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

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From: George_Tang@Dell.com
Date: Tue Oct 31 2000 - 18:15:56 PST


Alex,

All current paths in real circuits have inductance. That include the
capacitor and the trace. If you replace the capacitor with a continuous
trace, most likely, the trace will give you more inductance than the
capacitor, depending on the trace width. It is that inductance (for the
trace) that you use to calculate the trace impedance (Z = sqrt(L/C)). The
capacitor has inductance, and it also has parasitic capacitance to ground,
so you can determine the impedance, Z, for the capacitor. The 0.01uF cap
will work fine. But if you want to see sharp rising edges, you might want
to consider some capacitors with low loss dielectrics and low parasitic
capacitance to ground.

George

-----Original Message-----
From: Golian, Alexander [mailto:agolian@corvis.com]
Sent: Tuesday, October 31, 2000 1:45 PM
To: si-list@silab.eng.sun.com
Subject: [SI-LIST] : High Speed AC Coupling

Help!

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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