A Schmitt trigger is a type of electronic circuit that is used to convert an analog input signal into a digital output signal. It is commonly used in signal conditioning and noise rejection applications. The main function of a Schmitt trigger is to provide hysteresis, which means that it has two distinct threshold voltage levels, one for rising (positive) input signals and another for falling (negative) input signals. This hysteresis feature prevents the output from transitioning rapidly back and forth when the input signal is near the threshold level, reducing noise and jitter in the output signal.
The Schmitt trigger operates as follows:
Input Signal: The Schmitt trigger takes an analog input signal (usually voltage) and compares it to two threshold voltage levels, called the upper threshold voltage (Vth+) and the lower threshold voltage (Vth-).
Hysteresis: The key characteristic of the Schmitt trigger is the presence of hysteresis. The hysteresis band is the voltage range between the upper and lower threshold voltages. When the input signal rises above the upper threshold voltage, the output of the Schmitt trigger switches to a high (logic '1') state. Conversely, when the input signal falls below the lower threshold voltage, the output switches to a low (logic '0') state.
Output States: Once the output switches to a particular state (high or low), it remains in that state until the input signal crosses the opposite threshold voltage. This behavior ensures that the output doesn't change immediately when the input signal is near the threshold, but instead, it maintains its state until the input crosses the other threshold voltage.
Noise Rejection: The hysteresis property of the Schmitt trigger helps in rejecting noise and provides a more stable output when the input signal is noisy or experiencing fluctuations near the threshold levels. It prevents the output from oscillating rapidly when the input is close to the switching threshold, reducing false transitions.
Schmitt triggers find various applications, such as debouncing mechanical switch inputs, shaping waveforms, level shifting, and creating stable digital signals from noisy analog inputs. They are often used in digital circuits to clean up signals and provide robust and reliable switching behavior in the presence of noise or uncertain input conditions.