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TLK2711-SP AC COUPLING CAP

Kamal Najmi
Expert 3435 points
Other Parts Discussed in Thread: TLK2711-SP

HI,

i have a questiion from My customer ASTRIUM:

Question: We are using the TLK2711SP Wizard Link device. In the datasheet it is said, that a 1nF to 10nF capacitor for AC coupling should be used. We are using it at a data rate of 2.25 Gbps. Which capacitor value would you recommend? Which part dimensions (0805, 1206) would you recommend?

 Regards

kamal

  • 0
    TI__Mastermind 20820 points

    Hi Kamal,

    The data sheet values that you listed are valid for your data rate, we typically use a 10nF cap on our EVM designs. The size of the cap that you ultimately end up using should be determined by your layout. You want to pick a size that will give you a good match in width to your traces so to keep a good impedance match with as little tuning as possible. I will check with some of the engineers on my team tomorrow to see if there is any documentation out there on selecting capacitor values for high speed transmission lines.

    Regards,

    Mike

  • 0 in reply to Michael Peffers
    TI__Expert 3435 points

    HI Mike,

    i got the following email from my customer and he would like the best value for this cap  knowing the Fhz.

    According to the Datasheet 80MHz (1.6Gbps) to 125MHz (2.5Gbps) are supported.

    We intend to use the TLK2711-SP with a Clock Frequency between 115MHz and 125MHz.

    This clock will be connected to the TXCLK Pin.

     

    The resulting serial data rate is then 2.3Gbps to 2.5Gbps

    Which capacitor value do you recommend for the AC coupling capacitors in our application?

     

  • 0 in reply to Kamal Najmi
    TI__Mastermind 20820 points

    Hi Kamal,

    I recommend using a a 10nF (.01uF) capacitor for AC coupling the high speed serial links in your customers application.

    Regards,

    Mike

  • 0 in reply to Michael Peffers
    TI__Expert 3435 points

    hi Mike,

    I have one more question regarding the AC coupling capacitor:

    my customer  want to use a 0805 (CDR31) 1nF NP0 capacitor for the frequency range 115MHz to 125MHz.

    Do you have any concern regarding this selection? Can you please tell us, why you recommended the 10nF to us (due to the lower frequency resistance or are there other reasons)?

    regards

    kamal

  • 0 in reply to Kamal Najmi
    TI__Mastermind 20820 points

    Hi Kamal,

    To avoid any impedance mismatches the size of the capacitor (0805) should best match their trace width. If the size of the cap is somewhat larger than their trace width this should be dealt with in layout by tapering the trace to the pad to ensure a good match.

    The physical value of the cap will determine the corner frequency of the high pass filter that is created by the AC coupling cap and the termination resistor at the end of the link. For example:

    t = (R)(C) = (50ohms)(1nF) = 50nS

    2πRC = 2π(50nS) = ~314nS

    The bit time at 2.3Gbps is:

    1 / 2.3e9 = 434pS

    (434pS)(5) = 2.17nS (5 being the longest run of 1s or 0s that 8b10b data can support)

    So, 314nS is >>2.17nS so 1nF will work for this application.

    Regards,

    Mike