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CDCE62002: Crystal oscillator output to aux input?

Dan Schwartz
Prodigy 80 points
Part Number: CDCE62002
Other Parts Discussed in Thread: LMK03318

I would like to connect the output of a crystal oscillator to the aux input, rather than just connecting a crystal.  The purpose of this would be to get the fully specified accuracy of a packaged oscillator.  Can I do this?  The revision history in the CDCE62002 data sheet says "Deleted LVCMOS INPUT MODE (AUX_IN) section from Electrical Characteristics".  Does this imply that this capability is no longer supported, and I cannot use an oscillator?  If I can use an oscillator, should I use one with LVCMOS output levels?

  • 0
    TI__Mastermind 28550 points
    Hello Dan,

    You can connect the LVCMOS input to REF_IN+. When using LVCMOS on REF_IN+ connect REF_IN- to ground through 1 kohm resistor. Does this work for you?

    73,
    Timothy
  • 0 in reply to Timothy T
    Prodigy 80 points

    Thanks.  Yes, that's exactly what I want to do.  My only concern is that the datasheet seems to have been deliberately updated to remove support for this mode of operation (use of an external oscillator, rather than a crystal).  Am I misinterpreting the datasheet?  Are you confident that this is a supported solution?

  • 0 in reply to Dan Schwartz
    Prodigy 80 points
    Oh, I just realized that your answer is not what I want to do. I need two inputs to the chip -- one from a local oscillator, and the other from an external source. So, no -- your solution does not work for me. If I can't connect an oscillator to the aux input, I would have to use a crystal. My problems then become: a) How do I pick the optimum crystal (I find the data sheet to be a little vague on this), and b) How do I determine the worst case accuracy that I can expect from the crystal?
  • 0 in reply to Dan Schwartz
    TI__Mastermind 28550 points
    Hello Dan,

    I figure there is some level you could provide an input to the crystal input, but full swing LVCMOS is not ok and I don't know what those levels would be.
    - However simply using the crystal feature may be great for you. After all, it is that on board XO use case you are looking for. I'm going to have a colleague comment more on the crystal.

    One other thought, if you're not locked into CDCE62002, have you considered LMK03318?

    73,
    Timothy
  • 0 in reply to Timothy T
    TI__Mastermind 19780 points
    Hi Dan,

    I think Timothy has treated the use of an external XO to feed the AUX input of the CDCE62002.

    I believe the case you are seeking, which is supported in the datasheet still, is use of a crystal as the AUX_IN source.

    For frequency accuracy calculations, the pullability of the crystal will need to be known. The AUX_IN capacitive load is 8pF to 10pF. Some estimation will need to be done for the PCB traces.

    Your total accuracy will be frequency tolerance of the crystal plus the change in frequency per pF.

    There is some detail given in section 9.3.7 of the datasheet. If you have more questions about the crystal design params, please let me know.

    Best Regards,
  • 0 in reply to Rob Rodrigues
    Prodigy 80 points

    Thanks very much for the information.

    Dan