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LMK5B12204: External 10MHz signal reference input issue

Jiahui Gao
Expert 7485 points
Part Number: LMK5B12204


Tool/software:

Hi team，

For LMK5B12204, when an external 10MHz reference signal is input to the PRIREF pin or SECREF pin, must this 10MHz be an AC differential signal or can it also be a DC single ended signal

Thanks & Best regards，

Jiahui

  • 0
    Intellectual 321 points

    Hi Jiahui,

    Reference input signal can be differential as well as single ended.
    See datasheet LMK5B12204 table 9-2 and figure 9-7 for more details about termination.
    Also consider the slewrate of the ref, it must be at least 0.5 V/ns for single ended inputs for the best jitter performance.


    Regards,

    Octo

  • 0
    TI__Genius 16735 points

    Hi Jiahui,

    Yes, the input can be either AC-differential or DC-single ended.

    Table 9-2 that Octo is showing gives you the full list of input buffer termination options. As you can see from the table, both AC diff and DC single ended are supported.

    Repasting the table to include the last two rows:

    Regards,

    Jennifer

  • 0 in reply to Jennifer Bernal
    TI__Expert 7485 points

    Hi Jennifer, Octo,

    Thanks for your quick reply.

    But why is there this sentence in the TICS Pro configuration software, 

    3. Select interface type. AC or DC buffer is auto-seletected based on reference frequency. If reference frequency is below 5 MHz, then use DC buffer. Otherwise, use AC buffer. To select interface types for AC buffer, refer to the 'Interface Type Selection Tips' in the XO wizard page. The same can be applied to PRIREF and SECREF.

    And when entering 10MHz, it automatically configures to AC mode, which conflicts with the chip manual

    Best regards,

    Jiahui

  • 0 in reply to Jiahui Gao
    TI__Genius 16735 points

    Hi Jiahui,

    I can see the confusion with the sentence "Otherwise, use AC buffer". I've made note to clarify that text to say use AC or DC buffer for frequencies about 5 MHz.

    The GUI is meant to auto configure many settings to assist the user with register configuration. By default, AC input buffer is selected because it is what has worked well for applications > 5 MHz and helps reduce low frequency noise.

    What input frequency type do you plan on using (LVPECL, LVDS, differential, single-ended,...) ? I can help suggest the recommended reference input buffer type.

    Regards,

    Jennifer