Hi Bob,


Decoupling is really an interesting topic and there are few areas where
there are more
'bull'-talk than with ing the decoupling science.

I have not used the circuit you mention but I do know something about
decoupling.

THere are a LOT of wise guy statements around -all bases on some idea about
how the currents will flow
in the power system of the layout.
However, at Univ. Missouri ROlla, Prof. Todd Hubing and his team built 100ds
of test boards
and performed accurate meassurements to se where the facts where.
I urge you to take a look at his work.
Try http://www.emclab.umr.edu

Another good source of knowledge in this area is Lee Richey of Speeding
edge.
You reach him on [log in to unmask] -He works with consulting in the
high speed design area and
has bublished a lot of papers regarding this very topic.

Just to quickly summarise as an answer to your question:

It is NOT critical WHERE you place the decoupling  caps!

This thing with placing them close to the power (or ground pin ) is BULL!
 It doesn't work that way.
It is more important to use the right type of cap and to connect it to the
power/ground planes in a manner
that gives low inductance (via directly in the SMD pad is ideal but a short
lead will not kill you.

(I'm sure I've stirred up some feelings now. It would be interesting to hear
about  the experience from the list)

I can assure you that the decoupling effect will not vary much if you have
to place the caps a bit away from the circuit.
-Some enngineers might object but then you should ask 'Why?' and I can
guarantee you that the answer would be:
'If you don't place them at 'bla bla bla' then I won't guarantee
anything' -Well, that is not a statement
that will give your company a competitive edge is it?
The fact is that most engineers don't know much about physical design
(PCB/MCM) and even less about high speed
design. -Unfortunately this is also valid for MANY authors of Application
Notes from the circuit mfgs.
-Don't buy all whats written the app notes and don't buy all what the
engineers tells you.

Hope to have been to some help.

/Per

  --------------------------------------------------------------------------
-------------------
   Per Viklund                                  mailto:[log in to unmask]
   Technical Mgr.
   Dansk Data Elektronik AS          http://www.dde-eda.com
   -------------------------------------------------------------------------
-------------------

-----Original Message-----
From: Bob Vanech <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: Friday, June 18, 1999 11:49 PM
Subject: [TN] Escape path and decoupling for 225-Ball BGA (fwd)


>  To All..
>           Has anyone out there in Technet world have
>some hands on experience with Analog Devices' 225-ball
>BGA ADSP-21062. This part is being used on a multilayer,
>components both sides, fine pitch design. Since all the
>power and return balls are in the center of the package
>and the decoupling caps must be right at the power ball
>locations, I am wondering what is the best approach for
>capacitor locations and escape routes.
>            I think that I might have to use the 0402
>caps because of spacing, but the assembler would prefer
>the 0603. Since every pin-out is used, I cann't delete
>any signal pads, I am playing with ganging up the power
>and return pads, sharing vias and using the open area
>for the caps. That said, has anyone designed this part
>into your design and would you be willing to discuss your
>pro/con on the design approach. Thanks in advance and
>have a great day.
>                     Regards,
>Bob Vanech                     Bob
>Mango Computers
>Norwalk, Ct.
>(203) 857-4008
>x-108