Location of AC coupling caps on multi-board design

Thread Starter

sp2821

Joined Jan 8, 2014
23
Hi all,

I am working on a multi-board design where a simplified block diagram is shown below. The FPGA's transceiver block uses 0.95VDD and PHY uses 3.3VDD. So I need to use AC coupling caps to route SGMII between the FGPA and PHY.
1605555005961.png
I've seen multiple app notes about location of the AC coupling caps (TX side, RX side, or anywhere) but none of them provide any theoretical explanation. The FPGA's datasheet says it's ok to put the caps anywhere.

In summary I think there are 4 possibilities of placing AC coupling caps.
I'm sure each option has its pros and cons. Which way is most desirable way?
1605555382375.png
 

Irving

Joined Jan 30, 2016
2,311
From what I've read, SGMII implements a transmission line in each direction. My gut feel is the capacitors should be at the Tx end of the line to minimise reflections and make line matching easier, though the documentation is unclear on this...
 
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