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LMH1981: Glitches and missing HSYNCS

Simon Brown
Intellectual 340 points
Part Number: LMH1981
Other Parts Discussed in Thread: LMH1982,

I use the LMH1981 sync separator to pass HSOUT, VSOUT and FOUT to my FPGA.  These signals are then fed through to the LMH1982.  From time to time the LMH1982 loses reference, because there are glitches on the HSOUT signal, and more rarely, there is even a missing HSOUT pulse.  The LMH1981 is being fed by a SDI digital to analog converter, 1V p-p (after terminator) through a 1uF ceramic AC-coupling capacitor at the pin of the LMH1981.  The signal is PAL Black and burst.

The same circuit has worked fine in the past, so I'm wondering if in the last couple of years a batch change has happened to the LMH1981 which is causing this?  Or is the AC-coupling capacitor material critical?

Kind regards,

Simon

  • 0
    TI__Genius 16845 points

    Hi Simon,

    It's not clear why the circuit is misbehaving now after working fine in the past. Were any other modifications made to the circuit?

    Is this issue observed on a single system/device or multiple?

    Is the missing pulse observed directly at the output of LMH1981? Do you have an oscilloscope capture?

    Kind regards,
    Lane

  • 0 in reply to Lane Boyd
    Intellectual 340 points

    I've traced it to much higher noise on the supply or gnd.  I've added a bit more filtering on the power input to the PCB, but it's still not perfect.  Is there any filtering I can apply at the chip that will work with both bi and tri-level syncs?

    Regards,

    Simon

  • 0 in reply to Simon Brown
    TI__Genius 16845 points

    Hi Simon,

    Thanks for the update. It is good that the added power supply decoupling offers an improvement.

    The 75ohm termination combined with Cin would form a high-pass filter that works with bi and tri-level. I think the filter may be improved, but it is best to control the impedance to 75ohm @ DC for proper matching. For example, you might design a second order RC for example to achieve greater attenuation. 

    If the power supply noise is high frequency (relative to the frequencies of video input signal), RC bandpass filter might be an option.

    Kind regards,
    Lane