The comparator may appear straightforward at first glance. It compares two signal voltages and adjusts its output accordingly—high or low. But what happens when the two input signals are nearly identical? Even minor fluctuations from noise can cause the output to rapidly toggle between high and low states. To address this issue, increasing hysteresis is often the simplest solution.
Hysteresis introduces a form of memory into the system. It depends on its prior state, ensuring stability even when the input signals fluctuate slightly. Think about how your air conditioner's thermostat operates. Without hysteresis, the thermostat might repeatedly turn the AC on and off as temperatures hover around the setpoint, creating unnecessary wear and inefficiency. By introducing hysteresis, the thermostat maintains a consistent state longer, improving overall performance.
Some comparators come equipped with built-in hysteresis, typically a few millivolts. While this might suffice for certain applications, there are cases where additional external hysteresis is necessary. External hysteresis allows for precise control over the rising and falling thresholds, tailoring the system to specific requirements.
This hysteresis effect is achieved via positive feedback within the comparator circuit. Positive feedback is rare, but here it serves a purpose: instead of having a single threshold point, it establishes distinct rising and falling thresholds. This prevents the output from oscillating even when the input signal approaches the reference voltage. A comparator incorporating hysteresis through positive feedback is commonly referred to as a Schmitt trigger.
Consider the following example, showcasing the ON Semiconductor TL331 configured as an inverting Schmitt trigger. The TL331 is a low-power, single-channel comparator with no internal hysteresis. A resistor divider formed by R1 and R2 sets the reference voltage on the non-inverting pin and determines the switching threshold voltage. Since this is an open-collector comparator, attach a pull-up resistor to the output. A feedback resistor amplifies hysteresis through positive feedback, and values of at least 100 KΩ are recommended.
In this inverting setup, when the input signal falls below the threshold, the output goes high, pulling the threshold voltage upward via the feedback resistor. Minor fluctuations in the input signal won't affect the comparator's output until the input voltage crosses a higher, adjusted rising threshold. Once the input exceeds this threshold, the output switches low, pulling the threshold voltage down via the feedback resistor. This ensures the output remains stable until the input dips below a lower, regulated threshold.
Non-inverting configurations function similarly, relying on positive feedback to stabilize the system. Here, the threshold voltage established by the resistor divider remains constant regardless of the comparator’s state. Feedback adjusts the input signal at the non-inverting node instead.
These examples demonstrate how adding hysteresis requires just one or two external resistors. Adjusting their resistance values optimizes the threshold for the specific application. For comparators prone to input noise, increasing hysteresis is a practical method to mitigate instability when input voltages are close for extended periods.
In summary, hysteresis is a powerful tool for stabilizing comparators, preventing oscillations and improving reliability. Whether using built-in or externally added hysteresis, this technique ensures smooth operation in a wide range of electronic systems.
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