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LMK04826: LVPECL connection with dc-coupling

Kazuhiko Hirano
Genius 13278 points
Part Number: LMK04826

Hello,

 

My customer is connecting LMK04826 CLKout11 to another LMK04826 CLKin0 with dc-coupling as follows.

Actually, it seems to work well under my customer’s evaluation.

Though LMK04826 datasheet shows only ac-coupling example, does this dc-coupling work with no issue?

Best regards,

 

K.Hirano

  • 0
    TI__Mastermind 31220 points

    Hello Hirano-san,

    LVPECL termination voltage is defined as VCC-2V, and we use 240Ω to GND biasing to simulate 1.3V common mode. LMK0482x input voltage common mode is around 1.6V or 1.7V nominal (about VCC/2) and this changes slightly over PVT, maybe ±100mV. It is likely that DC-coupled LVPECL could still work for most parts as 1.3V common mode should be within acceptable range for the input stage, but we have not tested or characterized that configuration to say for sure. Be aware that in MOS mode, adding 100Ω across CLKinX will prevent LOS detection from functioning (if used).

    In summary, TI cannot recommend DC-coupled LVPECL configuration because it has not been fully tested or characterized, even though it would probably work okay. The configuration of datasheet figure 25 is TI's recommended LVPECL connection.

    HSDS 8mA output format has the same nominal common mode voltage as the CLKin/OSCin inputs, comparable phase noise performance to LVPECL (~4fs rms difference 100Hz - 20MHz), and only one external component for DC termination (the 100Ω resistor). If your customer does choose DC-coupling instead of any TI recommended configuration, HSDS 8mA is theoretically a better choice than LVPECL because the common mode voltage is closer to the self-bias voltage of the input.

    Regards,