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info of 74hc4051 maximum volatge needed

Discussion in 'General Electronics' started by michael nikolaou, Mar 30, 2009.

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  1. Hi

    I want to read many analog inputs from 0....24 V . I want o use one
    multiplexer.
    I think the 74hc4051 is the most common. My question is in case of
    overvoltage
    over the supply rails at the inputs can one get away without protection
    diodes with the use
    of appropriate input resistors ???. I mean the chip will go into latch-up
    the minute an
    overvoltage is detected or it needs appropriate current to create a latchup
    or other
    malfunction ?.
    Any help will be appreciated .



    Michael
     

  2. NO! NO! NO!

    The 4051 can't be used beyond +/- supply rails. If the voltage can
    occasionally go beyond the rail (even with the protection diodes and
    current limiting), there will be a really weird problem with the signal
    leak from one input to another. This happens with the generic HC muxes;
    the ADGs and MAXs don't behave like that.


    Vladimir Vassilevsky
    DSP and Mixed Signal Design Consultant
    http://www.abvolt.com
     
  3. KMoffett

    KMoffett

    696
    68
    Jan 21, 2009
    Have you looked at the datasheet?

    Absolute Maximum Ratings:
    Switch I/O Voltage (VIO) VEE -0.5 to VCC +0.5V
    Clamp Diode Current (IIK, IOK) ±20 mA

    This will not work as an analog switch for 24VDC analog signals.

    Ken
     
  4. Thanks for the reply Guys .

    Can i ask if i use schottky diodes tied to the supply rails so voltage
    is withing +-.2v within the supply rails will it be ok ?????.
     
  5. Bill Sloman

    Bill Sloman Guest

    It can be more subtle than that - when I once relied on diode clipping, with
    a 4051 everything looked as if it was working fine until I started checking
    linearity and precision, when the current that leaks out of or into the
    substrate
    gave me small but unnecessary errors when the signal was very slightly
    outside
    the rails.

    Easy enough to fix when you realise what is going on, but in my case3 it
    took
    a day or so of careful work before I noticed that I'd got aproblem.

    John Larkin called it correctly.
     
  6. You might be able to 'get away with it' (as in the chip would not
    immediately fail), but I would not advise it. Aside from latch-up
    (which is specified by the abs max ratings), it can affect the
    accuracy of the channel you are measuring -- you would be exeeding the
    recommended operating conditions (Vee <= Vin <= Vcc) and such things
    happen.

    If you put a voltage divider on each input st the output voltage won't
    exceed Vcc/Vee (eg. 10:1 with +/-5V supplies), then you can mux the
    divided-down voltages.



    Best regards,
    Spehro Pefhany
     
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