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LMH1981: Recommended buffer between Sync Separator and Video Clock Generator

Ryotaro Fukui
Intellectual 2345 points
Part Number: LMH1981
Other Parts Discussed in Thread: LMH1983,

Hello team,

We have received a question from a customer regarding the buffer between LMH1981 (Sync Separator) and LMH1983 (Video Clock Generator). I would appreciate it if you could give me some advice.

Question)
The customer wants to put a Buffer between LMH1981 and LMH1983.
The application is to branch the BP signal of LMH1981 output to LMH1983, FPGA.
This is to pass and determine the format on the FPGA side.

Is there a Buffer that can be used for this purpose?
Is it possible to use a general-purpose Buffer (for example, SN74AVC4T245RSVR)?

Best Regards,

Ryotaro Fukui

  • +1
    TI__Expert 5110 points

    Hello Ryotaro,

    The LMH1981 works well with the LMH1983, there should not be a need to have a buffer between these devices. As you can see in the LMH1983 datasheet the Typical Application Block Diagram even shows the LMH1981 and a the Sync Separator to LMH1983.

    The SN74AVC4T24RSVR is more useful as a voltage level translator. Please advise if there is confusion.

  • 0 in reply to Aaron Black
    TI__Intellectual 2345 points

    Hello Aaron,

    Thank you for the feedback.

    It is said that Buffer is not required between LMH1981 and LMH1983, but with this customer's design, the wiring pattern will be long due to the configuration of the device.

    LMH1981 → Pattern 9cm → Connector → Pattern 9cm → Connector → Pattern 9cm → LMH1983

    Question 1
    The total is close to 30 cm. Is it better to place a Buffer in this case too?

    Question 2
    Is the output current specified for the LMH1981 output terminal?

    Best Regards,

    Ryotaro Fukui

  • +1 in reply to Ryotaro Fukui
    TI__Expert 5110 points

    Hello Ryotaro,

    Question 1
    The total is close to 30 cm. Is it better to place a Buffer in this case too? Customer is using device BP signal - Tbp = 2.857MHz. All output signals should have a resistive load of about 10 kΩ and capacitive load of less than 10 pF. If these bounds can be met with 30 cm trace then it should work. I know some connectors might have bad capacitive load and this would be the biggest reason to worry about signal integrity.

    Question 2
    Is the output current specified for the LMH1981 output terminal? This is not explicitly mentioned in datasheet but if over current could be an issue each output can be protected by current limiting with a small series resistor, like 100Ω. 

  • 0 in reply to Aaron Black
    TI__Intellectual 2345 points

    Hello Aaron,

    Thank you for the answer. The customer is satisfied with the feedback and this case is now closed.

    Thank you for your support.

    Best Regards,

    Ryotaro Fukui