From: Degerstrom, Michael J. (email@example.com)
Date: Tue Apr 17 2001 - 14:46:14 PDT
You are AC coupling the output to the input. Make
sure that the output is 'back-terminated', i.e., that
is has 50 ohm resistors tied to the positive supply
on the chip itself.
Your CML is probably back-terminated. So here is another suggestion.
Transient simulations are difficult with large AC coupling
capacitors. You have to run the simulation for a long
time for the proper biasing to be seen at the input. Once
things have settled out then you can begin to analyze your
results. I think the 'RC' time constant will be about
2*Zo*C where Zo is the impedance of your interconnect
and C is your AC coupling capacitor. You probably want
to run for 2 or 3 time constants depending on your application.
By the way, I often setup DC biasing that switches off shortly
after the transient simulation begins. This way I don't have
to wait for transients to settle out.
Mike Degerstrom Email: firstname.lastname@example.org
Mayo Clinic; 200 1st Street SW ; Rochester, MN 55905
Phone: (507) 538-5462 FAX: (507) 284-9171
> -----Original Message-----
> From: Sreejith Varma [mailto:email@example.com]
> Sent: Tuesday, April 17, 2001 1:08 PM
> To: firstname.lastname@example.org
> Subject: [SI-LIST] : 2.5ghz simulations
> HI All,
> i am doing a differentail pair simulation using Cadence allegro
> tool. as it is diff. pair i am doing the post route sim on the
> allegro itself.
> it is a 2.5ghz signal (a clock). it is a CML interface. when i do
> the simulation, i see the differential signals, both input and output
> DC shifted, quite apart.
> the topology is something like this
> tx +/- ---> ac_coupling capacitor (10 nF) ---> trans. line ---> rx +/-
> the cml inputs are internally biased and terminated. when i run the
> rise time sim. the tool reports that the receiver doesnt meet the
> thresholds(2.2 v of vih and 1.76v of vil). what i see is a waveform
> of 500 mv peak to peak biased around 3 v (+ sig) and 2 v (- sig).
> another observation is that the rising waveform as per the IBIS model
> is not observed at the output pin, this also dc shifted and
> quite different
> in rise time than in IBIS model..
> any body have any idea what could be going wrong? or i am
> doing something
> grossly wrong !!!!!
> thanks in advance..
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