Choosing the right switch

Discussion in 'The Projects Forum' started by tifania, Mar 14, 2016.

  1. tifania

    Thread Starter New Member

    Mar 14, 2016
    Hello everyone,

    I am Stefania and I am new in this forum. I am working on a project for developing a sensor based on the principle of EIT (Electrical Impedeance Tomography).

    For the project, I need to inject into the material I am using a constant current between two electrodes placed at the periphery of the sensor and then read the corresponding voltages at the other electrodes (I have 8 electrodes).
    Then the current has to be switched between the remainig electrode pairs in a scanning cycle (as you can see in the picture).

    I am using 2 demultiplexer with 8 outputs to control the current injection. One multiplexer serves to control the current source and the other one for the ground in order to switch all the electrodes between ground and current source.

    The circuit is powered with 5V and the resistance of the sensor is around 30Ohm.

    I want to use switches connected to the multiplexers output, but at the moment I have only used relays which are a bit slow for my application. Ay suggestions? I could use mosfets but I don't know how and which one should be the best choice for my application.

    Also I post an image showing my idea.

    Thanks a lot,

    Stefania current.JPG mux.jpg
  2. #12


    Nov 30, 2010
    A mosfet is a good way, but nobody can know how to attach them without some sort of schematic from you.
  3. crutschow


    Mar 14, 2008
    You should be able to use multiplexers for all your switching.
    Why do you need the relays or MOSFETs?
    #12 likes this.
  4. AnalogKid

    Distinguished Member

    Aug 1, 2013
    The signal current is 17 mA, which is a bit high for most CMOS analog mux chips.

    1. What is the switching frequency and on time for each switch?
    2. How much resistance or voltage offset can you tolerate through a switch? Unlike relays, all electronics signal switches affect the signal more directly in the way of either appearing as a series resistor or a DC voltage offset.

  5. crutschow


    Mar 14, 2008
    What is the value of the constant-current?