Question

How does a hysteresis comparator reduce false triggering in noisy environments?

Answer 1

A hysteresis comparator, also known as a Schmitt trigger, is a type of electronic circuit used to convert a varying analog signal into a stable digital output. It is designed to reduce false triggering or noise-induced oscillations in noisy environments by introducing a form of positive feedback in its operation.

Here's how a hysteresis comparator works and how it helps to reduce false triggering:

Basic Comparator Operation:
A regular comparator without hysteresis has a single threshold voltage, typically set at the midpoint of the power supply voltage. When the input signal crosses this threshold voltage, the output of the comparator changes state. If the input signal is noisy or fluctuating near the threshold voltage, the output may switch rapidly and unpredictably, leading to false triggers or multiple transitions for a single input event.

Introducing Hysteresis:
A hysteresis comparator, on the other hand, has two different threshold voltages: one for rising edges (VTH+ or VUP) and one for falling edges (VTH- or VDOWN). The difference between these threshold voltages is known as the hysteresis voltage (VHYS).

Positive Feedback Loop:
The hysteresis is achieved by incorporating positive feedback into the comparator circuit. When the output switches state, the feedback from the output back to the input causes the threshold voltages to change. This means that once the input signal crosses one threshold (e.g., VTH+), the output will not change state until it crosses the other threshold (e.g., VTH-), creating a stable region where the output remains unchanged.

Noise Immunity:
The hysteresis effectively adds noise immunity to the comparator's operation. When the input signal is transitioning from low to high, it must exceed the higher threshold voltage (VTH+) to switch the output to a high state. Once the output is in a high state, the input signal must fall below the lower threshold voltage (VTH-) to switch the output back to a low state. This difference between the two threshold voltages prevents the comparator from reacting to small noise fluctuations that do not cross both thresholds, reducing false triggering.

Bistable Behavior:
The hysteresis causes the comparator to behave in a bistable manner, meaning it has two stable output states. This bistable behavior helps maintain the output in a stable state even when the input signal is close to the threshold voltage.

In summary, a hysteresis comparator reduces false triggering in noisy environments by adding positive feedback and bistable behavior to the comparator circuit. The hysteresis ensures that the output remains stable until the input signal crosses both threshold voltages, providing noise immunity and robustness to fluctuations in the input signal.