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LMH0324: SDI Signal Path: Inter-Board Component Location and Grounding Strategy

Steven zhu
Prodigy 60 points
Part Number: LMH0324


Tool/software:

I'm using the LMH0324 as an SDI signal input buffer, fed directly from an SDI cable. My setup involves three boards housed within an enclosure. The BNC connector needs to be mounted on Board 1 (B1), but the LMH0324 itself must be on Board 3 (B3).

My questions are:

1. Regarding the input capacitors C9 and C10, and the return resistor R28: Where should these components be located? Should they be installed on Board 3 (with the LMH0324) or Board 1 (near the BNC)?

2. The BNC connector ground (GND1) should be the chassis ground. The LMH0324 has its own signal ground (GND2). Where should these two grounds be connected together?

please find the schematic and box structure 

  • 0
    TI__Genius 14986 points

    Hi Steven, 

    Steven zhu said:
    1. Regarding the input capacitors C9 and C10, and the return resistor R28: Where should these components be located? Should they be installed on Board 3 (with the LMH0324) or Board 1 (near the BNC)?

    We recommend to place the components near the chip. 

    Steven zhu said:
    2. The BNC connector ground (GND1) should be the chassis ground. The LMH0324 has its own signal ground (GND2). Where should these two grounds be connected together?

    If these two grounds can be common grounded via board 2, that would be great. 

    Best,
    J

  • 0 in reply to J
    Prodigy 60 points

    Thanks J,

    Question 1, may you share the reason behind?   My thought was:  If we move C9, C10, and R28 to Board 1, closer to the BNC connector, C9 and C10 would effectively block external and ground interference from the coaxial cable. This would allow the SDI AC signal to transmit along a 75-ohm trace  to the chip. Ideally, we should aim to eliminate any interference.

    Question 2, the core issue is radiation susceptibility. We've observed that when the BNC connector's coaxial cable is exposed to external radio frequency (RF) signals, these signals couple onto the cable's outer layer. This interference appears to dissipate into the cable, causing a loss in the SDI signal. Do you have any suggestions for mitigating this?

    Question 3 (New): Since the SDI signal path from the BNC connector to the chip involves AC coupling through a capacitors C9 and C10 (blocking DC), we're considering the role of the ground connections. Is it essential to connect GND1 and GND2? We hypothesize that leaving these grounds (GND1 and GND2) disconnected might prevent external RF signals from interfering with the SDI chip. What are your thoughts?

  • +1 in reply to Steven zhu
    TI__Genius 14986 points

    Hi Steven, 
    Q1: This is because any external component you add causes impedance mismatch. By putting external component together, the impedance mismatch can be lumped together. 

    Q2: We recommend picking coaxial cables with proper shielding. It looks like the coaxial cables are not properly shielded so it is picking up RF signals. By picking properly shielded cables, you will be able to see reduction in the radiation susceptibility. 

    Q3: To ensure that the BNC connector and the device have same ground potential, we recommend to have the same ground for both. If you would like to mitigate some ground interference, you may want to have a chassis ground and connect the DGND and chassis ground with 1 million ohm resistor and 4700pF capacitor. 

    Best,
    J