[SI-LIST] Re: AC coupling placement
- From: Vinu Arumugham <vinu@xxxxxxxxx>
- To: benrothchild@xxxxxxxxx
- Date: Fri, 01 Feb 2002 14:20:47 -0800
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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