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TUSB542: AC coupling capacitor on TXAP and RXAP

Abhishek singh
Expert 2000 points
Part Number: TUSB542
Other Parts Discussed in Thread: TUSB8044A,

Hi, Team we are using  TUSB 542  for making  connection between the USB type C connector and  TUS8044A hub downstream port/. As per TUSB 542  datasheet, i  should put  AC  coupling  capacitor on both  TX  and  RX pairs, as per the TUSB8044A  datasheet we need to AC  coupling capacitor on their  TX  line.

Please confirm only one capacitor on each pair is okay. How  we  should place  the  AC  coupling capacitor during  placement  . 

  • 0
    TI__Guru**** 177396 points

    Hi,

    The hub datasheet is written assuming the hub is connected directly to the USB connector. 

    When connecting between the TUSB8044A and TUSB542, AC coupling caps need to be placed on both the SSTX and SSRX. The AC coupling caps need to be 0.1uF or 0.22uF for both the TX and RX.

    Between the TUSB542 and the USB connector, caps are required on TX and optional on RX. For TX, it is recommended to be 0.22uF and for RX, it is recommended to be 0.33uF.

    Thanks
    David

  • 0 in reply to David (ASIC) Liu
    Expert 2000 points

    Hi David  

    Thanks for your reply. Which capacitor we should place near the HUB and which towards TUSB542.

  • 0 in reply to Abhishek singh
    TI__Guru**** 177396 points

    Hi

    Please see the block diagram. The caps can be placed in the middle. 

    Thanks

    David

  • 0 in reply to David (ASIC) Liu
    Expert 2000 points

    Hi  David  

    Thanks for your reply. You can close this issue now. 

    But  I have one general query not related to any TI parts. Nvidia guidelines say I should use 0.22 UF to 0 .1uF  ( depending on speed for 10 Gbps 0.22uF)and TI  guidelines say 0.22UF. What will happen if I use less than a 0.22uF cap? Which parameter limits the minimum value of the capacitor. 

  • 0 in reply to Abhishek singh
    TI__Guru**** 177396 points

    Hi, 

    Whenever you pass a signal through any AC-coupled circuit there is always some tiny part of the signal that gets filtered out. This filtering effect induces a tiny difference between the informational part of the signal before and after filtering. In severe cases this difference manifests itself as the phenomenon of DC wander. This DC wander manifests itself as an additive noise and is proportion to the size of the AC coupling caps. 

    For more detail, I would refer to this IEEE paper: .

    Thanks

    David