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Registered: ‎09-06-2019

AC vs DC coupling (DVI-D/HDMI RX)


We're looking to incorporate the TMDS181 HDMI retimer/redriver into our design much like the ZCU106. One question we are having trouble answering is whether to AC couple incoming stream signals (pairs of DATA0, DATA1, DATA2, CLK) or to leave them DC coupled.

The ZCU106 utilizes the AC coupling capacitors on the line while the example in the TMDS181 datasheet does not use this scheme. We plan to implement our design similarly to the vcu_hdmirx example design provided in the 2019.1 VCU TRD in which these signals are routed to the video phy IP core. 

Do the signals have to be AC coupled when arriving to the Zynq Ultrascale+? My intuition tells me they must since the incoming DVI stream is DC coupled running at 5V like a typical DVI source in addition to the fact the these particular GT pins 

Thanks in advance!

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Registered: ‎07-09-2009

You need to look at the HDMI / DVI-D specification,

As I remember, the spec says capacitors at both the Tx and Rx ends.

Having said that,
Ive seen more than one design where the capacitors at the Tx are not present, just relying on the capacitors at the receiver.

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