piclist 2000\06\06\224027a >
Thread: Filter Capacitors
www.piclist.com/techref/logic/dsps.htm?key=filter
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face BY : David VanHorn email (remove spam text)



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>I've read that the long tiny leads on the caps in this case
>represent such a large inductance that this kind of socket is
>quite useless at higher frequencies.

Yes.


>Interesting observations here. But this makes it sound like going to
>"ground planes" would be the worst thing imaginable. I've read in the books
>that narrow buss traces might cause a problem regarding "return currents"
>for the "signals" routed from one chip to the next. Narrow traces -->
>larger inductive loops --> more EMI/noise transmission.

You're forgetting the bypass. You want the ground fat, and from the chip to
the cap fat, but track from the cap to the rest of the system thinner. The
loop in this case is very small.

{Quote hidden}

Narrow VCC, up to the bypass. Fat from bypass to chip, fat, and
interconnected ground.

Watch interconnection, it can bite too.  I had a thermal printer that took
pulses of 19A 300uS wide. I did NOT let it's ground return run through my
processor logic in a plane.
Can you guess why? :)   I ran separate co-planar power and ground tracks,
200 mils wide to the head. The ground track was isolated from the plane
until the point where it hit the supply caps for the printhead power.

The PH designer had a clue too, they provided power ground and logic ground
on separate pins.

It's a science, but there's some art in it too :)

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