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Original question:

TCA9617B: TCA9617B IBIS model for 3.3V

TCA9617B: TCA9617B undershoot for -0.5 V

Brian Chan
Expert 6630 points
Part Number: TCA9617B

Tool/software:

Hi Team

My customer wave form and datasheet wave form have 0.5V offset.

     

is it because SCH value wrong? 

BRs

Brian

  • 0
    Guru 317390 points

    The pedestal would go only to near 0 V. The voltages marked in the red circle go below GND; this is probably ringing due to the high speed of the falling edge. Anything larger than −0.5 V is harmless. (If your oscilloscope probe uses a ground clip, then this ringing might be happening in the cable loop and not on the board; a ground spring would allow better measurements of high-frequency signals.)

  • 0 in reply to Clemens Ladisch
    TI__Expert 6630 points

    Hi Clemens

    below is feedback from customer

    The R1385 and R1388 are adjusted to 510 ohm, but the undershoot is still near -0.5V.

    In addition, we don’t want  the value of these resistances to be too high. We hope the value will close to 0 ohm.  

    Under same schematic, the source from other vendors doesn’t generate the issue, so whether you can provide a solution for us to resolve

    it.

    falling of SCL

    Falling of SDA

    BRs

    Brian

  • 0 in reply to Brian Chan
    Guru 317390 points

    I do not see these resistors in the schematic.

    As I said, −0.5 V is harmless.

  • 0 in reply to Brian Chan
    TI__Guru 51896 points

    Hi Brian,

    In addition to Clemens' feedback, I do not seem to see R1385 and R1388 in the provided schematic as well.

    I presume they are series resistors and not pull-ups. If so, adjust closer to the transmission line (typically for 50 ohms). I.e. the output impedance of the device + series resistor should be closer to 50 ohms. 

    For example, output impedance of the device is 33 ohms with VOL of 0.1 V and IOL of 3 mA , R = VOL / IOL. Hence, series resistors of 17 ohms would suffice (50-33). Other vendor's output impedance can be closer to the transmission line used and may not have as much impedance mismatch to observe issue. You can confirm this if their rise / fall times are slower, thanks.

    Best Regards,

    Michael.