[SI-LIST] Re: Better choice for AC Coulplilng(calrification)
- From: "Ingraham, Andrew" <Andrew.Ingraham@xxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Mon, 23 Dec 2002 16:35:08 -0500
Bob wrote:
> These blocking caps are often utilized to prevent voltage spikes from
> damaging inputs when you hot swap modules.
I am curious how these AC coupling capacitors prevent voltage spikes
when you hot swap modules?
On the contrary, I can see that adding such capacitors could CAUSE a
voltage spike where none exists without the cap. Can you help me
understand how adding a cap can do the opposite?
On the other hand, if series caps are already used for the purpose of AC
coupling, and you want hot swap as well, you do need to be careful about
where the caps are with respect to the hot swap interface and
pull-up/pull-down resistors, so that you don't cause a voltage spike
when inserting/removing cards.
From a high frequency point of view, I think the fewer elements in the
signal path, the better. Each cap adds a potential resonant trap; or at
minimum, a frequency dependent series element, plus shunt capacitance
from vias+pads to planes. Also you could have an unfortunate situation
where the length of the line between the two capacitors is resonant at
the signal frequency or its harmonic.
If you need AC coupling caps on a point-to-point net, and things like
hot swap are not a concern, then I don't see any benefit to using two
caps in series.
There may be no particular advantage to putting the caps at the source
end or the load end or elsewhere, when hot swap is not a factor; but
each case will have different trace lengths and one might be better than
the other for avoiding resonances.
Regards,
Andy
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