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Original question:

TMUX1109: TMUX1109PWR application circuit for switching between two inputs

TMUX1109: Off-resistance diference between A side to B side

Rodrigo Vernini
Intellectual 290 points
Part Number: TMUX1109
Other Parts Discussed in Thread: TMUX1108,

I’m developing a multiplexer circuit for potentiostatic analysis using three TMUX1108PWR and one TMUX1109PWR, tested across three prototypes PCBs.

During validation, I observed an unexpected pulsed beep in continuity mode on the B side of the TMUX1109PWR — between pins DB and SnB. This behavior was not present on the A side.

To investigate further, I measured the resistance between Dx and Sx under three conditions:

  1. Powered PCB, switch OFF (EN, A0, and A1 measured at 0 V):

    • A side: 1.7 MΩ

    • B side: 260 kΩ

  2. Unpowered PCB:

    • Results discarded due to unstable floating logic inputs.

  3. TMUX1109PWR desoldered from PCB:

    • A and B sides both measured >7 MΩ

Despite the large difference in off-state resistance, I wouldn’t expect continuity mode to beep. So I measured current flow when shorting Dx to Snx (device powered and switch OFF):

  • A side: ~1 nA

  • B side: ~80 nA

Additionally:

  • DB (pin 9) is connected only to an output terminal — no other components or routing.

  • DA (pin 8) is connected to an output terminal and also to the D terminals of three TMUX1108PWR devices.

For reference, the TMUX1109PWR used in this design was sourced through TI’s sample program.

I plan to replace the TMUX1109PWR in the next prototype. However, I must present the current version to my client tomorrow, and unfortunately, this issue will affect the accuracy of the potentiostatic analysis.

Any other information about this? In the image bellow, Rn's are microcontrolled

  • 0
    TI__Mastermind 35638 points

    Hi Rodrigo, 

    Is the pulsed beep continuous or just when you make initial contact, then it goes away?
    Measuring with a multimeter isn't very reliable for these types of readings. When you measure the switch when it's in the off-state, you're effectively measuring the resistance across a capacitance, as the mux will behave as a capacitor in series across the open switch when it is in the off-state. 

    All that being said, the change may be from the extra leakage from the fact that you have 3 TMUX1108's connected to the TMUX1109. The leakage is higher than expected though. Are you using guarding techniques to be able to achieve the lower leakages? 

    Thanks,
    Rami

  • 0 in reply to Rami Mooti1
    Intellectual 290 points

    Hi Rami,

    Thanks for your reply.

    I understand that a single beep could be caused by capacitive discharge, but in this case it's a continuous pulsed beep — very consistent across all three PCBs and repeatable in every test.

    It seems to be right at the threshold of continuity detection, and based on my measurements, it's likely due to the ~80 nA current flowing through that specific switch.

    Regarding your point about leakage from the other TMUX1108s: that would make sense if I observed it on the A side, which is connected to the shared drain of the TMUXs. But the pulsed beep is actually happening on the B side, which is not connected to those other switches.

    Not using guard rings, but the problematic side should be much less prone to leakage because it has a direct and short track to terminal. 

    If there's any additional measurement or diagnostic you'd suggest, I’d be happy to investigate further.

    I could solder just the TMUX1109PWR and power to other PCB, and recheck, I`m pretty sure I`ll get the same results, it`s quite a simple PCB.

    Thanks again,
    Rodrigo

  • 0 in reply to Rodrigo Vernini
    TI__Mastermind 35638 points

    Hi Rodrigo,

    I definetely mis-read that and had the A and B sides connected backwards. Thanks for the clarification. 
    It could be damage but the fact that it's repeatable is concerning. 
    Can you remove the muxes and see the board leakage itself? The only thing that comes to mind here would be a layout issue or some damage, although I know you said the PCB is quite simple and consistent damage on 3 devices does indicate a more intrinsic issue. So it's worth verifying the board first. 

    Thanks,
    Rami