[SI-LIST] Re: Cap discontinuity

  • From: "Daniel, Erik S., Ph.D." <Daniel.Erik@xxxxxxxx>
  • To: mittalr@xxxxxxxxxx
  • Date: Thu, 16 May 2002 14:32:48 -0500

Rohit-

Ideally, the capacitor provides a capacitance in series which you *do* want
(presumably for AC coupling), but the large pads provide additional shunt
capacitance to the ground or power reference plane which you *don't*
typically want (can contribute to reflections, RC roll-off, etc.).  Yes, a
"real" AC coupling cap will have inductance and other nonideal features
which may also contribute to SI problems, but there is a good reason for
wanting to keep the pads as small as possible relative to the size of the
trace to minimize the additional parasitic shunt capacitance.

Ray's idea of adding the additional pad capacitance into your simulations
sounds like a good one to me.

                                        - Erik

==================================================================
Erik Daniel, Ph.D.                   Voice:  (507) 538-5461      
Mayo Foundation                      Fax:    (507) 284-9171
200 First Street SW                  E-mail: daniel.erik@xxxxxxxx
Rochester, MN  55905                 Web:    www.mayo.edu/sppdg/
==================================================================


> -----Original Message-----
> From: mittalr@xxxxxxxxxx [mailto:mittalr@xxxxxxxxxx] 
> Sent: Thursday, May 16, 2002 12:41 PM
> To: jian@xxxxxxxxxx
> Cc: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Cap discontinuity
> 
> 
>  Thanks for your comments, Jian.
> 
> However, I still don't know the answer - does the width of 
> the pad versus the trace matter. I know the cap will cause 
> discontinuity, as you mentioned below. Can we safely assume 
> that the discontinuity from the pad is much less than the the 
> cap. I can't find a simple simulation to do to verify.
> 
> Thanks
> 
> 
> 
> 
> --- On Thu 05/16, Jian X. Zheng  wrote:
> > Hi, Rohit:
> > 
> > I would like to make some comments on this subject.
> > 
> > From electromagnetics, any discontinuities in a uniform 
> transmission line
> > will create the reflection in the transmitting power. The reflection
> > normally is higher and higher when the size of the 
> discontinuities is
> > getting larger and larger compared to wavelength.
> >             -----
> > -----------/     \--------------
> > -----------\_____/--------------
> > incident   A     B
> > 
> > At low frequency, the difference between the pad size and 
> the TLN may not
> > cause much reflection because the reflection from A and the 
> reflection
> > from
> > B are out of phase and they will cancel each other when the distance
> > between
> > the A and B is small. However, the reflection will become 
> higher when the
> > distance between A and B is no longer very small compared 
> to wavelength.
> > Thanks!
> > 
> > Best regards,
> > 
> > 
> --------------------------------------------------------------
> ---------
> > Jian-X. Zheng, Ph.D
> > Zeland Software, Inc., 48890 Milmont Drive, 105D, Fremont, CA 94538,
> > U.S.A.
> > Tel: 510-623-7162, Fax: 510-623-7135, Web: http://www.zeland.com
> > 
> ---------------------------------------------------------------------
> > 
> > > -----Original Message-----
> > > From: si-list-bounce@xxxxxxxxxxxxx
> > > [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of 
> mittalr@xxxxxxxxxx
> > > Sent: Thursday, May 16, 2002 9:00 AM
> > > To: si-list@xxxxxxxxxxxxx
> > > Subject: [SI-LIST] Cap discontinuity
> > >
> > >
> > >
> > > Lot of books/app notes mention that if you have high speed lines,
> > > say 10G then you should make the pad size of the ac decoupling
> > > cap the same width as the trace.
> > >
> > > Can anyone give the reason why? It seems if you put a cap in a
> > > 10G line, there will be reflection anyway because the Z of the
> > > cap is not the same as the Z of the line. Then how much effect
> > > does matching the width of the pad to the line have anyway
> > >
> > > Thanks
> > > Rohit
> > >
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