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LMK04832: LVPECL Output Format only use one port

Zhizhao Niu
Intellectual 2315 points
Part Number: LMK04832

Hi Team,

I have a question that whether we can only use the LVPECL one output port? The matching circuit shown as below. Because the board layout have been fixed, so the the output can only set to differential output format can not set to LVCMOS. So I want to confirm that does the circuit 2 shown below can work? Thanks!

B.R.

Zhizhao

  • 0
    TI__Mastermind 28540 points

    Hello Zhizhao,

    Zhizhao Niu said:
    output can only set to differential output format can not set to LVCMOS

    Meaning that the outputs that support LVCMOS are connected to your load?  Because otherwise this circuit would support LVCMOS.

    --

    Actually the circuit on top is not the normal topology for AC coupled LVPECL either.  There should be capacitors between the emitter resistors (150 ohm) an the 100 ohm termination at load.  However it is ok, you just must consider the effect that your emitter resistor for one leg is in parallel with the load + emitter resistor of the other leg.  So if you increase your 150 ohm emitter resistor to 240 ohms, then it actually behaves as if you are using a 120 ohm emitter resistor.  So if your target was 150 ohms, you could go a bit higher than 240 ohm emitter resistor.

    If you want to run single ended, you could just the top schematic (use 240 ohm or higher emitter resistor) and remove the capacitor on one of the sides to the load.  So the N side for example.  This is my recommendation.

    The bottom schematic is not good.  Basically you now have an emitter resistor on one side even lower, 150 ohms || 100 ohms.  And 100 ohms is far from 50 ohms for a good single ended termination.


    73,
    Timothy

  • 0 in reply to Timothy T
    TI__Intellectual 2315 points

    Hi Timothy,

    Thanks. Actually this is for LMK04832 out 12. The out12 previous is a differential load which support LVPECL. But now I need to change to a LVCMOS load. That is why I need to use one output port of LVPECL and connect to the LVCMOS load. 

    From my understanding, you means, if only connect LVCMOS load to P. we need to do the matching shown as below? Thanks!

    B.R.

    Zhizhao

  • 0 in reply to Zhizhao Niu
    TI__Mastermind 28540 points

    What voltage level is your LVCMOS load?  3.3-V LVCMOS?  1.8-V LVCMOS? 

    Odds are you will not be able to meet the Voh and Vol with LVPECL.  But if you were, you'd have to (a) remove the capacitor or (b) bias the load side to the proper voltage.

    What I was previously suggestion is that the proper way to do AC coupled LVPECL as illustrated in fig 1.  The emitter resistors or isolated from the 100 ohm load by capacitors.  You don't want the circuit you illustrate above because the emitter output will see a DC path of 37.5 ohms to ground.  The emitter resistor should be >= 120 ohms.

    Now it is possible to do LVPECL as you have done with the load in parallel with the emitter resistor, but in this case I recommend the Re >= 240 ohm.  When using 240 ohm then the effective emitter resistance is 120 ohms after considering actual emitter resistance is approximately Re || (Rl + Re).  It's a bit more complicated than the equation given since it is connected to the opposite output.

       * I have used this approach in my recommendation for your circuit.  Using Re = 240 ohm, a differential 100 ohm termination, and then AC coupled to the CMOS load.  Note, I don't think you want the capacitor here for LVCMOS.  Use 0 ohms and then you will have a DC clock from LVPECL -- but it may not meet the Voh/Vol spec you need.  But given your layout, I think this is the best you can do.


    73,
    Timothy