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DS250DF230: Why does the output rise when the retimer is powered up, even though there is no input signal?

Takashi Yasuno
Prodigy 20 points
Part Number: DS250DF230

Tool/software:

When power is applied to the Retimer, the waveforms of the Retimer's power supply (ch1) and 100nF-decoupled output signal(ch2) look like the attached file.
At this time, no input signal is applied to the Retimer and the output is open.
Please tell me why the Retimer output rises.

  • 0
    TI__Mastermind 23707 points

    Hi Takashi-san,

    I believe this is due to DC bias getting applied to DS250DF230 output when power is applied.

    Thanks,

    Drew

  • 0 in reply to Drew Miller1
    Prodigy 20 points

    Hi Drew-san,

    Dose the output driver have CML scheme? 

    How much is the output impedance of the driver expected while writing the registers for initial setup?

     

    Given the output driver has CML scheme and output impedance of 100Ohm diff., 

    We assume that the 100nF-decoupled output signal on 100Ohm diff. termination would be several hundreds mV.

    But, We saw approximately 0V on it.

    Thanks,

    Takashi

  • 0 in reply to Takashi Yasuno
    TI__Mastermind 23707 points

    Hi Takashi-san,

    Can you clarify some details on your measurement?  Are you measuring single-ended or differential?  Are you measuring 0V immediately after power up, or after some time?

    I'd have to double check the output impedance of the driver.  The driver is typically muted while the retimer does not have CDR lock.

    Thanks,

    Drew

  • 0 in reply to Drew Miller1
    Prodigy 20 points

    Hi Drew-san,

    I measured the 100nF-decoupled output signal single-ended immediately after powering up and it measured 0V.

    Thanks,

    Takashi

  • 0 in reply to Takashi Yasuno
    TI__Mastermind 20160 points

    Hi Takashi-san,

    Drew is out of office today and can provide additional feedback later this week.

    I see in the datasheet electrical characteristics section that the typical differential output amplitude with TX disabled is 6.1mVppd. Your measurement of 0V single-ended immediately after power up seems to match this specification.

    Best,

    Lucas

  • 0 in reply to Lucas Wolter
    TI__Mastermind 23707 points

    Hi Takashi-san,

    I observed similar behavior as your initial scope shot if the scope is configured with high impedance termination.

    If the scope channel is configured with 50 Ohm termination, the signal from the DS50DF230 on power up is much smaller.

    High impedance termination:

    50 Ohm termination:


    Do you have any open questions or concerns at this point?

    Thanks,

    Drew