Hysteresis is a fundamental concept in Schmitt trigger circuits, which are widely used in electronics to create stable digital signals from analog input signals that might be noisy or fluctuating. Hysteresis adds a level of noise immunity and prevents rapid switching of the output state when the input signal is near the threshold.
In a Schmitt trigger circuit, hysteresis is achieved by having two different threshold voltage levels: one for the rising edge (upper threshold) and one for the falling edge (lower threshold) of the input signal. These thresholds are typically set at different voltage levels, creating a "hysteresis band" between them. The purpose of this hysteresis band is to ensure that the output state of the circuit remains stable even if the input signal is slightly noisy or fluctuating around the threshold levels.
Here's how hysteresis works in a Schmitt trigger circuit:
Initial State: Let's assume the Schmitt trigger circuit starts in a state where the output is low (0) and the input voltage is below the lower threshold.
Rising Edge: As the input voltage gradually increases, it will eventually cross the upper threshold. When this happens, the output of the Schmitt trigger circuit switches to a high state (1). However, the hysteresis band ensures that the input voltage must exceed the upper threshold by a certain margin before the output switches. This margin is the hysteresis voltage (Vh). So, the output will stay high even if the input voltage drops slightly below the upper threshold. This prevents rapid oscillations near the threshold due to noise or small fluctuations.
Falling Edge: Now, if the input voltage starts to decrease, it must fall below the lower threshold minus the hysteresis voltage (Vh) before the output switches back to the low state (0). Again, this hysteresis ensures that the output remains low even if the input voltage temporarily rises slightly above the lower threshold.
By using hysteresis in Schmitt trigger circuits, you can create a clear separation between the two output states and reduce the likelihood of false transitions caused by noise or unstable input signals. This makes Schmitt triggers particularly useful in scenarios where you need reliable and well-defined digital output signals from analog input signals that might experience fluctuations or noise.
In summary, hysteresis in Schmitt trigger circuits helps to improve noise immunity, stabilize output states, and minimize false transitions by introducing a defined voltage range where the input voltage must cross before triggering a state change in the output.