits and just about the first lesson one learned was to keep leads short (reduce unwanted inductance) and use decoupling capacitors everywhere. I recall some years later a young graduate engineer coming into my office with a rather involved circuit consisting of 30/40 TTL ICs and complaining that he'd double checked the…

My university made us use really crappy power supplies and dev boards. Nothing worked unless you first put a large bulk capacitor on the power supply's output, and small capacitors close to the components. Also I got bitten by parasitics in capacitors very early in my career: capacitors of different face value will resonate with each other to effectively kill the decoupling network at a specific frequency (resulting,…

Incidentally, in my post below on the MIT RadLab series I mention Vol 23. On p183 parasitic oscillation is mentioned. Also, I recall when working in the now defunct RCA prototype lab, one of the main cure-alls for parasitic oscillations was to place a ferrite bead on a transistor lead (between it and the PWA). It often worked wonders.

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

#63

Post by fps-hero »

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

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!

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

#64

Post by InUrNetz »

hilbert42 wrote: post #18

Ah how things have changed. When I was learning electronics we mainly dealt with radio and TV circuits and just about the first lesson one learned was to keep leads short (reduce unwanted inductance) and use decoupling capacitors everywhere. I recall some years later a young graduate engineer coming into my office with a rather involved circuit consisting of 30/40 TTL ICs and complaining that he'd double checked the…

I have an MSEE from a top university (from 20 years ago), this topic unfortunately is not really taught. The theory and analysis is taught, but the practical implications were not. I connected the dots in my first job out of school where some very talented gray beards taught me how the real world works. Which brings me to my point that EE really is a trade. It takes schooling at the beginning and in most cases a degree or two, but there is critical knowledge that you learn in the real world after school; and there are levels analogous to apprentice, journey man, and master.

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

#65

Post by ceejayoz »

hilbert42 wrote: post #18

Ah how things have changed. When I was learning electronics we mainly dealt with radio and TV circuits and just about the first lesson one learned was to keep leads short (reduce unwanted inductance) and use decoupling capacitors everywhere. I recall some years later a young graduate engineer coming into my office with a rather involved circuit consisting of 30/40 TTL ICs and complaining that he'd double checked the…

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.

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

#66

Post by bavell »

nielsbot wrote: post #28
JCTheDenthog wrote: post #14

Earlier quoted context omitted.

>And "leared" -- the (unintentional?) pun made me click. I assume it's a reference to the "Quality Learing Center" in Minnesota, one of the questionable daycares at the center of the alleged Somali daycare fraud scandal. Ever since some of the expose videos about it came out it's become a meme to say "lear" instead of "learn".

> questionable daycares If they don't find fraud, is it "questionable"?

Didn't the guy flee the country after posting bail? Doesn't exactly scream "innocent".

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

#67

Post by amelius »

fps-hero wrote: post #32
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…

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…

Explanations like that are a lot easier to understand if you can see the equivalent circuit with the parasitic components.

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

#68

Post by dgroshev »

ceejayoz wrote: post #65
hilbert42 wrote: post #18

Ah how things have changed. When I was learning electronics we mainly dealt with radio and TV circuits and just about the first lesson one learned was to keep leads short (reduce unwanted inductance) and use decoupling capacitors everywhere. I recall some years later a young graduate engineer coming into my office with a rather involved circuit consisting of 30/40 TTL ICs and complaining that he'd double checked the…

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-lab.net/

[3] https://www.youtube.com/watch?v=QG0Apol-oj0

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

#69

Post by exDM69 »

Slightly related note, the pictures in the articles show a handheld digital oscillscope. It's an Owon HDS200 series oscilloscope with signal generator and they are amazing and the lower frequency models are quite inexpensive.

I got myself one earlier this year and it does what it says on the tin. It can also be controlled from a computer via USB serial connection using a text based protocol (albeit poorly documented and a bit buggy). I used some python scripts to program the signal generator and then capture some measurements from the scope to check the frequency responses of some analog electronics circuits for guitar.

There is a small community around, there are a few repos on GitHub for using them and also this very long eevblog thred.

https://www.eevblog.com/forum/testgear/owon-hds-200-handheld...

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

#70

Post by WarmWash »

dragontamer wrote: post #23
WarmWash wrote: post #16

Earlier quoted context omitted.

The capacitor doesn't have a concept of "fast enough", it's a passive component. The signal is what determines what it does when it encounters the capacitor. Non-linearities and capacitor species aside, a good ole x7r 100nF would clean this up. In general you can just liberally dump 100nF caps all over your pcb power traces and quash most problems like this before even knowing they exist. I joke that you make a circu…

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 need to let perfect be the enemy of good.

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