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HD3SS3412 - PCIe Gen3 and AC Coupled Input

Kawai
Guru 19785 points
Other Parts Discussed in Thread: HD3SS3412, DS80PCI800

Hi,

Our customer is going to use HD3SS3412 for PCIe Gen2/Gne3 application. I have 2 question for this device.

[Question1]
About AC coupled input, I read the following thread. From this answer I understood that HD3SS3412 can support AC coupled inputs. Am I correct ?

AC coupling question thread
http://e2e.ti.com/support/interface/digital_interface/f/130/t/202580.aspx

[Question2]
Using in PCIe Gen3 application, there is Link Training in this standard.
Can the device support Link Training with the AC coupled input ?

When the input is AC coulpld, HD3SS3412 will use the voltage range under 0V. 
It seems that the device cannot have linear characteristic in this range.

Best Regards,
Kawai

  • 0
    TI__Guru* 76130 points

    Hello Kawai,

    Yes, the switch supports AC coupled signals.

    In order to avoid what you are calling as "voltage range under 0V" the AC coupling capacitors should be placed only on one side of the switch, in this way the other side of the link will provide a bias voltage to the switch.

    For example, in the below picture, the switch on the top is biased by the endpoint device and the switch on the bottom is biased by the host controller.

    Regards.

  • 0 in reply to Elias Villegas M.
    Guru 19785 points

    Elias-san,

    Thank you for your reply.

    I understood that  it will be fine if AC coupling capacitor  is on at just one side of the switch.

    Our customer also needs DS80PCI800 for signal conditioning. DS80PCI800 requires AC coupling on its Input and output. In this case, there will be AC coupling capacitors on both side of the switch.

    Root Complex ->Cap->HD3SS3412->DS80PCI800->Cap->CN->End Point

    In the above, DS80PCI800 is next to HD3SS3412. As DS80PCI800 requires AC coupling, it has input bias voltage.

    So, I think you can support PCIe Gen3 Link Training in this case, am I correct ?

    Best Regards,
    kawai

  • 0 in reply to Kawai
    TI__Guru* 76130 points

    Hello,

    I don't have expertise with the DS80P device, but if it is biasing the differential lines, then it should be OK.

    In general, when you require AC coupling capacitors on both sides of the 3412, you have to provide an external bias voltage, this voltage should be less than 2V.

    Regards.

  • 0 in reply to Elias Villegas M.
    Guru 19785 points

    Elias-san,

    I found DS80PCI800 had bias voltage around 1.6V on my evaluation board. So I guess we don't need external bias voltage.

    By the way, if external bias is needed what is your recommendation for the pull up resistor ?

    Best Regards,
    Kawai

  • 0 in reply to Kawai
    TI__Guru* 76130 points

    Hello,

    You can use 10K resistor.

    Regards.