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Question on 5150 termination resistor

Anonymous
Anonymous
Guru 17045 points
Other Parts Discussed in Thread: TVP5150, TVP5146, TVP5150AM1

Hi All,

 

I would like to ask a question on TVP5150 termination resistor.

 

 

 

 

In EVM6437 design which I am referencing, TVP5146 YOUT pins have 33 Ohm termination resistors near them, and at DM6437’s receiving end, YI[0..7] pins also have 22Ohm termination resistors near them. So on each path there is a total of 55 Ohm termination resistor.

 

Is this too large?

 

And I am replacing 5146 with 5150 and considering the appropriate termination resistor value. The tracks are of 5mil width, 1.4mil thickness. So how large termination resistor should I choose? Could anyone give a recommendation on this?

 

And may I place just one group of termination resistors near 5150, or need to place two groups, like in EVM6437 design?

 

 

 

 

Thanks,

Zheng

  • Zheng,

    The EVM was probably configured to use the Y port for either input or output.  When used as an input with the TVP, it is best to use one set of resistors at the TVP.  For the TVP5150AM1, 22-ohm resistors are commonly used when the trace inpedance is 50-ohms.  Some coustomer do not use series ternination on the TVP5150AM1 Y[7:0] traces, when the trace length is short.  The TVP data CLK is the most important signal and is best to always include series termination for this trace.  If board space is availble,  is it also best to include the resistors on the data lines.  This provides flexibility and can also help with noise surpression.

  • Anonymous
    0 Anonymous in reply to Larry Taylor

    Dear Larry,

    I see it, thanks very much.

    Zheng

  • Anonymous
    0 Anonymous in reply to Larry Taylor

    Dear Larry,

     

    If I am using

    Track width

    5mil

    Track height

    1.4mil

    Track to internal plane distance

    12.6mil

    Dielectric constant (er)

    4.8

     

    And http://www.pcb123.com/help/calculators/microstrip.html gives me Zo of 92.02 Ohm, is it a significant difference from 50 Ohm? And if I am still using 22Ohm termination resistor, will there be serious problems?

     

    Only when I increase the width to 20mil, which is far too thick, do I get Zo of 51.17 Ohm. Do you think the calculator is correct?

     

    I have in fact found most traces on EVM6437 are of 5mil width whose impedance by the above calculator is 92.02 Ohm,  and most termination resistors are between 22-33 Ohm. Is the matching correct?

     

     

     

    Zheng

  • Zheng,

    92-ohms looks pretty close for your data.   It could be significant if the signal rise/fall time is fast and the trace is long.  The TVP5150AM1 rise/fall is 4ns typcial, which is slow compared to other devices.  If trace lengths are short (4-5 inches), termination is less of a concern.  Larger series resistors could actually form RC with the load capacitance  and degrade rise/fall time.  Some customers do not use termination resistors on the TVP5150AM1 data lines.  The pixel clock is the most critical signal and should definitely include a series termentation, just in case it is needed.  One approach we take is to use R-packs for data and a discrete R for clock.  Clock and data can then be adjusted independently.  Having resistor footprints is the safe way to go, even if they are not needed.  Series resistors can also help reduce noise and EMI.  Final choice of resistor size will depend on final board trace length, trace impedance, and more importantly signal testing with an oscilloscope.  Make sure you probe the signal at the end of the transmission line at the receiving device.

    I am not sure about the EVM layer stack-up and signal height above ground plane.  I thought that the digital traces were designed for 50-ohm impedance.  22-33 ohms should be close for a 50-ohm trace when driver source impedance is included.

  • Anonymous
    0 Anonymous in reply to Larry Taylor

    Dear Larry,

    I will pay close attention to clock signal. Thanks very much.

    Zheng

  • Anonymous
    0 Anonymous in reply to Larry Taylor

    Dear Larry,

     

    Could you have a look at this post on impedance? I am very much puzzled and has no idea of it.

     

     

    Zheng