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LMK04832EVM: Differential signals polarity and naming

Alexander Sobko
Prodigy 30 points
Part Number: LMK04832EVM
Other Parts Discussed in Thread: LMK04828, LMK04832
Hello,
I looked on EVB schematics both LMK04828 and LMK04832 and put attention that external VCXOs' outputs are connected to LMKs' OSCin inputs with opposite polarity, i.e. negative VCXO output is connected to positive LMKs' OSCin and vice versa.
What is the reason? Is this a recommended connection?
 
BTW, I assumed that LMK's pin name with asterisk denotes negative pin in differential pair, while pin name with no asterisk – positive one. Like OSCin (positive) and OSCin* (negative).
Is this true?
Actually, this is not usual naming convention and it is not explained in IC datasheet nor in EVM user guide.
Thanks,
Alex
  • 0
    TI__Mastermind 33580 points
    Alexander Sobko said:
    What is the reason? Is this a recommended connection?

    It simplified EVM layout. There's no special reason or requirement for the polarity of the VCXO to match the polarity of the OSCin pins. In principle the polarity could matter if you care about the exact VCXO phase with respect to either the CLKin or the CLKout phase, but in most cases it isn't required, or it is more useful to control the phase of CLKout with respect to CLKin (and the VCXO phase is irrelevant).

    Alexander Sobko said:
    OSCin (positive) and OSCin* (negative).
    Is this true?

    Yes.

    Alexander Sobko said:
    Actually, this is not usual naming convention and it is not explained in IC datasheet nor in EVM user guide.

    Good point. We should document this convention somewhere, or else update the documentation to use more conventional P/N differential signaling. I'll add it to the list of planned document revisions.

  • 0 in reply to Derek Payne
    Prodigy 30 points

    Hi Derek,

    Thank you very much for this clarification.

    Alex