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66AK2H14: AC coupling requirement for Hyperlink signals

Madhu Sharma
Expert 2915 points

Part Number: 66AK2H14

Hi,

I am using hyperlink interface between two 66AK2H14 devices.

The hardware design guide ( figure 24, page 60) , mentions two devices must be connected using 0.1uF ac coupling capacitor.

Please let me know whether this ac coupling is required when both the devices are same. 

As per my understanding, ac coupling is needed if both the ICs have separate DC biasing. But here, since both the devices are same, DC bias voltage must be same.

Thanks & Regards,

Madhu

  • 0
    TI__Guru 65405 points
    Hi Madhu,

    I've forwarded your query to the hardware design team. Their feedback should be posted here.

    BR
    Tsvetolin Shulev
  • 0 in reply to Cvetolin Shulev-XID
    TI__Mastermind 46460 points

    Madhu,

    Yes, the capacitors are needed.  This allows the DC level of the receiver to be optimized.  Note that high-quality, wide bandwidth capacitors rates for multi-gigabit circuits should be used.

    Tom

  • 0 in reply to Tom Johnson 16214
    Expert 2915 points

    Hello Tom,

    Could you please elaborate above answer? 

    Due to lack of space on the PCB and due to dense routing, I am not able to place AC capacitors to hyperlink interface. Please let me know whether the interface will work without AC coupling.

    Thanks & Regards,

    Madhu

  • 0 in reply to Madhu Sharma
    Expert 2915 points
    Hi,

    Also, I saw below link where same topic has been discussed:

    e2e.ti.com/.../1006047

    In above link, it is mentioned that two Keystone I SOCs can be interconnected without using AC coupling.
    So Please let me know whether the same can be applied to Two Keystone II processors also.


    Thanks & Regards,
    Madhu
  • 0 in reply to Madhu Sharma
    TI__Mastermind 46460 points

    Madhu,

    That is correct.  We know that the KeyStone-I devices can be DC-connected for HyperLink.  However, the SERDES for KeyStone-II devices is very different.  Current guidance is to interconnect HyperLink on KeyStone-II devices using series capacitors.  I have asked the design team multiple times to validate this guidance but have not gotten a response.  Unless I hear otherwise, the guidance requiring use of series capacitors stands.

    Tom

  • 0 in reply to Tom Johnson 16214
    Expert 2915 points

    Hi Tom,

    Thanks for the reply.

    Also, could you please elaborate on capacitor characteristics that you mentioned in previous response ( Note that high-quality, wide bandwidth capacitors rates for multi-gigabit circuits should be used) ?

    Thanks & Regards,

    Madhu

  • 0 in reply to Madhu Sharma
    TI__Mastermind 46460 points

    Madhu,

    These are available from multiple manufacturers.  An example is the 520, 530 and 550 series caps from American Technical Ceramics (ATC).  These are readily available through distribution.  Standard ceramic capacitors are more of a notch filter at gigahertz frequencies due to their intrinsic package inductance.

    Tom

  • 0 in reply to Tom Johnson 16214
    Expert 2915 points

    Hi Tom,

    I am planning to use a 0201 package capacitor to reduce impedance mismatch near the pads ( and also due to space constraint on the board). So please let me know what are the capacitor characteristics to be considered, while selecting ac capacitors for hyperlink & SRIO.

    Thanks & Regards,

    Madhu

  • 0 in reply to Madhu Sharma
    TI__Mastermind 46460 points

    Madhu,

    The capacitors are part of the channel between the transmitter and the receiver along with the traces, vias and pads.  The Errata document SPRZ402E shows in Advisory 11 the required Insertion Loss and Return Loss for this composite channel.  It is a trade-off between PCB construction and routing and capacitor choice.  It is up to you how to balance this trade-off.  Then at device commissioning, you will need to tune the transmitter and receiver settings in the software to optimize performance of the link.  This can be done using the BER sweep tools as discussed in the SERDES commissioning document SPRAC37.  You can also use IBIS-AMI modeling to validate your board layout and/or a vector network analyzer to measure your channel.

    Tom