[SI-LIST] Re: Cap discontinuity
- From: Ray Anderson <Raymond.Anderson@xxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Thu, 16 May 2002 12:05:37 -0700 (PDT)
>
>Hi, Rohit:
>
>Yes. The cap is a much bigger discontinuities. It will cause the major
>reflection anyway. In that case, the difference between the pad size and the
>trace size is a much smaller discontinuities. In such a sense, it is not
>critical. The cap is critical. Thanks!
>
>best regrds,
>
>-----------------------------------------------------------------------
>Jian-X. Zheng, Ph.D
Jian-
In most cases, this is probably true. However he didn't specify
what kind of cap he is using.
If a garden variety X7R or COG 0603 or 0402 is utilized, the intrinsic
inductance of the cap is sufficient to make it look like quite a
discontinuity at the multi GB data rates that were quoted. But if a low
parasitic porcelain microwave cap like an ATC or single-pair opposed plate
cap (a couple $ per cap) were to be used, it _may_ be possible for the pads
to contibute more to the discontinuity than the capacitor.
I'd suspect that common caps would be used (if only for economic
reasons), in which case your comments are correct, but you
really can't make a blanket statement without knowing the details.
-Ray
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