[SI-LIST] Re: AC Termination Question

Doug,

I've used AC termination on clock signals as a cheap and dirty way to =
mimic
the advantages of a parallel termination to Vtt, without the need for a
seperate supply or regulator, where Vtt is the average voltage of the
signal.  You can get a nice balanced termination with minimial DC =
loading.
The reason it's suggested only on constant duty cycle and preferrably
nominal 50% duty cycle waveforms is that the effective Vtt moves up and =
down
with the average voltage level of the waveform, thereby changing the
dynamics of the termiantion. As you drift to one side or the other your
termination becomes non-symetrical. How much of this non-symetry is
tolerable is dependent on the application.=20

The size of the cap determines how much stable the effective Vtt =
termination
voltage is. So you have to do an RC time constant calculation and make =
sure
the Vtt voltage on the cap does not vary more than your desired range =
during
the time period of the terminated signal, and that the termination =
stays in
play during the transistion period.  I'm not sure I have the optimal
approach but I generally use a rule of thumb that the RC time constant =
of
the AC termination be on the order of 4 or 5 times the signal half =
period. A
typical termination on a 133MHz clock might be 50 ohms into a 270ps cap =
for
example. =20

Brian P. Moran
Signal Integrity Engineer
Intel Corporation
brian.p.moran@xxxxxxxxx


-----Original Message-----
From: Doug Brooks [mailto:doug@xxxxxxxxxx]
Sent: Thursday, June 27, 2002 9:18 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] AC Termination Question


 Gurus,

I read somewhere that AC Xline termination (e.g. 50 Ohm line with 50 =
Ohm R
inseries with a cap) required a more-or-less equal bit stream (equal =
number
of ones and zeros) so as not to charge up the cap. Now I'm not sure I
believethat.

Two Questions:

1. For a 50 Ohm RC terminated transmission line (for example) are there =
any
cap values that are better than any others?  If so, why?

2. Is AC termination effective for any bit stream? If not, why not?

Thanks --- Doug Brooks



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