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The datasheet for the SN64LVDS31 specifies to use 0805 decoupling capacitors (page 8):

image from datasheet

This seems awfully large. I typically use 0603 or 0402. Should I interpret this to mean a maximum size of 0805, or is there a valid reason to not go smaller?

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2 Answers 2

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The datasheet is from 2011. The part may be even older.

Back in 2011, or when the datasheet was first made, a 0805 capacitor was typical size and good enough for decoupling. But we can't really know the reason behind the suggestion because there is nothing that specifies why that is the suggestion.

A ceramic capacitor with smaller size has lower inductance which is better, but it may also have worse effective capacitance at the rated voltage. So if you are going to change the physical size or material of the capacitor, make sure the new capacitor is equal or better than the original.

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    \$\begingroup\$ If you're concerned about inductance, you can also get "sideways" capacitors in packages like 0508, with the contacts on the long side. These have lower ESR and ESL, at the cost of being less available (as they're less common) and more awkward to lay out on a PCB. I'm not sure I've ever seen any smaller than 0508 though; they might not be available in 0306. \$\endgroup\$
    – Hearth
    Apr 5, 2021 at 15:51
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Given the suggestion to use two different size capacitors in parallel, the circuit is attempting to have the lowest possible inductance on the decoupling capacitors. For that reason, using a smaller package size is not only ok, but probably a better idea than using 0805.

I would also strongly consider not populating both of those capacitors as a 0402 0.1uF cap is going to have a lower inductance than the parallel combination of two 0805s the datasheet suggests. The second cap is therefore useless at best, while putting two chip capacitors in parallel like that could (at least in theory) also contribute to resonances between them due to parasitic inductance.

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