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

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From: Ron Miller (rmiller@Brocade.COM)
Date: Fri Nov 05 1999 - 17:53:09 PST


Also note that ATC recommends that their microwave capacitors be
mounted on their side so that the resonances from the top layers and
bottom layers of the cap are all the same length from the input and output
trace, which shifts the SRF up somewhat.

For those caps that have very thin height it probably doesnt matter, but
for the cube shaped ones it does.

Ron Miller

Larry Smith wrote:

> > 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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