>I have several sources (including the Motorola FACT book) that say bypass
>caps should be placed as close as possible to the GROUND pin of the IC (not
>the power pin.) But I have seen few really specific discussions of why this
>is so.
>
>Some of my customers insist on having them placed as close as possible to
>the power pin.
>
>Can anyone give me a definitive explanation of where they should be placed,
>and why?  Thanks
>
>Doug Brooks
>
>

Bypass caps are used to filter and maintain a steady voltage to the device
they are near. Putting them near the power pin is a safer bet than the ground
pin. If they are near the ground pin or just in the vicinity of the device
they a just doing the job of general filtering. Using different values
and or different types of caps betters you chances of filtering out unwanted
noise. It depends upon the frequencies in the design as to which types of
caps should be used. ie: ceramic, alum, etc., etc.

        You can also say that your PCB is a CAP. The closer together your
power and ground plane the more capacitance. This can be calculated by your
board shop, and be factored in on your decision to use caps or not to (or
just on specific parts ie: memory). I have done this and have saved not only
room on my designs but ost as well. The cost of an average bypass cap after
it has been placed is usually about 7-8 cents.

Regards,

Jerry Schwartz
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"May The Schwartz Be with You"