Decoupling capacitors

antispam · 2020-12-10T14:55:29+00:00

I wonder about wisdom of paralleling ceramic decoupling capacitors. I some old text I have found advice to use 10uF (electrolytic), 100nF and 100pF in parallel. Several relatively new text write about combinations like 1uF and 100nF, both ceramic. In case of electrolytic capacitors things seem clear: electrolytic is mostly resitive in middle of frequency boound and ceramic gives pure improvement. However, datasheets of modern ceramic capacitors suggest that ESL is almost independent of capacitance (ESL groves with size but in given size seem to vary only a little). Simple RCL model shows that parallel ceramics will have parallel resonace peak when capacitance differ enough (about 3 times for modest peak, 10 times gives substantial peak). Parallel combination will have smaller impedance above self resonant frequency of smaller capacitor but the gain is modest. OTOH close to resonant peak we will have substantially higher impedance. For me it does not look like good deal. In fact, when low impendance at high frequences is important it looks better to connect several nominally identical capacitors. With usual tolerances they should give no parallel resonace and lower impendace due to paralleling of ESL. In principle one could play tricks with resonance frequencies putting parallel resonace at frequency not present in circuit and serial resonances at frequencies needing suppression. But with usual tolerances of ceramics this look impractical to me. Anyway, I wonder if I missed something and paralleling different decoupling ceramics gives some advantages? Or is this advice about paralleling just repeating old lore without understanding that world has changed? -- Waldek Hebisch

Dec 10, 2020 48 Replies

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Uwe Bonnes 5 years ago

Please try!

Thanks

Uwe Bonnes bon@elektron.ikp.physik.tu-darmstadt.de Institut fuer Kernphysik Schlossgartenstrasse 9 64289 Darmstadt --------- Tel. 06151 1623569 ------- Fax. 06151 1623305 ---------

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Lasse Langwadt Christensen 5 years ago

niptechnology.com:

It might depend on the package/device , table 3.1 list the required caps

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B.pdf

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Cursitor Doom 5 years ago

b) then. :)

Well it certainly beats what I've always done with DIL packages: strap an ugly looking, long-leaded ceramic over the top diagonally.

Yes sir. One less thing to worry about can't be bad.

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jlarkin 5 years ago

Hilarious. A CLG484 "requires" about 56 caps, about a dozen of which are 47 uF or more.

I use about 10x 1uF ceramics total on a Zynq 7020 with DRAM. That's probably overkill.

John Larkin Highland Technology, Inc The best designs are necessarily accidental.

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jlarkin 5 years ago

OK, I'll ask the boys to do that on Monday.

John Larkin Highland Technology, Inc The best designs are necessarily accidental.

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Rick C 5 years ago

ndsniptechnology.com:

ass

-PCB.pdf

Lee Ritchey talked about this in his class. I don't recall which vendor to ld him they didn't guarantee the design would work if he didn't use the exc essive number of caps recommended. His reply was, "Do you guarantee the de sign will work if I do follow the recommendations?"

In the case of Xilinx they don't know what your design is doing. They have to allow for every FF in the package changing state at the same time as we ll as every I/O changing in the same direction. What are the chances that will happen?

Rick C. ++ Get 1,000 miles of free Supercharging ++ Tesla referral code - https://ts.la/richard11209

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John Miles, KE5FX 5 years ago

I happen to have some of the CLG484 Zynqs on the shelf:

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If there are any caps in there, I sure don't see 'em. :( They usually jump out of the image, barium titanate being on the opaque side.

-- john, KE5FX

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John Miles, KE5FX 5 years ago

The ones I have came from a lot of 20 that I got from a surplus dealer in China. They look kosher enough, but I've never actually tried them. If you get a chance to X-ray yours, it'll be interesting to see if they match. It doesn't take much resolution to spot capacitors, given that no other features besides the balls themselves tend to be as opaque...

-- john, KE5FX

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Lasse Langwadt Christensen 5 years ago

p out of the image,

yeh I dound one on google,

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n_7_Fig9_HTML.png

I think what looks like caps are on the back of the board

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