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TVP5154A Impedance

MT12
Prodigy 120 points
Other Parts Discussed in Thread: TVP5154A, TVP5154

I am student working on a project using the TVP5154A. I am working on the board layout and due to my limited budget I do not have the money for a controlled impedance PCB. There is not much in the data sheet about the NTSC video input impedance and YCbCr 4:2:2 output impedance. I am assuming the NTSC component input requires 75 ohms impedance trace, am I correct?

Also does the YCbCr require a specified impedance? How strict are these impedances since they are not called out in the data sheet? Do you have any tips to achieve the required impedances on these line(s)?

Thanks 

  • 0
    TI__Mastermind 43875 points

    Mike,

    Whilst impedance controlled boards are preferred they are not really too critical as long as you  take care to maintain analog shielding, good grounding and keep the analog traces as short as possible.

    On the digital side I recommend including series termination resistors so that, if you do have any signal integrity issues, you can tune the resistors. I would start off with something in the 22R region and then see how that works out. I would make sure that the trace lengths for each output group are all within a few 100 mil of each other though.

    If you really want to learn about some of the details of PCB design I suggest that you look at the datasheets for both the TVP5154 and your target device, and look carefully at the min/max output delays for the TVP5154, and the max setup/hold times for your target design. Then you can examine the difference in trace lengths for the TVP output data and clocks, and knowing the signal velocity in the trace, you can then work out exactly if you have enough setup/hold time at your target device.

    What you should find though is that at 27MHz you should have plenty of margin.

    The impedance of any signal carrying medium is really about matching, and not the actual value itself. 75R is typically used for coaxial cable and RCA video cables. Whenever there is a change in impedance there is some energy lost/reflected which in turn causes slight distortions in the signal. For the digital outputs of the TVP to inputs of the target device the PCB impedance, whilst not controlled, will be for the most part constant. What becomes more important in this case is to match the TVP output impedance to the PCB trace impedance. Likewise at the target device input. Both these places are where there are changes in impedance. Adding series resistance can help compensate for these changes.

    I am by no means an expert in impedance control so suggest that you read up a little if you are really interested in the details. What I would say is that in many low speed digital designs (less than 50MHz) you can usually get away with not worrying too much about impedance matching. Analog is different and really depends on what type of signal you are working with as to whether you need to be as concerned. An excellent source of information on the importance of impedance control and matching etc... is pretty much any information or presentations by Dr. Howard Johnson.

    For the analog inputs you can usually 'tweak' the effective impedance by changing the trace width and/or spacing. You can also change the spacing of the ground reference plane to the signal by moving the reference ground to a layer further away from the signal (and cutting the ground plane copper area under the signal). Your PCB tool should then be able to help you analyze the trace impedance.

    Hope this helps a little.

    BR,

    Steve