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SD3GDAIII fault light

Richard Woodham1
Prodigy 210 points
Other Parts Discussed in Thread: LMH0307, LMH0395, LMH0376, LMH0303

I am working with the SD3GDAIII, feeding it with a signal from an HD-SDI pattern generator and connecting SDO- to a 75R terminated scope. In HD mode the fault light illuminates unless I place my finger over the SDO1- trace in the vicinity of the 'REV A' silk marking. Why is this?

The fault light appears to work correctly in SD mode (i.e. only illuminates when output or input cable is removed).

Richard

PS I modified the board to include a pull-up resistor on the cable driver /FAULT line as the LMH0307 datasheet describes this output as open drain and there doesn't appear to be a pull-up in the User Guide schematic (Figure 4).

  • 0
    TI__Guru* 88835 points

    Hi Richard,

    The LMH0307/3 termination fault detection was never designed to be precise; its purpose is to provide an indication when no cable is connected to the output (near end). The termination fault detection works by detecting reflections on the output.  The device measures the peak-to-peak output voltage.  The output amplitude is normally 800 mVp-p.  No termination results in 2x the output voltage (1600 mVp-p) due to the 100% reflection.  Also, there is a register, HDTFThresh, that sets the fault detection threshold. You can adjust this register as well.

    If possible please use 75 ohm termination to see if you still detect fault? I am thinking maybe the cable of scope termination perhaps could be causing the device to declare fault. You can also use HDTFThresh to adjust the threshold as well.

    Regards,,nasser

  • 0 in reply to Nasser Mohammadi
    Prodigy 210 points

    The 3 un-used channels (SDO+, SDO1+ & SDO1-) all have their 75 Ohm resistors soldered on (SD3GDAIII as supplied) and measure ~74.9R with a multimeter.

    Fault light shows with cable or termination on SDO-.

    I'd love to adjust HDTFThresh in the LMH0307, but how?  On the supplied GUI (Version:1.1.1.0), only the equalizer (LMH0395) & Reclocker (LMH0376) are listed in the selection list or in the register map.

    I'm not prepared to write my own GUI.

    PS On the Select Tags... button below this post the list of previously used tags stops fairly early on in the alphabet.  Is this right?

  • 0 in reply to Richard Woodham1
    TI__Guru* 88835 points

    Hi Richard,

    The present GUI does not support the cable driver SMBUS registers. This register is available and customers have used this register to fine tune the threshold settings based on their application requirements. Please refer to LMH0303 register map where this register is described in more details. There is a lot of flexibility in setting the threshold based on your application perhaps you can fine tune this once you receive your own board.

    Regards,,nasser