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Discussion in 'Electronic Basics' started by Animesh Maurya, May 12, 2004.

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  1. Hi all,

    How do NAND schmitt trigger CD4093 differ from usual NAND gate CD4011,
    thought they both perform the same function?

    Thanks
     
  2. Jan Pompe

    Jan Pompe Guest

    the CD4093 has a Schmidt Trigger on the input.

    Useful in noisy environments
     
  3. If a gate have a schmittrigger input, it means that it's able to handle
    analog input, ex audio.
     
  4. Guest

    I rephrase the question: What is a schmidt-trigger(ST)?

    A ST is a device that have a built in memory of one previous event (it is not a memory like a sram or eprom but it is a bistable (two-state) device).

    It works like this. The ST have a switching threshold window as below, also called hysteresis.

    (use fix width font to see figure)

    VDD--------------------------------
    ------
    / \ /\
    Vtu-----/--------\--/--\---------
    / \/ \
    Vtl---/------------------\--------
    / \
    / \
    VSS---------------------------------

    If the input voltage is below Vtl the ST will interpretate it as a '0'.
    If the input voltage is above Vtu the ST will interpretate it as a '1'.
    If the input voltage is between Vtl and Vtu ST will keep the last value.

    In this example it would be the sequence "010"

    If the NAND do not have a ST it will only have one threshold voltage Vt, see below.

    VDD--------------------------------
    ------
    / \ /\
    / \ / \
    Vt ----/----------\/----\----------
    / \
    / \
    / \
    VSS---------------------------------

    In this example the sequence would be "01010"

    So why and when to use ST-inputs?

    In the previous post above it is stated to use it to avoid false values due to noise.

    Another very important use is to speed up the transition time. Eg if the input signal have a long rise time it will affect the risetime on the output of the nand. But if the nand have a ST on the input the ST itself is constructed to have fast rise and
    fall times. I.e. the output rise and fall time from the nand will be more or less independent on the input rise and fall time.

    Hope this can be to some help

    /Pfna
     
  5. Bob Masta

    Bob Masta Guest

    The 4093 does indeed perform the same logic as a
    "normal" NAND gate like the 4011, and can probably
    be used to replace it in almost all logic functions.
    The difference is when you have input signals that
    don't have sharp transistions, or in fact may be sine
    waves or slowly varying voltages. The Schmitt trigger
    action of the 4093 means that you still get a sharp
    transition on the output, and since there is some
    hysteresis you don't get bursts of multiple transistions
    on slow, noisy input zero crossings like you would on
    a conventional gate. These are great for building all
    sorts of oscillators, one-shots, debouncers, etc. But
    their inputs will be seeing only fast outputs of other gates,
    there is no advantage over 4011.

    Hope this helps!


    Bob Masta
    dqatechATdaqartaDOTcom

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    www.daqarta.com
     
  6. grahamk

    grahamk Guest



    Have a look at http://homepage.ntlworld.com/g.knott/elect344.htm
     
  7. JeffM

    JeffM Guest

    What is a schmidt-trigger? [sic]
    The *trip* point and the *release* point are different levels.
    Another way to diagram Schmitt trigger hysteresis (loop):
    http://groups.google.com/groups?&threadm=
     
  8. Thanks to all repliers.
    (two-state) device).

    According to my preliminary knowledge, bistable device are the basic
    building block of sram.

    Thanks

    Best regards,
    Animesh Maurya
     
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