The comparator appears straightforward at first glance. It compares two input voltages and adjusts its output accordingly—high or low. However, when the two input signals are extremely close in value, even minor fluctuations from electrical noise can cause the output to rapidly toggle between high and low states. This issue can be mitigated by increasing hysteresis, which is often the simplest solution.
Hysteresis refers to a system’s tendency to rely on its previous state when determining output. In practice, this means setting a higher threshold for switching to the high state and a lower threshold for switching to the low state. If you think about it, this is akin to how a thermostat functions. Without hysteresis, the system would frequently toggle on and off as temperatures fluctuate slightly around the desired setpoint, leading to inefficiency and unnecessary wear on the device. Adding hysteresis ensures the system operates smoothly and conserves energy.
Some comparators come with built-in hysteresis, typically in the range of a few millivolts. While this may suffice for certain applications, there are scenarios where additional external hysteresis is necessary. External hysteresis allows you to fine-tune the exact rise and fall thresholds to suit your specific needs.
Hysteresis is achieved through positive feedback within the comparator circuit. This is one of the rare instances where positive feedback proves beneficial rather than detrimental. Unlike traditional threshold-based systems, hysteresis doesn’t rely on a single point of activation but instead establishes separate rising and falling thresholds. This prevents the output from oscillating, even when the input signal hovers near the reference voltage. A comparator configured with hysteresis via positive feedback is commonly referred to as a Schmitt trigger.
Consider the example of the ON Semiconductor TL331 configured as an inverting Schmitt trigger. The TL331 is a low-power, single-channel comparator with an open-collector design and no inherent hysteresis. By using a resistor divider consisting of R1 and R2, the reference voltage is established on the non-inverting pin, while the threshold voltage is determined by the comparator's output switching mechanism. As this is an open-collector comparator, a pull-up resistor must be connected to the output. The feedback resistor enhances hysteresis through positive feedback, and it’s generally recommended to use a feedback resistor with a value of at least 100 kΩ.
For this inverting configuration, when the input signal falls below the threshold, the output pin goes high, pulling the threshold voltage upward via the feedback resistor. Consequently, minor fluctuations in the input signal won't trigger a switch until the input voltage reaches a higher, adjusted rising threshold. Once the input signal surpasses this rising threshold, the output switches low, pulling the threshold voltage down via the feedback resistor. This ensures the output remains low until the input voltage dips below the regulated falling threshold.
Non-inverting configurations operate similarly but with a twist. Here, the threshold voltage set by the resistor divider remains constant regardless of the feedback loop. Instead, the feedback adjusts the input signal at the non-inverting node.
In both configurations illustrated, introducing hysteresis requires just one or two external resistors whose values can be tweaked to match the requirements of your specific application. When designing with comparators, adding hysteresis is a practical method to combat noise-related issues, especially when input voltages might converge for extended periods.
By incorporating these principles, engineers can create robust and reliable circuits that handle complex input conditions gracefully. Whether you're working on a thermostat, sensor interface, or any other project requiring precise signal processing, understanding and implementing hysteresis can significantly enhance performance.
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