Why does my demux give multiple outputs at the time ?

Discussion in 'Analog & Mixed-Signal Design' started by Ultravioletta, Mar 20, 2017 at 4:57 PM.

  1. Ultravioletta

    Thread Starter New Member

    Feb 17, 2017
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    I'm just trying out The 4067 to see how it works. The outputs here are all floating. I'm aware of that. what I don't understand is why I'm getting
    6v for the selected output pin and 5.99v for thoses whic are not? Can anyone explain to me what really is a demux impedance-wise ?
    [​IMG]
     
  2. crutschow

    Expert

    Mar 14, 2008
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    Where's the ground connection to the IC?
     
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  3. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Where is the power connection to the IC?

    The "real" impedance is on the datasheet; it varies with Vdd, Vin, and temperature.

    ak
     
  4. Ultravioletta

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    Feb 17, 2017
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    Hi! umm.. I don't really understand your question sorry :(
    What is there that I'm missing and that is to be connected to GND ?
     
    Last edited: Mar 20, 2017 at 5:39 PM
  5. Ultravioletta

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    Feb 17, 2017
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    As for the impedance here is what the datasheet says :
    Low “ON” Resistance - VCC = 4.5V. . . . . . . . . . . . . . . . . . . . . . . . . . . 70Ω(Typ) - VCC = 6V . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60Ω(Typ)
     
  6. AnalogKid

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    Note on the datasheet pins 12 and 24. These are the GND and power pins, as indicated in all of the circuit drawings. For your circuit, pin 24 connects to the +6 V battery, and pin 12 connects to ground.

    ak
     
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  7. AnalogKid

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    That is correct. Why do you not think this is the "real" impedance?

    ak
     
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  8. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    Not sure what tool was used to make the attached schematic. However in LTSpice depending on the library, the IC properties may need to be edited to properly define power. Comments about connecting this pin and that pin are meaningless if the model does not provide those pins. Which at a glance, this one does not.
     
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  9. Ultravioletta

    Thread Starter New Member

    Feb 17, 2017
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    It is all connected just as you described. yet, if my input is 4V for instance, I get my 4V on the elected pin and 3.996 v ! on the other pins !
     
  10. Ultravioletta

    Thread Starter New Member

    Feb 17, 2017
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    I do think it is the real impedance but I'm confused as to how is what I am seeing possible ? :confused:
     
  11. Ultravioletta

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    Feb 17, 2017
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    Hi! I'm using proteus and it has this option " goto to part in design Explorer" and I can see from there that the hidden pin 24 - VDD- is indeed connected to the VCC/VDD net which corresponds to 5V and the pin 12 is connected to the ground.
     
  12. djsfantasi

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    But your supply is 6V. Are you sure Vcc is 5V? Do you see a problem with an input pin getting a higher voltage than its supply?
     
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  13. AnalogKid

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    The non-selected outputs are floating. Depending on the software (and the physical measuring device), this can have different results, including what you are seeing. Add a 10K resistor from each of the 16 output pins to GND and measure again.

    Also - if the component has pins with defined net names, you need to make sure the nets connected to those pins have the same names. Vcc and Vdd are different nets, and neither one is assigned to a battery terminal by default.

    ak
     
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  14. djsfantasi

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    Ak. His inputs aren't floating. When the switches are open, the pin is connected to power through 10K resistors. Initially, I had the same reaction as you, until I took a closer look at the schematic. That is assuming his power symbols are defined correctly.
     
  15. AnalogKid

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    Said outputs, not inputs - the signal I/O pins used as outputs in this circuit.

    As I read the schematic, they all are not connected to any external load, and X1 is selected by the control pins. Do I think an unknown program/spice model is assigning a 5.99 mV value somehow. Sure, why not?

    What is the voltage on X1 when X is disconnected from B1 and connected to GND?

    ak
     
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  16. hp1729

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    Nov 23, 2015
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    V IN cannot exceed VDD. 6 V in? 5 V VDD?
     
  17. djsfantasi

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    Outputs? You said outputs!

    D'oh, missed that. You're right.
     
  18. Ultravioletta

    Thread Starter New Member

    Feb 17, 2017
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    I changed it to 4 V. I'm still getting the same prob.
     
  19. Ultravioletta

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    Feb 17, 2017
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    Thanks a lot guys! I'll try to figure it out.
     
  20. Ultravioletta

    Thread Starter New Member

    Feb 17, 2017
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    her* mind you :p . It actually works when I add up the 10k resistors which leads me to another problem: I need this circuit to feed some other circuit with a variable input impedance that is to be measured. so my Demux's behaviour will depend on the second circuit and that is not good at all.
    I'm going to need a voltage follower I suppose?
     
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