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LOG200EVM: log200 /EVM is susceptible to interference.

Chen Alex1
Genius 4545 points

Part Number: LOG200EVM
Other Parts Discussed in Thread: LOG200

hi sir

As shown in the figure below, the LOG200 Vlog_out signal exhibits 60Hz interference;
adding shielding helps mitigate this issue. 
Is the TI LOG200 IC itself particularly susceptible 
to external environmental interference at the PD input
(with the effect becoming more pronounced at lower currents)? 
I ask this because I did not encounter this problem when previously using the ADI ADL5304.

image.png

  • 0
    TI__Mastermind 19050 points

    Hi Alex, 

    Yes, the LOG200 is susceptible to 60Hz noise. I am not sure why you are not seeing this with the ADL5304. Have you determined if this noise is coming from the power supply? 

    How are you supplying the input current and what is the current range you are testing? 

    Best regards,
    Ashley

  • 0
    Genius 4545 points

    hi Ashley

    input by  the photodiode

    lighten(2%), waveform 1310nm, 1550nm, optical power -7dBm~ -60dBm, input current range  50pA~ 10uA
  • 0 in reply to Chen Alex1
    TI__Mastermind 19050 points

    Hi Alex, 

    Could you show me a waveform of the power supply? I would like to see if there is noise on the power supply that is coupling on the LOG200. 

    Best regards,
    Ashley

  • 0 in reply to Ashley Kang
    Genius 4545 points

    hi Ashley

    AC/DC PSU 12V output voltage ripple. 

    In the image above, the vertical scale is 20 mV per division with AC coupling, 
    and the signal components do not include a 60 Hz frequency. We tried switching to an isolated linear PSU,
    but the issue persisted. (Additionally, 
    our experiments showed that the Vlog output is affected when a laptop charger is brought close to the board.)

  • 0 in reply to Chen Alex1
    TI__Mastermind 19050 points

    Hi Alex, 

    Could you clarify your test image with the PSU? Is this the power supply voltage that is feeding into the LOG200? Can I also see how the LOG200 was connected inside the shielding? It's important to make sure the connections on the input and output are very short cables to minimize noise coupling.

    Chen Alex1 said:
    We tried switching to an isolated linear PSU,
    but the issue persisted.

    What "issue persisted" are you referring to? The 60Hz noise? 

    The laptop charger has a high frequency switching noise that can couple to the board and cause higher noise. This would impact both devices. 

    Do you have a comparison image of the ADL5304 output? Did you use the ADI EVM to test the output? 

    Regards,
    Ashley