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LMK04832: CML termination

Mark Guastaferro
Genius 11375 points
Part Number: LMK04832

Is there a rule of thumb as to when to use 50 ohm resistor pull-ups vs 68nH + 20 ohms for CML outputs?  Based on what I see in the data sheet, it looks like 50 ohms works best at lower frequencies and 68nH =20 Ohms enables higher amplitude output at higher frequencies, but I'm not clear on what frequency ranges would be recommended for each termination method.

A second related question - when selecting CML format, it looks like the frequency divider must be bypassed on the even number clock outputs. TICSPro doesn't appear to give a warning about this when configuring the output clocks.  I just want to verify that even number clock outputs cannot use the clock divider when CML output format is desired. 

  • 0
    TI__Genius 12560 points

    Hi,

    For the CML output 50 ohm termination provide the wideband performance but reduce the amplitude at high frequency due to the resistive loss, whereas the combination of inductor and resistor provide the high Q performance and higher amplitude.  

    The high-performance bypass path available on even-numbered CLKouts directly routes the clock distribution path to the CML output buffer, and uses significantly fewer muxes routing the signal to the output. Note that CML on even-numbered CLKouts is inaccessible unless using the high-performance bypass mode. CML output with divider can be used with resistive load (50 ohm) termination.

    Thanks!

    Regards,

    Ajeet Pal

  • 0 in reply to Ajeet Pal
    TI__Genius 11375 points

    Hi Ajeet,

    Thanks for the information.  I am confused by your last statement "CML output with divider can be used with resistive (50 ohm) termination." My understanding is that on even number CLKouts you can't use the divider with CML format.  Does your last statement refer to odd-numbered CLKouts or am I missing something?  

  • 0 in reply to Mark Guastaferro
    TI__Genius 12560 points

    Hi Mark,

    That's true. Sorry for the confusion.

    Odd-numbered CLKouts can be used with divider in CML mode and if want to have bypassed output then can change divider value to 1 and enable "DCC & HS" bit "high".

    Thanks!

    Regards,

    Ajeet Pal