Multiplexer output does not match the input

Hi there

I have some strange effects on an analog multiplexer from Analog Devices where I need your help to understand why this can happen. I'm using the ADG758BCPZ 8:1 multiplexer with 5V power supply and about 0.8V to 4V analog signal range at the input. The output of the multiplexer is open.

Now what happens: If I switch from one channel to another, the output changes immediately almost to the voltage level which is lying at the input of the channel I switched to. Only almost but not completely. In the first moment, the voltage level at the output is slightly beneath (or above, depending if I switch from a higher or a lower voltage level) the real voltage level of the input. The figure below shows what I mean.

The voltage difference between the input and output right after switching is about 30-40mV and the time until the output reaches the input voltage level is about 6us (I did not measure the time exactly, this is just an estimation). Can anyone explain why this effect happens and where it is specified? I need to understand this issue for evaluating another device. For this it would help me if I know where I can find the information about this effect in the datasheet.

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  • PB,

    1. Why would you need 0.1uF capacitors at unused S1 and S8? Or, can you at least change them to something like 1000pF?

    2. What is the width of PCB traces for 5VA1 rail (and for Ground)@? If it is not a plane but thin trace, you may want to add 10uF tantalum or aluminum capacitor. It would not hurt to have one such capacitor per rail even if there is a plane.

    3. As you already disconnected outputs and performance has not changed, I do not see the need removing the ADG758 from the circuit and testing it separately.

Reply
  • PB,

    1. Why would you need 0.1uF capacitors at unused S1 and S8? Or, can you at least change them to something like 1000pF?

    2. What is the width of PCB traces for 5VA1 rail (and for Ground)@? If it is not a plane but thin trace, you may want to add 10uF tantalum or aluminum capacitor. It would not hurt to have one such capacitor per rail even if there is a plane.

    3. As you already disconnected outputs and performance has not changed, I do not see the need removing the ADG758 from the circuit and testing it separately.

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