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TMUX7411F: Floating inputs and pull-down value on TMUX7411F analog switch

Lucien Th
Prodigy 10 points
Part Number: TMUX7411F


Tool/software:

Dear TI experts,

We are using a TMUX7411F 1:1 4-channel switch to enable/disable a CAN termination resistor.
Here is the schematic:

As you can see, switches 1 and 2 are used symmetrically.

Switches 3 and 4 are left unused and floating, with their control pins pulled up with 100k resistors to disable them (inverted logic).

My questions are:

1) Is 100k pull-up resistor a good value to keep the CMOS inputs pulled up on the unused SEL3 and SEL4?

2) Is it OK to leave the deactivated analog switches floating? Is there a risk of inducing noise to the CAN bus through the adjacent channels in the TMUX7411F?

Thanks in advance!

  • 0
    Guru 318050 points

    1. Yes. But 0 Ω would work just as well.

    2. Yes; you would need to care about noise only in high-frequency applications.

  • 0 in reply to Clemens Ladisch
    Prodigy 10 points

    Thanks for the answers.

    Can you elaborate a bit?

    Our client is worried that 100k is too high on the logic input and could cause noise.
    I am answering that 100k pullup is pretty standard albeit on the high side of value, and that with the 3µA max input current on the logic inputs, this is 0.3V drop max on the resistor, which connected to 3.3V drives the pin quite higher than the logic thresholds, preventing coupled noise to cause any problem. What do you think?

    Also, for the unconnected analog Sx and Dx, I read that internal circuitry in analog switches is not susceptible to noise pickup as a logic input is, but I'd like an expert's take on this. Can noise on these unconnected pins be carried to the pins that are used on the CAN bus?

    Thanks in advance!

  • 0 in reply to Lucien Th
    Guru 318050 points

    1. If there is noise that induces a current larger than 3 µA, then the voltage drop will be larger. I do not know how much noise is to be expected. In any case, 0 Ω is safe.

    2. Very high frequencies could indeed be picked up by an unconnected pin. But those are not used with CAN, and are likely to be prevented by filters on the CAN lines.

    See [FAQ] What to do with Unused Pins & Exposed Thermal Pads?