[SI-LIST] Re: AC coupling Capacitor

Nitin,

Regular surface-mount DC-blocking capacitors will create discontinuities in the 
differential lines, because most of our traces today are much narrower than the 
capacitor pad and body.  The typically strongest discontinuity is the parallel 
body capacitance to ground.  If you use two capacitors along eachline, you get 
two discontinuities, so do it only if there is a strong reason.  

Regarding their location along the lines: as a first-cut approach, their 
location should not (and does not) matter, you just want to place the 
capacitors on both lines at the same location symmetrically so that mode 
conversion is minimized.  If you (have to) take a closer look, the location of 
the discontinuity along the line may matters somewhat as it interacts with the 
losses and non-ideal impedances of the lines and transmit/receive impedances.  
If this matters, you need a simulator to find the answer.

Regards,

Istvan Novak
SUN Microsystems

From: jain.nitin@xxxxxxxxx
Date: Thu May 25 06:03:58 CDT 2006
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] AC coupling Capacitor


 Hello,

For a gigabit differential pair AC coupling capacitor is used. But what
exactly is the use of AC coupling capacitors? I believe that they are
used when the bias voltage of driver & receiver are different.=0D

Also where exactly AC coupling capacitors should be placed when the
differential trace is running on a backplane from one daughter card to
other (at driver end or at receiver end).

What will happen if two AC coupling caps are used. One at driver side
and other at receiver side.

With regards,
Nitin


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