Video DACs ADV7123-EP and ADV7393 output opamp driver circuit

I'm using Video DAC ICs ADV7123-EP and ADV7393 in my design to generate PAL signals. I need to drive two video multiplexer inputs from the output of each DAC. Please find attached the overview of design. Please help me choose opamp driver circuit such that the video signals do not deteriorate before sending the signals to outside world.

Thanks, 

Sandeep

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    •  Analog Employees 
    •  Super User 
    on Feb 3, 2020 2:46 PM

    There are a multitude ways to do this.  As an example look at the AD8044 and Figure 36.  Since the positive inputs are high impedance you can tie 2 of the inputs to the same source and generate 2 buffered outputs.

    1) Drive op-amps from +-5V rails so the outputs work will go below ground.  PAL signals go negative.

    2) Watch source impedance settings.  The source must always see a 75 Ohm load

    3) Make sure the op-amp output has 2x gain and drives into 150 Ohm load just like figure 36.

    There are many op-amps that can be used.  Just watch the gains, loading, bandwidth and be able to drive below ground and you should be good.  I can look at any solution you come up with.

    Keep in mind, this is for DC coupled video.  If AC coupled then other issues come up.

    Also the ADV9393 datasheet has some reference designs using the ADA4411

  • Thanks for the reply. There are PAL composite video signals coming into our board from outside world( 3 PAL signals coming through B2B connector as shown in figure). Is it possible to provide some kind of isolation for these signals(for eg. opto- isolation)? If yes, please suggest the circuit.

  • 0
    •  Analog Employees 
    •  Super User 
    on Feb 5, 2020 2:15 PM in reply to sandeep ms

    What kind of isolation are you looking for?  AC coupling, transformer isolation, ground loop prevention,  true opto-isolated is problematic for analog signals.

  • We are trying to isolate the PAL signal and its ground path from the signal's path inside the board. It may be galvanic isolation or opto-isolation. It will be an added advantage if we have AC-coupling as well. Please suggest your opinion. 

  • 0
    •  Analog Employees 
    •  Super User 
    on Feb 6, 2020 2:26 PM in reply to sandeep ms

    Attached is a app note on general video circuits including AC-coupling

     PDF

    For full galvanic isolation might be accomplished using video transformers however I do not have any experience with this path.  There are several transformer solutions used in CCTV applications.

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