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LVPECL's terminal problem

Ted Xu
Expert 4590 points

Question:

The topology change on the LVPECL terminal recommended is found in the different version of LMK04800EVK’s user guide.

                                                                                          LVPECL terminal 1

                                                                        LVPECL terminal 2

  1. 1.       In above LVPECL terminal 1, whether should  R107 be marked DNP? If it isn’t true, this terminal is different from the one in the LMK0400EVK user guide. Which one is correct?
  2. 2.       What are the difference between terminal 1 and terminal 2?

Answer:

Question 1) R107 is not used in the circuit because R106 and R109 are not placed.  I placed in during the build, because if we wanted to enable the far end termination, it was one less component to place.

Question 2) Difference between LVPECL Termination 1 (TOP) and LVPECL Termination 2 (BOTTOM).
LVPECL Termination 1 (TOP) is a traditional emitter resistor AC coupled type termination.  The emitter resistors should be placed close to source.  Termination will occur at destination.  Since AC coupled, the DC bias level should be set at destination.  Many high speed/high performance inputs operate in this manner.  For example, LMK.

LVPECL Termination 2 (BOTTOM) uses what is called a Y-Termination for LVPECL.  We have not published this in our literature.  It's method of working is to generate a voltage of ~ Vcc - 2V (or whatever the desired termination voltage is) by using two voltage dividers:  CLKout high - 68 ohms (R85) - 62 ohms (R91) - GND.  And CLKout low - 68 ohms (R93) - 62 ohms (R91) - GND.  When clock polarity switches, the same circuit is achieved, but in the opposite direction.  In this way a constant termination voltage happens above R91.
   > This termination could be used to properly DC terminate an LVPECL output (so that LVPECL outputs desired voltage levels) close to output, as shown.
   > Or more commonly, this termination could be used at destination AC or DC coupled.  Since it provides a 50 ohm termination to the Vcc set point voltage.
       - When using DC coupled, receiver must be able to accept the LVPECL Vcm.
   > Note our values above (68 ohms, 68 ohms, 62 ohms) were adjusted to make a trade-off between traditional termination and power consumption.

  • 0
    TI__Mastermind 28550 points

    Hello,

    Question 1) R107 is not used in the circuit because R106 and R109 are not placed.  I placed in during the build, because if we wanted to enable the far end termination, it was one less component to place.  (This circuit was best for terminations at the end of the PCB for DC level measurements)

    Question 2) Difference between LVPECL Termination 1 (TOP) and LVPECL Termination 2 (BOTTOM).
    LVPECL Termination 1 (TOP) is a traditional emitter resistor AC coupled type termination.  The emitter resistors should be placed close to source.  Termination will occur at destination.  Since AC coupled, the DC bias level should be set at destination.  Many high speed/high performance inputs operate in this manner.  For example, LMK.

    LVPECL Termination 2 (BOTTOM) uses what is called a Y-Termination for LVPECL.  We have not published this in our literature.  It's method of working is to generate a voltage of ~ Vcc - 2V (or whatever the desired termination voltage is) by using two voltage dividers:  CLKout high - 68 ohms (R85) - 62 ohms (R91) - GND.  And CLKout low - 68 ohms (R93) - 62 ohms (R91) - GND.  When clock polarity switches, the same circuit is achieved, but in the opposite direction.  In this way a constant termination voltage happens above R91.
       > This termination could be used to properly DC terminate an LVPECL output (so that LVPECL outputs desired voltage levels) close to output, as shown.
       > Or more commonly, this termination could be used at destination AC or DC coupled.  Since it provides a 50 ohm termination to the Vcc set point voltage.
           - When using DC coupled, receiver must be able to accept the LVPECL Vcm.
       > Note our values above (68 ohms, 68 ohms, 62 ohms) were adjusted to make a trade-off between traditional termination and power consumption.  Traditional values may use 50 ohms, 50 ohms, and ~46 ohms.

    ---

    We have chosen LVPECL Termination 1 (TOP) for our eval boards for simplicity.  The Y-termination could be used... but not best on evaluation board because we cannot place next to receiver.

    73,
    Timothy