🚀 go-pugleaf

In article <7480126@hpfcso.FC.HP.COM> myers@hpfcso.FC.HP.COM (Bob Myers) writes:
>>     The answer is THEY must go between VCC and GROUND on the SAME
>>     chip.  The reason for a despiking cap is the momentary
>>     current rush caused by switchIno@in DRAMs and some other
>>     chips.  The cap supplies the current needing for these chips
>>     and prevents them from degrading the powersupply.  By
>>     placeing them not Immediatly to the chips ground you are
>>     totally defeating thier purpose.
>> 
>
>No, they do NOT need to go on the same IC if that means longer cap leads
>than simply tying to the nearest ground point.  Assuming a board with
>power/ground planes, or a power/ground "grid" as Henry mentioned (easily
>"do-able" on a wire-wrap board, this may mean that the caps tie to power and
>ground *between* adjacent ICs.  Distributing the capacitance across the
>power plane is the key, not tying the caps to a given chip.
>
>
>Bob Myers  KC0EW   Hewlett-Packard Co.      |Opinions expressed here are not
>                   User Interface Tech. Div.|those of my employer or any other
>myers@fc.hp.com    Fort Collins, Colorado   |sentient life-form on this planet.
Actually, you're both right. The fact is, it depends on several factors. Yes,
in many cases anywhere in the neighborhodd on the power/ground planes is close
enough, but for really high frequency / high current / low noise applications,
you have to do better than 'close enough'.  A typical arrangement would be to
have the cap adjacent to the VCC lead of the IC, connected by a VERY short 
trace. The ground lead from the chip is then connected to the other side of the
cap, and then to the ground plane. By doing this, the cap is placed as close as
possible to the point where the chip is drawing current out of the power 
distribution network.

Another point to ponder is that there are resonances to consider. Almost all
the time this can be ignored, but, IMHO, a resonance in the power system is
one of the biggest pains in the butt to nail down.

Phillip
--
Phillip M. Vogel, President             | "I like pussy, but I don't like Bush"
Bartal Design Group, Inc.               | Domain: phillip@bartal.com
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