The first time I saw a complex number used with units of resistance, I was like, huh?
How I leared what a decoupling capacitor is for, the hard way
31–40 of 91 posts
Re: How I leared what a decoupling capacitor is for, the hard way
#32This 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 vias improve performance, either by having vias spaced close together and in the right order (https://learnemc.com/decoupling-for-boards-with-widely-space...), or arranging capacitors in alternating or doublet layouts (https://incompliancemag.com/decoupling-capacitor-design-on-p...).
As you say, just having power planes and directly connecting to them is almost always going to be superior to using a trace, despite seeing this all the time, especially in datasheet example layouts. It made sense for 2 layer boards, but not today. Just think, the inductance of the planes is practically zero, and distance to the plane from the components is going to be on the order of 0.2mm, round trip 0.4mm. Is there any way I could place the capacitor 0.4mm away from the pins to achieve an equivalent inductance? And even if you could, you can't add extra vias to lower inductance, and you don't benefit from mutual inductance.
Re: How I leared what a decoupling capacitor is for, the hard way
#33Meta; typo in title, should be "learNEd".
Quality *Learing* Center
1-800-FRAUD
Re: How I leared what a decoupling capacitor is for, the hard way
#34Ah 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, for me, in an amplifier with a nice hole in its frequency response).
Re: How I leared what a decoupling capacitor is for, the hard way
#35Ah 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…
> 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 place, from power rails to completely unrelated signals that are supposed to be stable. Using actual TTL, there was another funny lesson what this weird "fanout" value in the datasheet meant.
A similar lesson I learned that way (and a very memorable one :-)) was about flyback diodes.
Re: How I leared what a decoupling capacitor is for, the hard way
#36Seems like a missed opportunity to try adding a capacitor dead-bug style onto the board to see if it cleans it up.
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 being said, I'm not 100% convinced this is a 20MHz++ noise issue.
> 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.
Re: How I leared what a decoupling capacitor is for, the hard way
#37Ah 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 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 learned when analogue circuitry was the norm. I learned when digital circuitry..."
What's not taught properly these days is that ALL electronics is analog at the physical/circuit level.
For you digital types that's OSI Model Layer 1 — Physical layer (look it up on Wiki). Nothing in electronics works unless that's working properly—ICs, tunnel diodes, transistors, inductors, resistors, capacitors, cables and antennas are all analog devices at that level. That includes the heart of the most advanced digital ICs. For example, the upper clock speeds in processors are limited by transit times/electron mobility, inter-electrode and stray capacitances, unwanted inductance, etc.—all of which are analog effects and they must be accounted for.
Like it or not, the physical analog world is alive and well! The Noughts & Ones Brigade unfortunately seems to have forgotten that fact.
Re: How I leared what a decoupling capacitor is for, the hard way
#38leared = learned ?
The O'Reilly book "Designing Embedded Systems" covers this pretty well with a story very similar to yours.
Great to be able to learn something new.
Re: How I leared what a decoupling capacitor is for, the hard way
#39Ah 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…
Re: How I leared what a decoupling capacitor is for, the hard way
#40Earlier 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…
Ah, but that may well be because of your scope probe's leads! The sharper the edge the more likely that will happen. That's what those shitty little springs are for that come with your scope probe: you disconnect the ground wire and put that spring on the naked scope probe pin around the ground collar. Then where you want to measure you use the pin to go to the signal and the little spring to reach the nearest ground. Presto: clean signal (or at least, much cleaner). Also, make sure to tune your probe (that's what the little plastic screwdriver with metal tip is for, there is a small trimmer in the probe you can reach through a hole and that is critical at high frequencies) and avoid probes with switchable 1/10 like the plague, over time the switches go lame and then you'll be tracking all kinds of weird gremlins.