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Lead length to bypass cap "must exceed 20mm" ??
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adamcord:
Hi all, I'm thinking of using the ADuM4120-1 isolated gate driver for a project, and I came across a curious line in the datasheet, on page 13, in the "PCB Layout" section. At the end of that section appears the following (talking about bypass capacitor placement):

"The total lead length between both ends of the smaller capacitor and the input or output power supply pin must exceed 20 mm."

I'm hung up on the "must exceed 20mm" bit. My usual thinking is the closer the better with bypass caps. Is this a typo, or is there something I'm missing?  Perhaps the designers found that there needs to be some tiny extra inductance between the bypass cap and the pin for high-frequency response? If anyone has some insight on this, I'd love to hear it!
MagicSmoker:
99.9% chance that is a typo. No gate driver benefits from extra inductance in the loop between it and its bypass capacitors.

T3sl4co1l:
It could almost make sense actually, in that those things use quite high frequencies internally (100s MHz) and maybe they want you to stay away from it a little bit.... but much more likely yeah it's a typo, and that's precisely why it needs to be close up.

On that note, keep that in mind -- they can generate significant emissions, so a little filtering outside of that bypass cap (a ferrite bead and another cap, with everything laid out on ground plane) may be desirable.

Tim
David Hess:
High frequency analog ICs sometimes have requirements like that but in this case I suspect it is a typo and they meant maximum instead of minimum lead length.
coppercone2:
why does the power supply pin for the IC require a minimum 'choke' inductance ? That would be approximately 20nH I think.

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