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Switching between two LVDS (DS90UR907) sources using a CML mux (DS25MB100)

Kouji Hamanaka
Prodigy 110 points
Other Parts Discussed in Thread: DS25MB100, DS90UR907Q-Q1, DS25CP102, DS90UR908Q-Q1

I am currently designing a board that is intended to switch between two LVDS sources. The sources are from the DS90UR907. The plan is to select between two 720p video sources. The only IC's that I could find that supported LVDS inputs and outputs only supports a maximum data rate of 1500Mbps. Which is very close to the rate needed for 720p uncompressed, without the DC balancing encoding that the DS90UR907 includes. I am currently contemplating using the DS25MB100. The data rate is high enough and the data sheet states that the receivers are compatible with AC coupled LVDS lines, which the outputs of the DS90UR907 are. However, I need to deserialize after the mux, and that is where I am unsure of things. I currently have the DS90UR908 deserializer in my design. Would the output of the DS25MB100 be capable of driving the receivers on the DS90UR908? Are there any special concerns that I should have with the entire circuit?

Just to clarify this is the setup of the circuit.


DS90UR907Q (x2) --->DS25MB100 ---> DS90UR908Q

  • 0
    TI__Expert 4400 points

    The DS25MB100 will work for this application.  You could also use a 2x2 LVDS crosspoint switch such as the DS25CP102 (in the 2:1 configuration) to switch between the two sources.  Just be sure to AC couple the signal at the DS90UR907Q-Q1 output and the DS90UR908Q-Q1 input according to their datasheets.

    Gary Melchior

  • 0 in reply to Gary Melchior
    Prodigy 110 points

    Thanks for the help. Would the inputs and outputs of either of the above mentioned switches also need to be AC coupled? This is my first time working with AC coupled signals, and I am unsure if there is supposed to be coupling capacitors on both ends of the lines, or if there is a single capacitor per line. The data sheets for the DS90UR907/908 have an applications diagram which shows coupling capacitors on the LVDS driver side and the receiver side. However, much of the technical literature i have seen, seems to only show a single coupling capacitor per line, between the driver and reciever.

  • 0 in reply to Kouji Hamanaka
    TI__Expert 4400 points

    I would recommend to AC couple both the inputs and the outputs of the crosspoint switches.  The AC coupling requirement is for the DS90UR907Q-Q1 and DS90UR908Q-Q1: The DS90UR907Q-Q1 requires AC coupling on its output (DOUT), and the DS90UR908Q-Q1 requires AC coupling on its input (RIN), per the respective datasheets.

    DS90UR907Q (x2)  --||--  SWITCH (2:1)  --||--  DS90UR908Q

    Gary Melchior

  • 0 in reply to Gary Melchior
    Prodigy 110 points

    Thanks again Gary. Not sure if I asked the question clearly. What I meant to ask is if the circuit should look like what you replied with in your last post, a single capacitor or like the figure below, or does it even matter how many capacitors there are as long as there is one, since it's function is to block the DC component of the signal?

    DS90UR907Q (x2)  -||----||-  SWITCH (2:1)  -||----||-  DS90UR908Q

  • 0 in reply to Kouji Hamanaka
    TI__Expert 4400 points

    The AC coupling capacitors are only necessary at the LVDS driver output of the DS90UR907Q and at the LVDS receiver input to the DS90UR908Q.  It is not necessary to also include AC coupling right at the crosspoint switch I/O.  It would not hurt to include these extra AC coupling capacitors in the line, but I do not think it would be necessary.

    Gary Melchior