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TMUX646: Tie unused pins to GND?

Kevin Shia
Prodigy 20 points
Part Number: TMUX646
Other Parts Discussed in Thread: TMUX1574, TMUX136

Hello, 

On the datasheet, it recommends tying every unused signal pin to 50 Ohm resistor to GND. However, I was wondering if it is possible to just have them tied directly to GND to reduce component usage. 

Best. 

  • 0
    Guru 316920 points

    High-frequency signals need termination with the proper impedance. In lower-frequency applications, these pins may be left floating. See [FAQ] What to do with Unused Pins & Exposed Thermal Pads?

  • 0
    TI__Mastermind 35528 points

    Hey Kevin,

    Just to add a little bit to Clemens comment (which is correct). The datasheet recommendations is more of a system level recommendation given the assumption that if you're using the TMUX646, you'll be working with signals upwards of a few GHz. In this case it is generally recommended to have termination. If you're using the TMUX646 at lower frequencies (typically the cutoff is roughly 500MHz-1GHz depending on who you ask) you can consider removing the termination all together without much concern for performance loss.

    Thanks,
    Rami

  • 0 in reply to Rami Mooti1
    Prodigy 20 points

    Hey Rami, 

    Thanks for the response. 

    Do you happen to have another part that only uses 4-Channel MIPI instead of 10-Channel MIPI?

    Just wondering since if we use this part, we will have to use a lot of terminating resistors due to unused channels. 

    Also, given the better availability for smaller resistors, can 51 Ohm be used instead?

    Regards,

    Kevin

  • 0 in reply to Clemens Ladisch
    Prodigy 20 points

    Then can you explain why in this thread, where the bandwidth is just as high, they state that it's okay to tie them to GND or float them? e2e.ti.com/.../ts3dv642-unused-lane-tie-to-ground-or-floating

  • 0 in reply to Kevin Shia
    Guru 316920 points

    For low-frequency signals, it does not matter whether the unused pins are floating or have any other voltage.

  • 0 in reply to Clemens Ladisch
    Prodigy 20 points

    What is considered "low-frequency" signals? We might run this up to 1.5 GHz.

  • 0 in reply to Kevin Shia
    Guru 316920 points

    There is a certain parasitic capacitance between the channels. At high frequencies, the impedance becomes low enough to get noticeable effects. The specified crosstalk is −40 dB at 1.25 GHz; also see figure 6-17.

    If you are using digital signals, you might decide to that you have enough noise margin. But at 1.5 GHz, the noise is probably high enough so that you cannot ignore it.

  • 0 in reply to Clemens Ladisch
    Prodigy 20 points

    Thanks for the info, we will put 49.9 Ohm resistors to GND on all unused channels.

    However, do you have any alternate ICs that are 4 channel instead of 10 channel for MIPI DSI applications?

    Regards,

    Kevin

  • 0 in reply to Kevin Shia
    TI__Mastermind 35528 points

    Hey Kevin,

    Perfect, at this frequency it's definitely recommended for those unused channels!
    So we don't have a 4channel 2:1 that I would recommend at 1.5GHz. The closest 4 channel is the TMUX1574. While it has a 2GHz bandwidth, this is the 3dB bandwidth so at 1.5GHz you're looking at closer to 2.5dB insertion loss. You can determine if this is acceptable but typically I would think it wouldn't be pushing the limits a little too much.

    I would instead recommend a multi-device solution here. 2 of the TMUX136 would provide the 4 channels you need and has more headroom in the bandwidth so that the insertion loss is minimized at 1.5Ghz. The chip itself is pretty small (1.50 mm × 2.00 mm) so 2 of them shouldn't add too much space to the design. Also has flow through routing (sources all on one side, drains on another) so the trace routing should be a lot easier to handle. Especially since you'll be using 2 of them.

    Thanks,
    Rami 

  • 0 in reply to Rami Mooti1
    Prodigy 20 points

    Crazy thought, given that the CLK channel and Channel 1 are relatively "far away" from Channel 3 and Channel 4, do you see it being an issue if we terminate Channel 2, but ground channel 3 and 4?

  • 0 in reply to Kevin Shia
    TI__Mastermind 35528 points

    Hey Kevin,

    Being relatively further away will help and you will see less crosstalk the further those pads and your traces are but I would still recommend safe practices and terminate them simply because I can't guarantee that you wouldn't see much there. It may be find just grounding this way is sufficient, which wouldn't surprise me, but ultimately it would be your call if you it meets your standards. You may want to order a part and test it to see if it's up to the standards you're looking for.

    Thanks,
    Rami