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Original question:

DAC37J84: LVPECL termination for Clock and Sysref

DAC37J84: about DAC3XJ8XEVM-SCH_D SYSREF TERMINATION

user5983069
Intellectual 320 points
Part Number: DAC37J84
Other Parts Discussed in Thread: LMK04828, , DAC38J84

Hi

We try to connect SYSERF SIGNAL between LMK04828 and DAC37J84 as LVPECL DC coupling  connection which shows in DAC3XJ8XEVM-SCH_D SYSREF TERMINATION.

But we cannot understand the effect of (R98,R89) / (R106.R105)/(R115,R116) 

Would you please show us what for?

thank you

  • 0
    TI__Guru 66104 points

    Hi,

    R98, R89 form the necessary pull-down bias resistors needed for the LCPECL driver. R106/R105 and R115/R116 forms the resistor divider to divide down the common mode to 0.5V necessary to the DAC38J84 sysref receiver circuit.

    The overall impedance (Thevinin looking back into the SYSREF driver) is close to 100ohm differential. This creates the source impedance needed to match the DAC38J84 SYSREF load termination of 100ohm.

    -Kang

  • 0 in reply to Kang Hsia
    Intellectual 320 points

    Hi Kang

    I understood.

    thank you very much.

    -Takeshi