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Original question:

LMG1020: Why is input pulse width substantially larger than output pulse width

LMG1020: Ringing

Sushmit Dhar1
Prodigy 80 points
Part Number: LMG1020

I have soldered the laser diode on the board, also R1 and R2 are 0 as out of the box. Still, there is a lot of ringing in the falling edge, and it takes a lot of time to switch off. Is there anything more I can do to reduce this?

  • 0
    TI__Genius 9371 points

    Hi Sushmit,

    Thank you for your question. Could you please clarify: are you referring to the LMG1020-EVM from TI? 

    Best regards,

    Leslie

  • 0 in reply to Leslie Marquez
    Prodigy 80 points

    Yes, Leslie, I am referring to LMG1020-EVM.

  • 0 in reply to Sushmit Dhar1
    TI__Genius 9371 points

    Hi Sushmit,

    Thank for the clarification. Before soldering the laser diode, we recommend to first fine tune the gate drive pulses using resistors R5 to R8. Then once you obtain the pulse you desire, you can remove the resistors and solder the laser diode. Please review section 4 "EVM Operation Out-of-the-Box" of the EVM user's guide with detailed procedure on how to do this. 

    Link: EVM user's guide

    Best regards,

    Leslie

  • 0 in reply to Leslie Marquez
    Prodigy 80 points

    Hi Leslie,

    Yes, I have done and received the desired pulse before removing the R5-R8 resistor.

    Thanks and Regards,

    Sushmit

  • 0 in reply to Sushmit Dhar1
    TI__Genius 9371 points

    Hi Sushmit,

    The gate drive resistors, R1 and R2, can be used to control the turn on and turn off drive strengths. In your case, since you are having trouble with the turn off ringing, you can use a 2 Ohm resistor at least on OUTL to help with inductive ringing. 

    We recommend using at least a 2 Ω resistor at each OUTH and OUTL to avoid voltage overstress due to inductive ringing. 

    If you are still having issues after this, please share the waveforms of the signals observed at the input of the driver and at the gate so we can help you debug further.

    Best regards,

    Leslie

  • 0 in reply to Leslie Marquez
    Prodigy 80 points

    Thank you for your reply. My present waveform(optical output, measured with a PMT) looks like this while trying to generate 1ns pulse, with out of the box R1 and R2. So, do you think placing 2 Ohms R1 and R2 will improve the waveform further?

  • 0 in reply to Sushmit Dhar1
    TI__Guru**** 189399 points

    Hi Sushmit,

    Leslie will get back with you by Monday.

  • 0 in reply to Sushmit Dhar1
    TI__Genius 9371 points

    Hi Sushmit, 

    Is the waveform you shared the voltage across the laser diode?

    Could the PMT be contributing to the noise?

    When taking oscilloscope measurements, we normally recommend to use the tip and barrel of the probe to make the measurements instead of using the regular probe clip/hook to minimize the noise introduced by this connection. It would be good if you can measure the signals with an oscilloscope at both the inputs and also at the outputs of the driver using the probe's tip and barrel, so we can help debug further. The reason to measure the input as well is because the noise could be actually caused/introduced at the inputs. 

    Let's try this first to analyze the signals before you switch the resistors if you haven't changed them yet. 

    Best regards,

    Leslie

  • 0 in reply to Leslie Marquez
    Prodigy 80 points

    This is the waveform of the received signal at the PMT. Maybe the distortion is caused by the PMT probes then, I will check.

    And one more thing, for TOF calculation, how to do you generally measure the start time, as in when the light is emitted from the diode? As I could not find evidently any output on the board, or is it the trigger from the waveform generator? 

    Thank you 

  • 0 in reply to Sushmit Dhar1
    TI__Genius 9371 points

    Hi Sushmit,

    Thanks for the follow up. For ToF calculation information, I recommend that you check the sensor or laser diode manufacturer's website/documentation to see what they recommend in terms of ToF measurement.

    Best regards,

    Leslie