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THS7374: Passing RS170 NTSC Video

Richard Elmquist
Mastermind 45250 points
Part Number: THS7374

Is there anything special that the customer needs to do to simply pass RS170 NTSC video directly through the device?

Do we have a schematic available that the customer could use that shows the components that need to be used if this is the application that they want to do?

Can this be done with this chip?

Thanks for your help with this!

Richard Elmquist

  • 0
    TI__Genius 15975 points
    Hi Richard,

    Yes, it is possible to use the THS7374 for passing the RS170 NTSC video through the device by setting the channels in 9.5MHz low pass filtering mode. The customer can look at Figure 64 in the datasheet for the external components required for this application.

    Best Regards,
    Rohit
  • 0 in reply to Rohit Bhat
    TI__Mastermind 45250 points
    Rohit,
    Thanks for your quick response!
    I will let you know if the customer has any further questions.
    Have a great day!
    Richard Elmquist
  • 0 in reply to Richard Elmquist
    TI__Mastermind 45250 points
    Rohit,
    By looking at the THS7374 EVM Item 3 in Table 2: (C1, C2, C3, C4 and C19, C20, C21, C22 and C7, C8, C9, C10) capacitor values are not shown in the schematic and not in the BOM. Can you please get back to me as soon as possible on what values are recommended for the mentioned caps? If there are some calculation needed could you point me to the proper calculations to use to derive the values needed.
    Thanks for your help with this!
    Richard Elmquist
  • 0 in reply to Richard Elmquist
    Guru 140200 points
    Hi Richard,

    I hope I'm not impolite towards Rohit when I try to answer your question?

    In the text below figure 59 of datasheet of THS7374 you will find indications when an external capacitor can be added in parallel to the termination resistors. This cap can be useful to filter overshoot problems. Figure 53...55 show the impact of this cap on the frequency respsonse.

    But in the text above figure 61 of datasheet it is mentioned that the stray capacitance appearing directly at the THS7374 output pins should be kept below 20-pF to maintain stability. So, I think, that in a standard application it is best to omit the caps C7...C10.

    C1...C4 and C19...C22 can also be used to provide an additional filtering. But in a standard application it's not wise to have them installed, because they can easily erode the 75R termination.

    Kai
  • 0 in reply to kai klaas69
    TI__Mastermind 45250 points
    Kai,
    Thanks so much for your response!
    I will let you know if the customer has any further questions.
    Have a great day!
    Richard Elmquist
  • 0 in reply to Richard Elmquist
    TI__Mastermind 45250 points
    Kai,
    The customer is saying that the video is black and white and fuzzy.
    He is wondering if he needs to include the filter capacitors (C1 -C4 and C19 -C22?
    Is there anything that he should test first before he tries to add these caps?
    If he decides to add the caps, are there standard values or does the customer need to fine the frequencies of the noise source and then add appropriate caps? Can you give me a method to do this without eroding the 75ohm termination?
    Thanks for your help with this!
    Richard Elmquist
  • 0 in reply to Richard Elmquist
    TI__Mastermind 45250 points
    Kai,
    Have you been able to look at the questions from the customer?
    Thanks for your help with this!
    Richard Elmquist
  • 0 in reply to Richard Elmquist
    TI__Genius 15975 points

    Richard,

    Can you confirm whether the THS7374 is being used in a 9.5MHz low pass filter mode or the bypass mode? For NTSC video, the 9.5MHz low pass filter mode would have to be used. 

    Can you also confirm that the video signal does not have any sync information? Sync information is there usually when the input signal goes below GND or 0V to -300mV. If there is sync, then the customer would have to use the inputs in AC coupled mode by replacing the Z1 -Z4 with 0.1uF instead of 0-ohms.

    The C1-C4, C7-C10 and C19-C22 are not needed in the actual system implementation, and are only present for the device evaluation.

    Best Regards,

    Rohit

  • 0 in reply to Rohit Bhat
    TI__Mastermind 45250 points
    Rohit,
    If this does not solve the issue can you possibly call the customer?
    I will let you know what happens.
    Thanks so much for your help!
    Richard Elmquist
  • 0 in reply to Richard Elmquist
    TI__Genius 15975 points
    Richard,

    Yes, it should be possible to have a call with the customer if this does not solve their issue.

    Best Regards,
    Rohit