hysteresis voltage

Discussion in 'General Electronics Chat' started by nileshkhupase, Jun 23, 2011.

  1. nileshkhupase

    Thread Starter Member

    Mar 22, 2011
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    Hi,

    what is significance of Hysteresis voltage in schmitt trigger?

    i know that it improves noise immunity of circuit...but does it mean that

    higher the hysterisis voltage higher is the noise immunity..or better for design..


    and what about positive threshold voltage and negative threshold voltage...do both should same?if they are different why is the provision made like this..?
     
  2. bertus

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  4. MrChips

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    A Schmitt trigger will be designed to have positive threshold voltage (Vt+) to be higher than negative threshold voltage (Vt-).

    For example, for a 74LS14
    Vt+ = 1.6V
    Vt- = 0.8V

    The hysteresis voltage is the difference = 0.8V.

    What this means is that after you have crossed Vt+ your signal must fall below Vt- before the circuit returns to the previous state. Think of it as "backlash" like in your house thermostat.

    This gives it greater noise immunity.

    If Vt+ and Vt- were the same there would be no hysteresis and your output would "chatter" like an analog comparator with no positive feedback or you wouldn't want your house furnace or A/C to be switching on/off like crazy.
     
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