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SN75LVPE4410: AC Coupling Capacitors

Gillian Lee
Prodigy 25 points
Part Number: SN75LVPE4410

We want to add a redriver for the PCIe signals going between two card modules via a backplane. In the top half of the attached PDF block diagram, we were able to connect the PCIe channels between the cards without redrivers and without any additional capacitance other than on what is already mounted on the left-side card without any major issues (so far) on a much smaller prototype.

When incorporating the SN75LVPE4410s to redrive, what is the recommendation for where to place any AC coupling capacitors? Currently, I have them only on the "right" side of the redrivers (i.e., on the TX/RX going to the right-side card, marked in red in the bottom half of the PDF), but I'm not sure if that's correct since the SN75LVPE4410 specifies that they should be placed close to the RX pins. Should only the AC caps on the TX pins be specified >1in away from the pin, while the RX pins should be as close as possible?

Additionally, the switch specifies 100nF capacitors to be placed on every TX and RX line, whereas the SN75LVPE4410 recommends 220nF. Is having a capacitance mismatch problematic? 

 pcie redriver.pdf

Thanks for your help,

Gillian

  • +1
    TI__Guru* 93290 points

    Hi Gillian,

    It is best to put the AC coupling Caps close to the receiver or a connector. This is because putting these Caps close to the TX would degrade or cause reflections for the TX side.

    220nF Cap value is the preferred value. This allows more signal DC content.

    Regards, Nasser

  • 0 in reply to Nasser Mohammadi
    Prodigy 25 points

    Hi Nasser,

    Thanks for the response! To clarify, I should add additional 200nF capacitors on the "left" and "right" of the redriver on the backplane? If I add them on the left side, should I be concerned about the decrease in total series capacitance on the left side?

  • +1 in reply to Gillian Lee
    TI__Guru* 93290 points

    Hi Gillian,

    In PCIe applications, Caps are placed on TX side only - not on the RX. Also, in layout this should be put close to the receiver - not the transmitter.

    Regards, Nasser

  • 0 in reply to Nasser Mohammadi
    Prodigy 25 points

    Hi Nasser,

    Since we can't change the left or right cards because they've already been designed, is the recommended course of action to do the following:

    1. For the left-to-right redriver:

    a) Add 220nF capacitors on the TX side of the redriver, as close to the backplane connector as possible, for the right-side card's receiver since it has no capacitance?

    b) Add 220nF capacitors on the RX side of the redriver? This would be series capacitance in addition to the 100nF already on the lines, so would the decrease in total equivalent capacitance on the line be an issue?

    2. For the right-to-left redriver: (these seem pretty straightforward, unless I misunderstood everything)

    a) Redriver's TX side is okay without any additions

    b) Add 220nF capacitors on the RX side of the redriver, as close to the redriver pins as possible

    I think I might be getting confused because TX for the receiver is the RX for the switch/FPGA, and vice versa, so then placing a cap on the TX side but far away ends up being close to an RX pin. Could you draw where you're recommending the capacitors be placed? 

  • +1 in reply to Gillian Lee
    TI__Guru* 93290 points

    Hi Gillian,

    1a). Your understanding is correct.

    1b). Adding additional Cap would decrease the total capacitance and this is not recommended. 

    2). For PCIe switch to the redriver your Caps are correct.

    Regards, Nasser

  • 0 in reply to Nasser Mohammadi
    Prodigy 25 points

    Great, thanks so much for all the help!