Re: [SI-LIST] : Comment on Johnson's article

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From: Larry Smith (ldsmith@lisboa.eng.sun.com)
Date: Fri Nov 05 1999 - 16:59:50 PST


> Date: Fri, 05 Nov 1999 16:39:47 -0800
> From: Howard Johnson <howiej@sigcon.com>
>
> If you want to achieve an even lower impedance, try smaller capacitors,
> or go with some of the more advanced packages (for
> example the new "IDT" package from AVX claims to
> deliver an installed inductance of 0.1 nH per component.)
>
> Best regards,
> Dr. Howard Johnson

Most of the inductance is not in the capacitor but is in the pads
and vias to the power planes. Our studies show that the inductance
in the capacitor itself is between .2 and .4 nH for 0805 capacitors,
depending on the height. The mounting inductance usually ranges
from 0.3 to 2 nH depending on the depth of the vias to the power
planes and the spacing between vias.

The cost of low inductance capacitors is not worth it unless you
are going to commit substantial board area for multiple vias to
the power planes and can tolerate the loss of wiring channels.

best regards,
Larry Smith
Sun Microsystems

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