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SN65EPT22 output biased with 51ohm to GND

John11513
Expert 5720 points
Other Parts Discussed in Thread: SN65EPT22

Hi team,

The output of PECL driver such as SN65EPT22 is typically biased with 50ohm to Vcc-2V, or 150ohm to GND. My customer's previous design use 150ohm biasing. However, large EMI was observed due to bad PCB layout. To reduce EMI they tried to bias the output to GND through 51ohm. The EMI is significantly reduced and signal is good enough now. However this is not a typical biasing solution. I am wondered is there any potential issue with this kind of design? Can this design work reliablly? Below is the schematic ckt:

Thank you,

John

 

  • 0
    TI__Guru 62920 points
    Hi John,

    It is OK to operate the SN65EPT22 device with a 50-Ohm load to ground, although it will not meet all of the rated specs under this set-up. LVPECL uses an asymmetrical driver structure which will pull up one side of the differential link and allow the other side to be pulled low by the load termination. Using a 50-Ohm load to ground rather than Vcc - 2 V means that the "low" side of the differential signal will be much lower than usual. The "high" side voltage will be reduced as well, since the pull-up source in the driver will now have to source more current.

    The higher output current should not cause any reliability risks, since it should be below the 50 mA absolute maximum continuous output current rating. So, as long as the differential voltage that results from the new high and low levels is within the input range of the receiver module this should be fine.

    Please let me know if you have any other questions.

    Regards,
    Max
  • 0 in reply to Miguel Robertson
    TI__Expert 5720 points

    Hi Max,

    Thank you for your reply. The output signal amplitude, as engineer measured is arround 1.5Vpp, Basically same as typical 150ohm load.

    Regards,

    John