hysteresis
/ HISS-tuh-ree-sis /
Hysteresis is the trick of using two thresholds instead of one, so a decision, once made, sticks. Think of an air-conditioner thermostat: it switches the cooling on at 25 degrees but does not switch it off until the room falls to 23. That 2-degree gap stops the unit from rapidly clicking on and off every time the temperature wobbles by a tenth of a degree. The output depends not just on the input now, but on which way it came from.
In a comparator, hysteresis means the trip point moves depending on the current output state. When the output is low, it takes a fairly high input to flip it high — say 2.6 V. But once it is high, the threshold drops to a lower value — say 2.4 V — so the input must fall well below where it switched on before it switches back off. The gap between the two, here 0.2 V, is the hysteresis band. Any noise smaller than that band cannot bounce the output, because after the first transition the threshold has already moved out of the noise's reach. You build it with a little positive feedback: a resistor from the output back to the + input nudges the threshold each time the output flips.
Hysteresis is the standard cure for chatter, and it appears everywhere: thermostats, battery chargers that stop at one voltage and restart at a lower one, the snap-action of a wall light switch, and the input buffers of logic chips. The honest tradeoff is that hysteresis makes the device deliberately less sensitive — it ignores changes smaller than the band, and it adds a small lag between input and response. You size the band to be comfortably bigger than your noise but smaller than the real signal swing you must detect.
A comparator with thresholds at 2.6 V (rising) and 2.4 V (falling) has 0.2 V of hysteresis. A noisy 2.5 V signal with 50 mV of jitter cannot cross either threshold once latched, so the output stays rock-steady instead of chattering.
Two thresholds, a gap noise cannot jump — the cure for chatter.
Hysteresis trades sensitivity for stability: a band wider than your noise stops chatter, but it also means the input must travel the full band before the output responds again. Too much hysteresis can miss small real signals.