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DS90UB921-Q1EVM: COAX and differential FPD LINK lane

Elec Cheng
Expert 7055 points
Part Number: DS90UB921-Q1EVM


Hi Expert,

I get a little confused about the single and differential FPD link lane. Under single mode, only one single is transferred? How about the GND? Also for differential mode, no need for common GND?

Another question, can I use two SMA interface for differential working mode? Because I use 921 and 926 for testing but 926 cannot support coax mode but I don't have another HSD connector. So I want to use two SMA directly. 

BR,

Elec Cheng

  • +1
    TI__Mastermind 38040 points

    Hi Elec,

    Most of our FPD-Link products are capable to support Coax or STP (Shield Twisted Pair) configuration. The serializer and deserializer can use the interconnect coax or STP to connect from serializer to deserializer. In STP configuration, the P and N signals are all tightly coupled in both separation and equal length to benefit from the electromagnetic field cancellation. When the differential pair are symmetrical and have equal length, it has an excellent noise immunity where you can transmit the information across the link. The goal here also is to maintain 100Ω differential impedance to prevent any reflection or discontinuity. For Coax configuration, N signal can be terminated with 50Ω to ground as close as possible to the input/output connector with a series capacitor. It is very important to pay attention to the entire system channel specs for both serializer and deserializer.

    The recommendation is to use HSD connector where it has equal electromagnetic cancellation and better performance of signal integrity. Yes, you may use two SMA cables to connect from serializer to deserializer.

    Hope this helps. Let me know if you have any questions.

    Aaron

  • 0 in reply to Aaron Heng
    TI__Expert 7055 points

    Hi Aaron,

    Below screen shot comes from DS90UB921 EVM User Guide. After comparing with 927 User Guide, I find the SMA connectors circuit is different. In 921, if I use SMA connectors, the coupled inductor and cap will not be applied. But I 727 EVM, the coupled inductor and cap are shared for SMA and HSD. Does it matter? 

    Another question, if I use SMAs for 927 in differential mode, there is a 50Ω resistor R93 in J2. should it be removed or not? Thanks a lot!

    BR,

    Elec Cheng

  • 0 in reply to Elec Cheng
    TI__Mastermind 38040 points

    Hi Elec,

    For coax configuration, you do not need to use CMC for your circuitry. The CMC is needed when you have differential pair so that it helps to improve the skew as well as reducing noise. For the AC coupling cap, the N channel will have different value for coax configuration, please refer to datasheet toward to the end.

    Aaron

  • 0 in reply to Aaron Heng
    TI__Expert 7055 points

    Hi Aaron,

    Just let you know some background, I don't have HSD interface so I want to use two coax to achieve the "differential pair" working mode. That why I compare the coax config. Based on this, maybe you can understand my question above. Thanks a lot!

    BR,

    Elec Cheng

  • 0 in reply to Elec Cheng
    TI__Guru** 110570 points

    Hello Elec,

    Please refer to the DS90Ux949-Q1 Reference designe in the User's Guide, on page 39. There we have implemented both options, single ended and STQ.

  • 0 in reply to Hamzeh Jaradat
    TI__Expert 7055 points

    Hi Hamzeh,

    Thanks for hep! It shows the the CMC is need in differential mode, no matter the cable is two coax or STP.

    BR,

    Elec Cheng