[SI-LIST] AC Decoupling pad design
- From: "Grasso, Charles" <Charles.Grasso@xxxxxxxxxxxx>
- To: "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
- Date: Tue, 24 Feb 2015 15:42:51 +0000
Hello all,
My question has to do with pad design for AC Decoupling caps (but I suppose can
be translated to
any high speed signal line that has to terminate into a component).
Untreated, the pad presents a very low impedance at its location (albeit for a
short time) to the intended signal impedance.
Common practice is to cut out the ground plane underneath to increase the pad
impedance to match the intended signal
impedance. So - if one just examines the numbers then we would have a 50ohm
trace terminating into a 50 pad.
However the structures are quite different dimensionally (thin trace over a
plane vs a thick pad over a hole and a distant plane).
It is this disproportion that leads to my questions:
1. Will the EM wave propagate undisturbed through the combined structure?
2. Is this structural mismatch (but electrically matched) significant?
(Or should I park this in the right angle bend trash pile!!??)
3. Given that the pad is usually 4x the trace width would "tear dropping"
(i.e gradually increasing the trace width to match
the pad with a corresponding void in the plane underneath) be an advantageous
design feature?
Before I launch on an extensive simulation exercise, getting the opinions of
experts in the industry will point me in the
right direction. Thank you all for your generosity in sharing your expertise.
Best Regards
Charles Grasso
Compliance Engineer
Echostar Communications
(w) 303-706-5467
(c) 303-204-2974
(t) 3032042974@xxxxxxxxx
(e) charles.grasso@xxxxxxxxxxxx
(e2) chasgrasso@xxxxxxxxx
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