What is a sensor's hysteresis and why is it important?

• A. The temperature coefficient of sensor
• B. The period of sensor oscillation
• C. The maximum range of sensor measurement
• D. The difference between sensor output at rising and falling input; affects repeatability and noise immunity.

Answer: (D)

Hysteresis in a sensor is the difference in output when the input is rising versus when it is falling. This happens because the sensor’s response depends on its past input, not just the current value, due to physical and electronic lag in the system. If you plot output versus input, you get a loop rather than a single line, showing a higher threshold to switch on and a lower threshold to switch off.

This history-dependent behavior is important because it creates a deadband that makes the sensor more resistant to small, random fluctuations (noise). By requiring a noticeable change in input to flip the output, the sensor avoids rapid on/off chatter, which improves noise immunity. It also helps with repeatability of switching events: once the input crosses one threshold, the output won’t flip back until the input moves past the other threshold, giving stable, predictable behavior in changing conditions.

The other common sensor traits—how the output changes with temperature, how quickly the sensor responds (period of oscillation), or the overall measurement range—describe different aspects of performance and are not what hysteresis refers to.