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LMK04832-SP: LVPECL Interface Circuit Qustions

Joseph Sanchez
Intellectual 360 points
Part Number: LMK04832-SP
Other Parts Discussed in Thread: LMK04832, LMK04828

Tool/software:

Hello,

We're interfacing the CLKOUT to FIN1 or CLKIN of an LMK04832 to LMK04832 at 960MHz.  We chose to make this interface LVPECL (Fig. 9-2).  Hyperlynx simulations show that the Pull-Down Resistors work better at 75Ω and below.  Is it okay to use a lower value resistor than the 240Ω as shown in the app notes?  If so, how low can we go?

Thanks,

  • 0
    TI__Mastermind 33800 points

    I wouldn't go below Re = 120Ω, because at some resistance below 120Ω the output driver will trip internal overcurrent/short circuit protections. This isn't guaranteed to be at the same resistance on all devices, but it should be well below 120Ω - I've tested 68Ω at room temperature before (on one part). I recall on LMK04828 that the current limit settings are much more aggressive on OSCOUT specifically; this difference is likely carried forward into the LMK04832 as well (though I haven't tested to confirm). You mentioned using CLKOUT so I don't think this is applicable to your use case, but I figured I should mention it just in case.

    We've done some experiments to test the differences in performance between 120Ω, 150Ω, 180Ω, and 240Ω drivers. Generally, lower resistance modestly (~0.5dB) improves phase noise performance at lower frequencies (<300MHz), and there is minor reduction in output amplitude at higher resistances; but rise/fall times or signal integrity (provided the emitter resistor is close to the driver pins) are not substantially affected. On the other hand, 240Ω greatly reduces the current consumption compared to 120Ω: 240Ω is about 22mA vs 120Ω at about 36mA.

    I also wouldn't put too much stock into IBIS modeling of the LVPECL driver (edit to add: at conditions deviating greatly from the simulation used to derive the model). There's a lot more going on in the driver (overcurrent protection, feedback for amplitude control, internal supply voltage droop, unmodeled startup behaviors, etc) that isn't captured in IBIS modeling. It's not clear from your post what about the 75Ω and below conditions constitutes "working better" in your simulations, but I suspect the actual behavior of an LVPECL driver at 75Ω and below would not correlate well with the simulations.