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LMX2581 PCB layout

Alber, Moon
Intellectual 720 points
Other Parts Discussed in Thread: LMX2581

Hello
I'm Alber as FAE in Arrow Korea.
My customer will design the pll using LMX2581.
but, they can't design that pad0 connect to GND due to PCB size(12mm x 12mm).
if they operate the pll without connection(Pad0 - GND), what do you expect any side-effect?
or how much do you expect degradation at output performance?

  • 0
    TI__Expert 4085 points
    Hi Alber,

    Thank you for your interest in our products. I forwarded your query to the applications engineer most familiar with this device.

    Regards,
    Julian
  • 0 in reply to Julian Di Matteo
    TI__Guru** 111250 points

    Hi Alber,

    The ground paddle is important and it must be ground properly. First of all, it is being used to dissipate heat. The device will consume at least 180mA current and may be up to 300mA depending on the use case configuration. Heat dissipation is necessary. Secondly, the ground paddle will provide isolation between each row of pins.

  • 0 in reply to Noel Fung
    Intellectual 720 points
    Hi
    Thank you for your answer.
    My customer asked another one.
    Do you have any test result using 10MHz refernce input?
    There's only explanation on datasheet using 100MHz reference input .
    My customer said, the performance(spur) is so bad on EVK when they use 10MHz reference input.
  • 0 in reply to Alber, Moon
    Intellectual 720 points
    Would you please give me any feedback?
  • 0 in reply to Alber, Moon
    TI__Guru** 111250 points
    Hi Alber,

    Would you describe the test configuration? Also could you provide a screen shot of the test result?
  • 0 in reply to Noel Fung
    Intellectual 720 points
    Hi, Noel

    I have not the test result.
    But I remembered that the phase noise was not good.
    If I get a test result from customer, I will share it.

    Following is the test conditions.
    ------------------------------------------
    Test Board : LMX2581 EVK
    REF_IN : 10MHz from signal generator(Agilent) / On board VCXO was power down.
    VCO Frequency : 2800MHz
    PFD : 10MHz
    Output Divider Setting : 1
    Output Frequency : 2800MHz(A port on/ B port Off)
    ------------------------------------------

    Thanks
  • 0 in reply to Alber, Moon
    TI__Guru** 111250 points

    Hi Alber,

    Do the following when using external reference clock.

    1. Change L3 to 0ohm resistor (L3 is located next to OSCin SMA connector)

    2. Short GND_OSC to GND

    Furthermore, use the 10MHz out at the rear panel of spectrum analyzer instead of RF out from the signal generator, its quality is bad at low frequency.

    Original layout (bottom side):

    Short GND together:

  • 0 in reply to Noel Fung
    Intellectual 720 points
    Hi, Noel

    Thank you very much for your feedback.

    When they tested using external Ref clock(10MHz), R15 moved to R13 and R6 removed.
    And they used 10MHz from rear pannel of signal generator.
    I think it is same the qualilty of 10MHz between Signal Generator and Spectrum Analyzer of rear pannel.

    As you mentiond,
    1. Change L3 to 0ohm resistor (L3 is located next to OSCin SMA connector)
    ==> L3, is this right Ref No.?
    2. Short GND_OSC to GND
    ==> If they changed L3 to 0 ohm, it connected to GND already.

    Anyway, I will let them know this information(GND_OSC connect to GND), and ask the test result again.
    After I get a test result, I will share it here.

    Thanks.
  • 0 in reply to Alber, Moon
    Intellectual 720 points

    Hi, Noel

    Would you please give your feedback about my reply?

  • 0 in reply to Alber, Moon
    TI__Guru** 111250 points
    Hi Alber,
    The on-board XO power supply should be disconnected, so remove R6. R14 shall also be removed to disconnect XO output.
    R13 and R16 should be change to 0 ohm, while R15 should be changed to 50 ohms.
    L3 is the ferrite bead connecting GND_OSC and GND. in the pcb layout, GND_OSC and GND are separated. Simply shorting these ground together by L3 is not enough, that's why I suggest shorting these ground together at multiple place.