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DS90UB914A-Q1: impedance issue

Hirofumi Moriyama
Prodigy 55 points
Part Number: DS90UB914A-Q1

Hi team,

We measured TDR our DS90UB914A-Q1 board as shown bellow.

1. Why DS90UB914A-Q1 has sharply drop to  about 20Ω from pin. 

2 Why actually about 60Ω when  termination resistance control( register 26h/46h) sets to 50Ω in DS90UB914A-Q1

 (Why actually about 50Ω when  termination resistance control( register 26h/46h) sets to 37.7Ω in DS90UB914A-Q1)

Best Regads,

Hirofumi

  • 0
    Prodigy 55 points

    Hi team,

    Sorry to missed insert TDR image.

    Could you please tell me how shuld I fix or investigate this issue.

    Best Regards,

    Hirofumi

  • 0 in reply to Hirofumi Moriyama
    TI__Expert 3390 points

    Hi Hirofumi,

    Have you tried to capture the TDR of just the board itself? with a 50 Ohm termination at the pin instead of the 914A? If not, can you try that. In terms of the set-up with the 914A, what is exactly being done? The device is powered and you are checking the TDR from the connector to the device?

    Regards,

    Mandeep Singh

  • 0 in reply to Mandeep S
    Prodigy 55 points

    Hi Mandeep-san,

    Thank you for reply.

    Yes, TDR image I send before is board itself.
    And I've asked this TDR measurement to dedicated company so I think it's exactly correct.
    (the device was measured with power)

    I'll try late 50 Ohm termination instead of 914A.

    914A EVM's TDR shown below looks like 30 Ohm, where same point of 20 Ohm of our board.
    Is this difference within specifications?

    NOTE:EVA 914A's date code is ?8AF7LUG3, and our board is 97CCJXUG3
    And Could you please tell me how range of impedance (max and min) at thie point?

    Best Regards,
    Hirofumi

  • 0 in reply to Hirofumi Moriyama
    TI__Expert 3390 points

    Hi Hirofumi,

    If you are going to have the device powered then you need to ensure that the back channel and forward channel drivers are turned off. Those drivers toggling will have an impact on the performance because the TDR essentially transmits an incident wave and compares that to the reflected wave and if the drivers are toggling then the capture will not be a true representation of the TDR. I do want to mention that it is not typically the part that has issues because we have internal termination. If it was a part issue, it would be intermittent, so you certainly wouldn't see it on every board. It is normally the board performance that has an impact on the signal integrity and TDR alone won't tell you the performance. I recommend you do a Return Loss profile as well to better understand the performance over frequency.  

    Regards,
    Mandeep Singh

  • 0 in reply to Mandeep S
    Prodigy 55 points

    Hi Mandeep-san,

    Thank you for your advice.
    I'll re-measure the TDR refer to your advice.
    before that, Could you tell me how do I turn forward channel off on 914A?

    Best Regards,
    Hirofumi

  • 0 in reply to Hirofumi Moriyama
    TI__Expert 3390 points

    Hi Hirofumi,

    I'll take that offline and send you an internal email.

    Regards,

    Mandeep Singh

  • 0 in reply to Mandeep S
    Prodigy 55 points

    Hi Mandeep-san,

    I'm just waiting for your internal email.

    Best Regards,
    Hirofumi

  • 0 in reply to Hirofumi Moriyama
    TI__Expert 3390 points

    Hi Hirofumi,

    I sent an email on July 2nd. Can you please check your work email again and see if it didn't end up in junk mail.

    Regards,

    Mandeep Singh

  • 0 in reply to Mandeep S
    TI__Expert 3390 points

    That's July 2nd on the West Coast (USA). Not sure what time zone you're in but email is from Mandeep Singh.