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MUX509: Question about MUX509 unused pin

Part Number: MUX509
Other Parts Discussed in Thread: ADS125H02

Hi Team,

I check that the video about unused pin, just wondering that if MUX509 analog input is floating, then we will read the random number in the software since the state of analog input is not fixed[no integrated PU/PD resistor]. Am I understanding it right? Please correct me if I am wrong, thank you.



  • Hey Frank,

    It almost sounds like you're implying that you're expecting the mux to drive a signal. Just to be clear this is an analog mux, not a digital mux. Where a digital mux may redrive your signal an analog mux simply provides a passive path from one input to another. Therefore an analog I/O's will be floating or pulled to whatever is on the passive path - whatever is seen on the inputs (S1 for example in the MUX509) will be seen on the other (D for example in MUX509) whenever the control pin logic corresponds with the associated channel from the truth table :

    1 and 0 are defined when the control pins (A0, A1, EN) are above or below the logic thresholds noted in the datasheet.

    When the control pins (A0, A1, EN) are somewhere between these VIH and VIL values, then the state is unknowns, so we know for certain which channels are turned on. This is why it's necessary to have the control pins biased to a high or low voltage above or below the VIH/VIL.

    With regards to the actual analog I/O's (S1-S8 and D), these can be left floating at lower speeds. At higher frequencies it is recommended to tie to ground. This is because when a channel is unselected or unused it will be effectively floating and acting as an antennae picking up noise and crosstalk from higher frequency signals on used channels. It is common practice to redirect these to ground through a 50ohm (assuming that the board is 50ohm impedance matched, to reduce reflection).

    Let me know if this answers what you're questions or if you need anything else clarified.


  • Hi Rami,

    Thanks for the detailed explain!

    I saw that you said analog I/O's can be left floating, just wondering if the I/O's is floating(no matter high/low frequency), when I try to read the ADC, then the ADC should read a random number, right?

    I am asking this because I am reading the reference design which is combine MUX509 and ADS125H02 for a multi-channels application. I did test on the board and saw if the analog I/O's are floating, I can read different value on the software. I would like to have your comment on this case to see if this case is a normal one.

    The reference design webesite:



  • Hi Frank,

    That is correct, when the I/O pins are floating you will measure a random number from the ADC, depending on the noise of your system and environment. If you want to avoid this scenario, i would recommend a weak pulldown on the I/O pins, so that they are always connected to some known voltage like ground.