RE: [SI-LIST] : Decoupling between non-ground power rails, yes or no??

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From: Ingraham, Andrew (Andrew.Ingraham@compaq.com)
Date: Mon Jan 22 2001 - 19:15:55 PST


> I think it is a matter of proportion. ...
 
> I don't think that planes fall into this category (not enough C).
 
I agree.

Discrete bypass capacitors might be 0.01 to 0.1uF each, times a few dozen.

Intrinsic plane capacitance is perhaps two to five orders of magnitude less.

Voltage regulators likely respond in the timeframe of microseconds; i.e.,
loop dynamics in the under-to-low MHz range. Above the loop's bandwidth,
it's pretty much a don't-care; it doesn't affect the closed loop stability.
But add enough capacitance in the right place, and the phase response in the
low-MHz range IS affected, and the response blows up.

Andy

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