Analogue Multiplexers

Discussion in 'Homework Help' started by Steve1992, Jan 21, 2012.

  1. Steve1992

    Thread Starter Senior Member

    Apr 7, 2006
    Do you have any experience with analogue multiplexer ICs.
    I was surprised to see the typical resistance of the switch is around 100 ohms (dependent on
    input voltages) when compared to mechanical rotary switches.

    If I connected a DVM to the common output to measure, for example 12 volts, and
    the resistance of the switch, for example was 100 ohms, because of the high input
    resistance of the DVM, the voltage drop across the switch is negligible. Therefore a negligible change
    to the measured reading?

    12V / (10M + 50 ohms) = 1uA

    1uA*50 ohms = 50uV drop across switch.

    Am I right with these assumptions?
    NB. I attached the datasheet just for description use - not for specific data.
  2. Adjuster

    Well-Known Member

    Dec 26, 2010
    It is fairly important to keep the load resistance for such a switch within a reasonable range. With a very high resistance, as presented by a DMM, the voltage dropped across the switch element in the "on" state will indeed be negligible, but leakage through it in the "off" state may be significant, particularly capacitive leakage at higher frequencies.

    A lower load resistance will mean more noticeable losses with the switch on. As you have noted, the element resistance is voltage-dependent. This is significant because it can cause distortion. On the whole, the load resistance should be as large as possible consistent with adequate isolation with the switch off
    Steve1992 likes this.
  3. Steve1992

    Thread Starter Senior Member

    Apr 7, 2006
    I looked at the prices of multi channel mux - they are relatively expensive!
    I think I will stick to mechanical switch - K.I.S.S.
  4. crutschow


    Mar 14, 2008
    You can't really make a blanket statement like that. ;) It depends entirely on the application.

    Relays are fine for low speed switching of low to high current signals or loads and they do provide isolation of the control signal from the switched signals. But they are large, have a limited life, and require a relatively large amount of power to operate.

    CMOS multiplexer/switches user very low power, can switch very rapidly (such as would be needed if you wanted to sequentially monitor may sensor outputs with a microprocessor), and have virtually unlimited life.