Even though a comparator might seem straightforward at first glance—comparing two signal voltages and setting its output accordingly—it can encounter issues when the input voltages are extremely close. In such cases, even minor noise on the inputs can lead to erratic oscillations between high and low logic states. One of the simplest solutions to this issue is to increase the hysteresis.
Hysteresis refers to a system's reliance on its prior state when making decisions. For instance, imagine a thermostat that adjusts based on whether the temperature rises or falls past certain thresholds. Without hysteresis, a thermostat might toggle the air conditioner repeatedly as temperatures fluctuate slightly around a set point, leading to inefficiency and unnecessary wear. By introducing hysteresis, the system stabilizes and operates more efficiently.
While some comparators come with built-in hysteresis, often just a few millivolts, many applications demand additional external hysteresis. This can be achieved by fine-tuning the rise and fall thresholds externally to suit the application's needs.
Hysteresis is typically implemented using positive feedback within the comparator circuit. Although rare, there are instances where positive feedback proves beneficial. Instead of having a single threshold, hysteresis establishes distinct rising and falling thresholds. This ensures the output remains stable—either high or low—even when the input signal hovers near the reference voltage. A comparator equipped with hysteresis via positive feedback is commonly referred to as a Schmitt trigger.
Take, for example, the ON Semiconductor TL331 configured as an inverting Schmitt trigger. As a single-channel, low-power, open-collector comparator without internal hysteresis, its threshold voltage is set by a resistor divider formed by R1 and R2. Connecting a pull-up resistor to the output facilitates positive feedback via a feedback resistor, thereby enhancing hysteresis. A feedback resistor value of at least 100 kΩ is recommended.
In this inverting setup, the output remains high while the input signal stays below the threshold. The feedback resistor pulls the threshold voltage high, preventing small input fluctuations from triggering the comparator until the input voltage surpasses the adjusted rising threshold. Upon reaching the rising threshold, the output switches low, pulling the threshold voltage down via the feedback resistor. This maintains the output in the low state until the input voltage dips below the regulated falling threshold.
A non-inverting configuration operates similarly but differs in how the threshold voltage behaves. Here, the feedback regulates the input signal at the non-inverting node, ensuring consistent behavior regardless of switching conditions.
These examples demonstrate how adding hysteresis—via a couple of external resistors—can effectively address noise-related challenges. When designing comparators, increasing hysteresis provides a practical method to mitigate issues arising from input voltages that frequently approach each other.
By adjusting the resistance values appropriately, the thresholds can be optimized for specific applications. Overall, incorporating hysteresis into comparator designs not only enhances stability but also ensures robust performance in noisy environments.
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