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cable driver for the MADI audio standard

Jon Bean1
Expert 1130 points
Other Parts Discussed in Thread: DS30BA101, LMH0001, LMH0034, LMH0394, LMH0366

Hi

I am looking for a cable driver chip that can be used with the MADI audio standard. It needs to be able to drive a 125 Mbit signal of 0.3V to 0.6V p-p into a 75 ohm load. I found the part DS30BA101 that looks like it may do the job. Thought I would just check that someone agrees with me or can suggest something else.

Thanks

Jon

  • 0
    TI__Genius 13475 points

    Jon

    The DS30BA101 would work, as would most any of our cable drivers designed to support the SDI (Serial Digital Interface) standards used for broadcast video.

    MADI has very broad specs for amplitude and edge rates, whereas SMPTE specs are much tighter - the one issue that you might have with the DS30BA101 is that it is designed to support very fast data, (3 Gbps), and has very fast edges, which may cause issues at 125Mbps if lots of care is not taken in matching impedances and shielding for EMI, the devices directed for the SDI standard have a switch for slew rate, (SD/HD), and by using the SD (Standard Definition) mode, the edge rates will be slower and the device will generate less EMI.

    Mark

     

  • 0 in reply to Mark Sauerwald88474
    Expert 1130 points

    Mark

    I have just looked at a SD part the lmh0001. It says that you can alter the output level with a resistor. It doesn't seem to say how to calc the resistor value for the output level. It just gives some typical examples. It seems that using the suggested value would give an output that is too large. So do you know how to determine the resistor value?

    Thanks

    Jon

  • 0 in reply to Jon Bean1
    TI__Genius 13475 points

    Jon

    For the bulk of the range, the resistor value is roughly proportional to the output amplitude, so for MADI levels, I'd try a resistor value of ~330Ohms.

    MADI has a very wide range of output amplitudes, so you will have quite a bit of latitude in resistor value.

    Mark

  • 0 in reply to Mark Sauerwald88474
    Expert 1130 points

    I have been looking at the LMH0034 cable equalizer too as I also need to implement a MADI receiver. I want to connect this to an FPGA using LVDS an input. But I am thinking that I will have to convert the output level of the part to a LVDS compatible range. I guess it could be done with some capacitors and resistors, but are there any parts in the TI range that would do this for me?

    Thanks

  • 0 in reply to Jon Bean1
    TI__Genius 13475 points

    Jon

    How long of a cable do you need to equalize? - One of the issues with using a cable equalizer designed for the SMPTE standards is that they expect that the initial launch amplitude of the signal was within the SMPTE spec (800mV +/-10%)  MADI does a very poor job of controlling input amplitude, which makes it much harder to design a cable equalizer which will properly equalize the signal.  If a cable equalizer is required,  I would suggest the LMH0394, which from a speed standpoint is way overkill, but it has an additional loop to compensate for launch amplitude variation as well as cable length.   I would also take a look at the LMH0366 reclocker, which will lock at MADI rates.

    Mark