Re: Expansion board decoupling specs

[Ivor Bowden <ivor@peritek.com>]

Dimiter,

Not a good idea. The decoupling requirement doesn't have anything
to do with power, it has to do with signal integrity. You notice
how the connector is laid out that signals are paired with power
pins? ALL the power pins are necessary as AC grounds. And if they
aren't connected to anything, just antennas hanging out open ended,
they aren't going to be of any use as AC grounds. There is a reason
for the spec, follow it.

Craig,
I think it would take quite a bit of plane area, along with controlled
spacing and dielectric to make sufficient capacitance. Using 0603 (or
0402!) smt caps shouldn't hurt routing much. Note you don't have to
bus those unused power pins together as long as you bypass each one.
Best of luck.

-Ivor

At 02:46 AM 9/17/99 +0300, you wrote:
>Craig,
>
>> In section 4.4.2.1, there is a statement regarding decoupling of unused
>>power pins to ground via 0.01uF per Vcc pin. On our board we are only
>>using the +5V power pins, and thus have many unused power pins (all 3.3V
>>and all VI/O pins).
>
>Personally I would follow common sense and ignore this part of the spec.
>What if the slot is empty? Will the (unused) power lines be more
>decoupled?
>
>Dimiter
>
>Dimiter Popoff
>Transgalactic Instruments, Gourko Str. 25 b, 1000 Sofia, Bulgaria
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