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I am very new to electronics (software engineer background).

I want to send an analog signal (5V) to one/many input wires. The input wires are connected to a 16 channel mutiplexer. My arduino then reads all of the 16 channels.

Right now the issue is: If I connect only one pin to the 5V output of the arduino many other of the multiplexer input pins also read high voltage values even tho only one pin is actually connected.

I am pretty sure this issue is a duplicate - please point me to the right direction.

I have read about decoupling (if that is the case - please explain it for newbies without background).

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    It is not clear at all what you are doing. Please show a schematic of exactly what you have connected on the multiplexer (including power, ground, selectors), and what/where you are measuring.
    – Mat
    Jul 16, 2021 at 6:19
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    Please show a schematic or a picture of your setup. What multiplexer are you using? Also understand that an unconnected input has an undefined value and might read high or low or even oscillate between them. It’s much better to connect an unused input to either ground or Vcc to give it a defined value and not have it pick up noise.
    – StarCat
    Jul 16, 2021 at 7:54

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As I understand it you have a multiplexer with one of the inputs connected to 5 volts and the rest are open. What you are reading is leakage in the IC and associated wiring. There are many reasons they tell us not to leave inputs open, you just found one. This is especially problematic with CMOS devices because of there extremely high impedance. The first thing the TI publication SCBA004D–July 1994–Revised September 2016 tells you is power consumption is becoming a major issue in today’s market. However, when designing systems using CMOS and BiCMOS devices, one must understand the characteristics of these families and the way inputs and outputs behave in systems. It is very important for the designer to follow all rules and restrictions that the manufacturer requires, as well as to design within the data-sheet specifications. Because data sheets do not cover the input behavior of a device in detail, this application report explains the input characteristics of CMOS and BiCMOS families in general. It also explains ways to deal with issues when designing with families in which floating inputs are a concern. Understanding the behavior of these inputs results in more robust designs and better reliability.

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