Hysteresis refers to the difference in voltage levels between the detection of a transition from a logic low level to a logic high level and the transition from a logic high level to a logic low level. The following figure illustrates hysteresis for high and low logic levels:

All digital logic devices have some level of hysteresis on their digital inputs. The hysteresis magnitude for a particular device can be determined by the following formula:

Hysteresis ≈ VIH - VIL

On a rising edge of the digital signal on the input, the device detects a transition from a logic low to a logic high at VIH. Conversely, the device detects a transition from a logic high to a logic low when the voltage at the input of the device crosses VIL.

Hysteresis is a useful property for digital devices because it provides some amount of natural immunity to high-frequency noise in your digital system. This noise is generally caused by reflections from the high edge rates of logic level transitions. High-frequency noise might cause false transition detections by the digital device. These false detections occur when only a single voltage threshold determines a change in the logic state. The following figure illustrates this occurrence:

After applying hysteresis the first sample is acquired as a logic low level. The second sample is also a logic low level because the signal does not cross the logic high level threshold. The third and fourth samples are logic high level, and the fifth is logic low level.

For devices with fixed voltage thresholds, the NIM and hysteresis of your system are determined by your choice of system components. Some devices allow you to control both your system NIM and hysteresis. Both system NIM and hysteresis give your system levels of noise immunity.

For a specific logic family, increasing hysteresis reduces the NIM and decreasing hysteresis increases it.

To set voltage thresholds appropriately, carefully examine the signal quality in your system:

  • If your system requires greater noise immunity for valid logic levels, aim for a higher NIM.
  • If your system needs more noise immunity during logic level transitions, prioritize greater hysteresis.