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LMH0384: Unable to see output coming what could possible go wrong

subeg sran
Prodigy 40 points
Part Number: LMH0384
Other Parts Discussed in Thread: SD384EVK,

I am using 384EVK kit which SDI cable equalizer. I dont see output coming.

I have Tektronix waveform generator 2300 which is driving cable driver SD303EVK kit and then connected with 75ohm SDI compliance cable and then signal is fed into cable equalizer 3.3V kit and finally signal goes into Tektonix waveform generator.

Problem is when i connect power supply 3.3V to the SDI cable equalizer module i do  see on Agilent supply 3.3V set correctly. But as i connect 75 ohm cable to the equalizer kit. Agilent supply read 1.2V not sure why this is happening. It seems when i connect 75ohm cables with BNC connector due to some reason voltage is not stable.

It worked for me for 3 days and then i see this issue and could not resolve.I am thinking this kit could be defective and didn't last long.

Any ideas.

  • 0
    TI__Mastermind 29635 points
    Hi Subeg,

    Based on your description, something may have become shorted out, since you mentioned that the board was working for you a few days ago and is now only drawing 1.2V. To determine if there is a shorting issue with the EVK, I have a few questions:

    1. Can you power just the SD384EVK and let us know what the current reading is for the board without anything connected? Also, what is your current limit setting for the power supply?
    2. Does the reduction from 3.3V to 1.2V on the supply only occur when plugging in the cable with active signal?
    3. Do you have a picture of the board to clearly show us all the jumper settings you are currently using?

    Thanks,

    Michael
  • 0
    TI__Guru* 92040 points

    Greetings,

    If i understand you correctly, you are saying power supply to the LMH0384EVM shows 3.3V. Then when you connect coax cable to the SDI input of the LMH0384EVM supply drops to 1.2V. Is my understanding correct?

    If this is the case and my understanding is correct, i am thinking perhaps LMH0384 power supply could be current limiting. Please check to make sure you are setting the supply current limiting is at least 0.5A or 1A. 

    Otherwise, if current limiting is set to 0.5A or 1A and you are still seeing this issue, perhaps LMH0384EVM may have been damaged.

    Regards,,nasser

  • 0 in reply to Michael Lu (Santa Clara)
    Prodigy 40 points

    Hi Micheal,

    I power up SD384EVK kit again. I am settings voltage on Agilent power supply to be 3.3V and Current to be 1 Amp.

    Currently it seems something is shorting with connected kit to any external connection.

    I read 1.02 V when current limit is set to 1amp. For some reason i think kit is getting short. I followed all protocols and put legs on the EVK kit and also my lab is ESD compliance lab. I am still surprised part got damage so easily. I am think it could be kit.

    Here is default jumper settings:

    Let me know if you have any questions.

    Thanks,

    Subeg

  • 0 in reply to subeg sran
    Prodigy 40 points
    Sorry i mean:
    Currently it seems something is shorting WITHOUT connected kit to any external connection
  • 0 in reply to Nasser Mohammadi
    Prodigy 40 points
    Hi Nasser,
    Thanks for response. If you think part has damaged.
    What could be possible reasons. Any guidelines from TI experience.
    From my actions. I don't think i did something incorrect. I would not like to repeat this and my be need to order another part if it is damaged.
    We still need to root cause.

    Thanks,
    Subeg
  • 0 in reply to subeg sran
    TI__Guru* 92040 points
    Hi Subeg,

    It seems somehow LMH0384 may have gotten damaged. I believe you are just powering up the board without any connection (other than power supply connection) and voltage drops to 1V. Also, check for short across power supply coupling caps as well.

    Regards,,nasser
  • 0 in reply to Nasser Mohammadi
    TI__Mastermind 29635 points
    Hi Subeg,

    The only root cause that comes to mind at the moment is that there may be a faulty power supply. I have seen in the past that, if the power supply is not functioning correctly, it is possible for the supply to emit a large transient spike at turn-on before settling at the desired voltage level. Something similar can happen right before turning off the power supply. This transient can damage the device over time.

    Please check on this by applying a scope probe on the supply rails and then triggering on power up and power down to see if such a spike is observed. This test can be done in addition to the recommendations Nasser suggested to debug the problem.

    Regards,

    Michael