RE: [SI-LIST] : series capacitor placement

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From: Prasad Venugopal (Prasad_Venugopal@pmc-sierra.com)
Date: Wed Apr 11 2001 - 13:08:14 PDT


My 2 cents

 1 cent.
 PRO: put it at the tx

 Assume that the configuration is as shown below and also assume that all traces are stripline, without buried vias or any other structure that reduces capacitive and inductive parasitics, but costs more.

 driver--via---via-cap-via----via--connector--via ------backplane----via--connector-via---via--receiver
where ------ is a transmission line.

 in this case if the driver is matched to the transmission line then any reflections off the ---via-cap interface will be absorbed by driver.

 if you change the above config to
 driver-via-----via--connector --via ------backplane-----via--connector--via----via-cap--via----via--receiver
 
 then any reflections at the ----via--cap will be re-reflected by the connector--via------ interface and depending on how it interferes with the signal will cause the receiver to see the effects of the reflections.

 1 cent.
 CON: do not put it at the tx

 If you put it at the driver, since the edge is very sharp, then the ----cap discontinuity generated reflection, will be absorbed to the extent that the driver matches the transmission line.

 If you put it at the receiver depending on the length of the transmission line the edge has already been softened, therefore, the reflections at the ----via---cap interface in the second case shown above will not be very bad.

 
 I think these effects become particularly evident as speeds reach 2.5 Gb/s, 400ps eye opening. SerDes devices from many vendors require ac coupling between rx and tx.

 BOTTOM LINE: model, simulate, build, measure.... :-)

 I look forward to comments and experiences from others.

 Thanks.

-----Original Message-----
From: Saeen Malik [mailto:saeens@hotmail.com]
Sent: Wednesday, April 11, 2001 11:59 AM
To: si-list@silab.eng.sun.com
Subject: [SI-LIST] : series capacitor placement

Hello Gurus,

I have another question. How critical is the placement of the
series capacitor. Some people think it is critical to place them
close to the load while others think the placement is really not
critical. What do you people think and is there a quantatative reason????

Saeen
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