[SI-LIST] : RE: Charge moving from decoupling capacitors

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From: Barry Ma (barry_ma@altavista.com)
Date: Mon May 15 2000 - 13:32:48 PDT


Thanks a lot for your inputs.

All responses to my second question are only concerned with the inductance due to “long” distance between chip and decap. Nobody seems to agree imposing another constrain to the distance. My question was

“Do we really have to limit the distance letting the charge have enough time to move from the cap to the chip during the rise time interval? I doubt it.”
 
But I really read an article implying this extra concern.

George, you wrote:
> This is true if you have only DC current. For AC, you may have water in the pipe but
> no water out of the faucet if the faucet is switching out of phase from the water in
> the pipe.

Thank you for reminding me of Frequency Domain analysis. Yes, I should have described and analyzed a transient problem (charge travel during Tr) in both TD and FD, and then correlate the results. Let me have a try this time:

It is generally acknowledged that decaps and plane cap are complementary (supposing a 10 mil or less spacing between pwr and gnd planes). Decaps cover low end of frequency range, while the plane cap takes care of high frequencies. Thus the interplane cap would play more and more important role in high-speed PCB design, as the speed gets faster and faster. On the other hand, nobody objects closer distances from decaps to the chip, if possible. ..... When a chip drains necessary charges from pwr/gnd planes during Tr, decaps would supply charges to pwr and gnd planes on lower frequencies, while interplane cap can respond itself on higher frequencies.

Best Regards,
Barry Ma
bma@ANRITSU.com

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