A hysteresis comparator, also known as a Schmitt trigger, is a type of electronic circuit used to convert an analog input signal into a digital output signal. It is designed to provide noise immunity and stability for input signals that may be prone to fluctuations or oscillations near the threshold levels.
The basic idea behind a hysteresis comparator is to introduce positive feedback to the circuit, creating two distinct threshold levels: a higher threshold (VTH+) and a lower threshold (VTH-). The positive feedback ensures that the output of the comparator remains in one of two stable states until the input signal crosses a certain threshold, at which point the output switches to the other state. This behavior is often represented as a hysteresis curve on a graph.
Here's a simplified explanation of how it works:
Rising edge: When the input voltage rises above the upper threshold level (VTH+), the output of the comparator switches from a low state to a high state.
Falling edge: Conversely, when the input voltage falls below the lower threshold level (VTH-), the output switches from a high state to a low state.
Hysteresis effect: The key feature of the hysteresis comparator is that the upper threshold (VTH+) is higher than the lower threshold (VTH-). This difference between the thresholds is called the hysteresis width (VH). It prevents the comparator from rapidly oscillating when the input voltage is near the threshold levels.
The hysteresis effect adds noise immunity to the comparator, making it less sensitive to small fluctuations or noise around the threshold levels. Once the output changes state, the input voltage must cross the opposite threshold level plus the hysteresis width before the output will switch again. This prevents the output from rapidly toggling when the input is near the threshold region.
Hysteresis comparators are widely used in various applications, including noise filtering, signal conditioning, level detection, and digital-to-analog conversion, where stable and noise-tolerant switching behavior is required.