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Registered: ‎08-09-2013

DC Coupling on the GTH (KU040 to KU040)

Dear High Speed Experts.


Our Board design is a dc coupling between GTHs on the board (KU040 to KU040, Same FPGA).

I think that there was no problem between Same FPGA with DC coupling.

However, I heard that there are problem in case of DC coupling even though same GTH using.


How to set the termination when we are using dc coupling between GTH and GTH.


Thanks and Regards,



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3 Replies
Registered: ‎07-09-2009

Re: DC Coupling on the GTH (KU040 to KU040)

The correct answer, is to simulate, IBIS etc.


why did you decide not to ac coupled ? 


if both chips are on the same board, with the same power, then you should be ok,

    just treat it as normal, on chip termination,


though the power might be a little higher, 


if the chips are not on the same power supply or board, then you are in a 'different world' of hope and pray as any offset on the power supplies or ground noise, is going to be imposed on the serdes signal.


<== If this was helpful, please feel free to give Kudos, and close if it answers your question ==>
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Registered: ‎02-24-2014

Re: DC Coupling on the GTH (KU040 to KU040)

There is an answer record which recommends these GTH settings:


For GTH Use Mode 3, set:
1. RX_CM_SEL[1:0]  to 2'b11 (programmable)
2. RX_CM_TRIM[3:0]  to 4'b1010 (800 mv)
3. RXDFEAGCTRL.[4:3] to  2'b10


But you should check this against which protocol you are using.   Check this answer record for your protocol.

Don't forget to close a thread when possible by accepting a post as a solution.
Xilinx Employee
Xilinx Employee
Registered: ‎11-29-2007

Re: DC Coupling on the GTH (KU040 to KU040)


unfortunately in the IBIS-AMI simulation only the differential signal is simulated and there is no way to set the RX termination bias.

We find DC connection principally in cases of

  • schematic errors
  • oversampling or when the DC level should be transmitted
  • difficult layout with no room for capacitors

DC connections has many drawbacks, in particular the absence of a protection against DC currents. Currents outside the Datasheet specification will damage the part. Plug in/out and TX/RX power sequences become a concern.


In the rare cases where DC connection is used, typically the TX drives a common mode of 0.7-0.9V and the RX bias is floating. Because the signal common mode is related to the signal swing, DC connection limits the flexibility to tune the transmitter for a proper signal quality at receiver.