[SI-LIST] Re: AC coupling placement
- From: "Perry Qu" <perry.qu@xxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Mon, 04 Feb 2002 11:46:24 -0500
Hi!
I found this thread interesting. We placed series cap either near driver or
receiver, so that we introduce minimum number of vias on the series link,
e.g., we may be able to share vias for series cap with vias for
termination/biasing resistors.
Regarding value of capacitance, I did some simulation in frequency domain
and I can see that by using larger caps, we basically allow more lower
frequency to pass, compared with a smaller cap. In this case, the series
cap act as a high pass filter and the cut off frequency is determined by
the cap value. I see many cases where 10 nF or larger cap is used. In my
simulation it shows that 10 nF pass through most of the frequency component
above 465 kHz (0.5 dB loss); whereas a 470 pF cap pass through frequency >
95 MHz . Ideally series cap should only block DC component and let all
other frequency pass. That's why we see > 10 nF is used more frequently.
In time domain simulation, I have not seen major differences between large
or small caps, although the convergence time for 10 nF is much longer and
takes a lot of simulation time. If anyone have any idea about shortening
the convergence time in SPICE, I will be very intereted to know.
Regards
Perry
Vinu Arumugham wrote:
> Ben Rothchild wrote:
>
> > Vinu,
> >
> > Thanks for the input. I forgot to mention
> > in my last mail that all these links are
> > differential PECL. Any idea what is run length
> > for 155-STM1 and 622-STM4.
>
> I am not familiar with these transmission standards.
>
> > The 1.25G and
> > 2.5G are 8B/10B coded so run lengths should not
> > exceed 8.
> >
> > The value recommended for the AC coupling cap
> > is 0.01uF. Now with an ESL of say 4nH
> > we have w=1/(0.01uF X 4nH)^0.5=500Mhz
> > for the case of series resonance,
> > or f=79Mhz. How is this going to work
> > for even 155M leave alone 1.25G/2.5G.
> > I seem to have got something wrong. Could
> > you or somebody correct me.
> >
>
> At 2.5Gbps, the series impedance is dominated by the mounted inductance
> of the capacitor. While the impedance of the capacitor is not at its
> lowest at this frequency, with proper layout/stackup, it will be low
> enough to work at 2.5Gbps.
>
> >
> > Thank you once again,
> >
> > Ben
> > --- Vinu Arumugham <vinu@xxxxxxxxx> wrote:
> > >
> > > Usually the AC coupling capacitor would need a
> > > couple of vias to connect
> > > to the lines. Reflections from this structure can be
> > > lowered by placing
> > > it near the receiver where the edge rate is slower
> > > than at the
> > > transmitter.
> > >
> > > If you do not have very long run lengths (>100
> > > bits), a 0.01uF will
> > > work.
> > >
> > > Thanks,
> > > Vinu
> > >
> > > Ben Rothchild wrote:
> > >
> > > > Hello Experts,
> > > >
> > > > I have a question on AC coupling.
> > > > While AC coupling a high speed serial
> > > > link to match levels, where should
> > > > the AC coupling capacitor be placed.
> > > > Close to transmitter/close to receiver/
> > > > or doesn't matter, can be placed
> > > > anywhere on the link.
> > > >
> > > > What should be the value of the AC
> > > > coupling capacitor or rather how
> > > > do I calculate it. Is it dependent on
> > > > frequency. I am trying to AC couple
> > > > signals of 155M, 622M, 1.25G and 2.5G.
> > > > Any special care to be taken at these
> > > > different frequencies.
> > > >
> > > > Thanks for any advice.
> > > > Ben
> > > >
> > > > __________________________________________________
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> > > >
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--
Perry Qu
Product Integrity | 600 March Road
Alcatel Canada | Ottawa, ON K2K 2E6, Canada
DID: (613) 7846720 | FAX: (613) 5993642
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