RE: [SI-LIST] : High Speed AC Coupling

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From: Zabinski, Patrick J. ([email protected])
Date: Tue Oct 31 2000 - 18:15:29 PST


You're probably right about a multilayer chip cap operating in the
"inductive" region at these frequencies. However, this is not
necessarily a show stopper. What's more important for AC coupling
applications is that the impedance magnitude is "low" relative
to the termination impedance. Most commonly, 50 ohm lines are
used, and I can generally get away with staying 1/10th of that,
so as long as the cap's impedance magnitude is less than 5 ohms,
you should be okay (regardless if it is in the capacitive or inductive

I don't have convenient access to one of the shareware routines
on cap parasitics (AVX, Kemet, and ATC have good ones), so I'll
leave the exercise up to you. However, keep in mind that it is
much more important to keep the cap's impedance lower than the
line's characteristic impedance (in this application) to keep
a handle on reflection coefficient.


> I am designing with an optical transponder that is designed
> to accept a PECL
> level electrical signals at 2.5GB/s. The driver outputs CML level
> electrical signals. I have to AC couple the signal to make
> the voltage
> levels compatible. The vendor said any old 0.01uf capacitor
> would do, but I
> can't beleive this. I would expect a general purpose
> capacitor at 0.01uf to
> become increasingly inductive past 1GHz already. I would
> expect that the
> capacitor would have to be good out to 12GHz as to not
> attenuate the first 5
> harmonics. Does anyone out there have a good suggestion? I
> have seen some
> very wide band DC blocks. What, if any, is the difference
> between a DC
> blocking cap, and a AC coupling cap?
> Thanks
> Alex

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