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LMH0346 LOCK_DET issue

Robert Savage
Prodigy 40 points
Other Parts Discussed in Thread: LMH0346

Hi.

I have a design utilising a number of LMH0346 devices. I am using the LOCK_DET signal from the device to enable the Output Cable Driver. The input to the 0346 comes from a Switch (AC Coupled). In certain situations the LOCK_DET signal becomes intermittent or is lost completely when a number of outputs from the switch are enabled. How sensitive is the Lock Detection circuit? I have followed what I believe to be a good layout around the Differential routing but I have a concern I may be getting crosstalk between routes to the re-clockers. This is only an issue on one device of the eight I have in-circuit, although when this one output fails some of the other output channels fail. This is compounded by another anomaly in that if some of the other outputs are then enabled from the switch to the re-clockers, all outputs then work. I have the LOCK_DET connected to the /OP_MUTE, could this be an issue? When the re-clocker does fail the LOCK_DET output becomes intermittent. The input to the re-clocker is AC Coupled and has a local 100Ohm parallel termination. I am also wondering if I may have an issue with a damaged\intermittent  switch. I do have the option to drive the Control signals to the cable drivers from a Processor but this would mean re-laying the board.

Thanks.

  • TI__Mastermind 29655 points
    Hi Robert,

    Thanks for your post. We are currently working our way back from being out on holiday from the Thanksgiving holiday. We will respond in the next 1-2 days.

    Regards,

    Michael
  • in reply to Michael Lu (Santa Clara)
    TI__Guru* 93670 points

    Hi Robert,

    After reading your post a couple of times,

    1). Does LMH0346 devices share the same power rail as the switch? Could there be a case when cable drivers and LMH0346 devices plus switch is turned on we could be starving current? You mentioned one of eight devices has this problem. Under the same condition can we use a lab supply for this device to check on this?

    2). You mentioned you have several LMH0346 devices. Does this problem occurs at any video rate? How about if you interleave 3G and SD. Does the problem worsens? If this is the case then perhaps there could be a cross talk or cross coupling between differential traces.

    3). LMH0346 has loop filter cap 56nF. How about on the problematic device what happens when you touch 56nF Cap one side or another. Does the problem goes away? I am thinking perhaps noise is getting injected into the loop filter when you have multiple parts running.

    Regards,,nasser