Free Schmitt Trigger Hysteresis & Resistor Tool

Theory of Schmitt Trigger Comparators with Hysteresis

A bare analog comparator without hysteresis exhibits erratic output chattering when the input signal hovers near the threshold voltage due to thermal noise and ground bounce. Adding positive feedback creates two distinct switching thresholds—an Upper Trigger Point ((V_{UTP})) and a Lower Trigger Point ((V_{LTP}))—forming a deadband known as the hysteresis window ((V_H)).

Inverting Schmitt Trigger Equations

For an inverting Schmitt trigger, the input signal connects to the inverting input ((V_-)), while positive feedback is fed back from output to the non-inverting terminal ((V_+)):

$$V_{UTP} = V_{ref} + (V_{OH} - V_{ref}) cdot rac{R_{in}}{R_{in} + R_f}$$

$$V_{LTP} = V_{ref} + (V_{OL} - V_{ref}) cdot rac{R_{in}}{R_{in} + R_f}$$

$$V_H = V_{UTP} - V_{LTP} = (V_{OH} - V_{OL}) cdot rac{R_{in}}{R_{in} + R_f}$$

Non-Inverting Schmitt Trigger Equations

For a non-inverting Schmitt trigger, the input connects through (R_{in}) to the non-inverting terminal, and feedback resistor (R_f) connects from the output:

$$V_H = (V_{OH} - V_{OL}) cdot rac{R_{in}}{R_f}$$

$$V_{UTP} = V_{ref} cdot left(1 + rac{R_{in}}{R_f} ight) - V_{OL} cdot rac{R_{in}}{R_f}$$

$$V_{LTP} = V_{ref} cdot left(1 + rac{R_{in}}{R_f} ight) - V_{OH} cdot rac{R_{in}}{R_f}$$