I think I have a lock on the values of decoupling capacitors. When I do layouts, I normally use 0.1uF caps unless the datasheet says otherwise. THis has served me well so far. I’m just wondering if there is any reason to use one size over another? I had done mostly through hole designs in the past, at pretty low speed. I’m looking to get into faster, smaller designs though.
HI. 0.1 uF is typical for me as well. But as @EEVblog once suggested it’s a good idea to use combination of caps. Because each cap has different ESR and capacitance, each will be able to decouple different surge frequencies. I like to use 10nF, 100nF, 1uF for main MCU inputs.
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As a general rule just put down one of each?
Since the wording is a little ambiguous in the first post, you are talking about the capacitance? Or the physical size? (1206, 0805, etc)
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that certainly depend on design, MCU type, in-rush current issues, board space, etc., but mostly Yes, I use combination of at least two caps, and sometimes three.
Wow I never considered using more than one. Makes sense though. But I would like to know if there is some kind of rule of thumb on when this might be important.
And which capacitor to use.
For example, when using battery powered devices, yes or no.
Or when using high speed signals
Or when using power hungry LED’s
Or…
And like Chris points out, also the physiscal size seems to matter. I was recommended for a last design to use 0805 instead of my commonly used 0603’s for the main power capacitors. Although they were the same value and power rating (100nF).
Would love to know more about this.
This tutorial form Cypress gives some insight on when to use multiple caps, basically when you have variable clock frequency.
Excerpt from p.5: Use this multiple decoupling capacitor method only when a wide range of frequencies must be bypassed around a single integrated circuit and adequate range cannot be achieved by a single capacitor.
as far as I know, as the package size gets smaller, ESR increases.
This is a question I have asked myself too! I was listening to a DeskOfLadyada Video. She noted a .1 and 10 µF: DeskOfLadyAda Decoupler advice…
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That cypress link is great! I didn’t get the “system” view in Figure 2 though. Why do they draw all of those components in line?
I wanted to try image upload on this new forum, so I’ll do that here:
It’s illustrating that when you add a capacitor to your circuit that you’re also adding some inductance and resistance from the capacitor package
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So I actually was talking about the physical size of the capacitors, but all of this information has been great. I suppose as long as the capacitor is the value (or range of values, like some people are talking about), then it doesn’t really matter what size of capacitor I use?
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As far as I understand it, below a certain size you won’t be able to get certain capacities. Like I looked for a 10 uF capacitor in a 0201 on digikey and nothing came up:
Ah, so below a certain size, it’s harder to get certain amounts of capacity. I’m guessing there is some kind of relationship there.
There is a bit more to it than just physical size vs. capacitance availability. For ceramic and other capacitor types, the capacitance will decrease as the voltage across the capacitor increases. This effect gets worse as the physical size of the capacitor decreases or the nominal value of the capacitor increases, as shown in this graph:
Dave Jones / EEVBLOG also shows and measures this in one of his videos: https://www.youtube.com/watch?v=2MQyQUkwmMk
A really in-depth video on the various capacitor types and their idiosyncrasies can be found here: https://www.youtube.com/watch?v=ZAbOHFYRFGg
That said, one can make excellent circuits without knowing much of this 
Regards,
Niels.
This is an excellent thread. I would like to make one addition that I have not seen mentioned before and that is the microphonic effect, especially for ceramic capacitors. If you tap the capacitor, it introduces the “sound” into the circuit.
Normally that is a very small problem, but there are circumstances where it matters. Consider electronics that are part of a moving or shaking system. You might end up with decoupling caps that actually introduce more ripple on the power lines than they eliminate, albeit probably at different frequencies.
The effect can be quite astonishing, as Dave Jones demonstrated in a video about oscilloscopes:
microphonics in ceramic capacitors