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DS90UB948-Q1: The input capacitor's recommendation is different in our datasheet,which one should I follow?

Sherry Liang
Genius 10255 points
Part Number: DS90UB948-Q1

Dear team,

When the cable is coax, there are two recommendations about the RIN-'s AC capacitor. Below second picture recommends 30nF-100nF, but the third picture recommends 15nF-47nF, which one should I follow? Do the two capacitors C1 and C2 have to be equal? If c1=33nF, C2=15nF, do they will impact the signal quality?

Thanks & Best Regards,

Sherry

  • 0
    TI__Mastermind 25955 points

    Hi Sherry,

    Please ensure the network matches that of the serializer.

    Assuming that the serializer is a 941AS.  With the EVMs, please use 100 nF for C1/C3 on both sides, and 47 nF for C2/C3.  Rterm will remain 50 ohm. 

    These may be reworked on the 948 EVM while populating the 0 Ohm resistors to enable the SMA connectors (or coax connector of your choice).

    Sincerely,
    Bryan Kahler

  • 0 in reply to Bryan Kahler
    TI__Genius 10255 points

    Hi Bryan,

    Thanks for your reply! I still have two doubts about the selection of capacitors.

    1. The range of C1 and C3 is 33nF~100nF. What is the difference between c1&c3=33nF and c1&c3=100nF? The larger the capacitance, the better the signal quality？

    2. If we set c1=c2 & c3=c4, we can get the better signal quality, right?

    Thanks & Best Regards,

    Sherry

  • 0 in reply to Sherry Liang
    TI__Mastermind 25955 points

    Hi Sherry,

    The suggested value of 100 nF and 47 nF above fall in the range listed in the datasheet of the 948.  

    The suggestion to switch the 33 nF caps populated on the 948 EVM to 100 nF and 47 nF is so that both boards match for coax mode.  The 50 ohm resistors are also critical on both boards for coax mode.

    You're correct - signal integrity is a concern here.  Please ensure insertion and return losses are kept to a minimum.

    Sincerely,
    Bryan Kahler