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DS80PCI102: AC capacitor placement in PCIE redriver

Francesco Focardi
Prodigy 20 points
Part Number: DS80PCI102 
Hello everyone,
I am currently designing a card with a 1x PCI Express connector. The card can be mounted inside a computer directly or through a 1m express PCI extension cable.
On the board an FPGA will have to communicate with the CPU through a pci express 1x GEN2. I have to use the DS80PCI102 product as a redriver for pci express signals.
My concern is about the position of the AC capacitors for both links (redriver <-> FPGA and redriver <-> motherboard).
The datasheet clearly states:  "The AC coupling capacitors should be placed closer to the Rx on the channel". This clashes with what I've read in many application notes and with what I've done in past projects. 
I always use a single pair of AC coupling capacitor per link placed near the transmitter device.

My idea is to put AC caps in these positions:
  • TX from Redriver to PCI-e connector: in the middle of this two components (connector and redriver are about 1 inch away).
  • RX to Redriver from PCI-e connector: Like above. I think that the motherboard already have her owns capacitor but i believe they are necessary becouse hot-plug issues.
  • TX from Redriver to FPGA XCVR: near redriver.
  • RX to Redriver from FPGA XCVR: near FPGA.

Do you think I'm doing the right thing? Am I missing some "rule" about AC capacitor?

Thanks for your support.

  • 0
    TI__Mastermind 43795 points

    Hi,

    There needs to be an AC coupling capacitor in each Rx and Tx signal.  As a rule in PCIe the AC coupling capacitor will be placed on the same PCB as the Tx.

    On the motherboard:

    There should be capacitors on the CPU Tx line going to the Rx redriver.

    On board with redriver and FPGA:

    Place capacitors on the redriver Tx line going to the CPU.  Placing between connector and redriver Tx is good.  Use 0402 size capacitor.

    Place capacitors on the FPGA Tx going to the redriver.  Ok to place anywhere between parts - recommend >1 inch from Tx if possible.

    Place capacitors on the redriver Tx going to the FPGA . Ok to place anywhere between parts - recommend >1 inch from Tx if possible.

    Regards,

    Lee 

  • 0 in reply to Lee Sledjeski
    Prodigy 20 points

    Hi Lee, thanks for the answer.

    I know there should be a capacitor (only one capacitor) for each line. I'm trying to figure out which place is the best, along the line, to place AC capacitor. Near TX? near RX? In the middle of both, as you state?

    So what does the datasheet refer to when it reports "Place AC-coupling capacitors near the receiver end of each channel segment to minimize reflections"? Reading your suggestions it does not seem that placing the capacitor near RX is a good idea.

    Is the datasheet wrong, or inaccurate, this time?

    Thank you again.

    Francesco

  • 0 in reply to Francesco Focardi
    TI__Mastermind 43795 points

    Hi Francesco,

    Placing the capacitors closer to the receiver end of a channel is ideal.  Placing them right next to the Tx is not ideal as it is more likely to cause reflections on the transmission line.  If possible keep the capacitors close to the Rx side and definately keep them away from the Tx pins. 

    Regards,

    Lee