When an input signal to a digital circuit un-matches the characteristics of a digital signal it can have some sort of noise. This circumstance require a specialized circuit that will modify a signal and make it to actual or true digital shape. The required circuit can be said as Schmitt Trigger. But the trigger for this circuit to change forms is the input voltage state, instead of a digital pulse or more simply we can say that the output state depends on the input state.
It switches the output negative when the input goes up through a positive reference voltage.
The Schmitt Trigger is also called Zero Crossing Detector or sometimes Multiple Crossing Detector.
Why we go for Schmitt Trigger :
Although the simple comparator circuit using either an ordinary operational amplifier if the input waveform is slow or has noise on it, then there is the possibility that the output will switch back and forth several times during the switch over phase as only small levels of noise on the input will cause the output to change.This is called MULTIPLE CROSSINGS.
This may not be a problem in some circumstances, but if the output from the operational amplifier comparator is being fed into fast logic circuitry, then it can often give rise to problems.
The problem can be solved very easily by adding some positive feedback to the operational amplifier or comparator circuit. This circuit is called Schmitt Trigger.
Characterstics of Schmitt Trigger:
The effect of the new feedback resistor is to give the circuit different switching thresholds dependent upon the output state of the comparator or operational amplifier. When the output of the comparator is high, this voltage is fed back to the non-inverting input of the operational amplifier of comparator. As a result the switching threshold becomes higher. When the output is switched in the opposite sense, the switching threshold is lowered.
Schmitt trigger inputs—
Almost all other input pins are of Schmitt trigger design. A Schmitt trigger has different transition voltages, depending on whether the input signal is changing from high to low or low to high.
Calculation of feedback resistance…
The centre voltage about which the circuit should switch is determined by the potential divider chain consisting of R1 and R2. This should be chosen first. Then the feedback resistance R3 can be calculated.
Application and Advantage
The benefit of a Schmitt trigger over a circuit with only a single input threshold is greater stability (noise immunity).
The Schmitt trigger is a comparator application which switches the output negative when the input passes upward through a positive reference voltage.