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Comparator Hysteresis (Schmitt Trigger) Calculator

Calculate comparator hysteresis trip points for Schmitt trigger circuits, upper and lower threshold voltages, and design resistor values for a desired hysteresis percentage

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Formula

VT+=Vref×R2/(R1+R2)+Vcc×R1/(R1+R2),VT−=Vref×R2/(R1+R2)V_T+ = V_ref × R2/(R1+R2) + V_cc × R1/(R1+R2), V_T− = V_ref × R2/(R1+R2)
V_ref— Reference voltage (V)
V_cc— Supply voltage (V)
R1— Feedback resistor (Ω)
R2— Input resistor (Ω)
ΔV— Hysteresis band (V)

How It Works

Comparator hysteresis calculator computes upper and lower threshold voltages for noise-immune switching — essential for signal detection, zero-crossing circuits, and level conversion. Embedded systems engineers, sensor interface designers, and power electronics engineers use this to prevent oscillation when input signals cross the threshold slowly or with superimposed noise. Per Horowitz & Hill 'Art of Electronics' (3rd ed., p.231), hysteresis creates a deadband where the comparator ignores small input variations, requiring the input to exceed V_TH+ to switch high and drop below V_TH- to switch low. The hysteresis voltage V_hyst = V_TH+ - V_TH- should exceed peak-to-peak noise by at least 2× for reliable switching. Standard comparators (LM339, LM393) require external resistors to set hysteresis; integrated Schmitt triggers (74HC14) have fixed hysteresis of 0.4-0.9V depending on supply voltage.

Worked Example

Design a 3.3V comparator circuit with 100mV hysteresis centered at 1.65V for a battery voltage monitor using LM393. Required: V_TH+ = 1.70V, V_TH- = 1.60V, V_hyst = 100mV. Using positive feedback topology: R1 (input divider) = 10kΩ, R2 = 10kΩ establishes V_ref = 1.65V. For 100mV hysteresis with Vout swing = 3.3V: R_fb = R_parallel × (V_out / V_hyst) = 5kΩ × (3.3V / 0.1V) = 165kΩ. Select 160kΩ (E24 series). Actual hysteresis = 3.3V × 5kΩ / 160kΩ = 103mV. With 50mVpp noise on input, this provides 2× noise margin — signals crossing threshold trigger clean output transitions without chattering.

Practical Tips

  • ✓For 5V systems, 74HC14 provides 0.9V hysteresis with no external components — ideal for debouncing and signal conditioning at digital logic levels
  • ✓Calculate feedback resistor: R_fb = R_eq × (V_swing / V_hyst) where R_eq = R1||R2 of input divider; larger R_fb = smaller hysteresis
  • ✓Open-drain comparators (LM339) require pull-up resistor; push-pull comparators (TLV3201) don't — check output type on datasheet

Common Mistakes

  • ✗Setting hysteresis narrower than input noise — results in multiple output transitions (chatter) during single input crossing; hysteresis should exceed 2× peak-to-peak noise
  • ✗Using op-amps as comparators — op-amps have recovery delays of 1-100μs when output saturates; dedicated comparators (LM339, LM393) recover in 100-300ns
  • ✗Ignoring comparator propagation delay — LM339 has 1.3μs delay; for high-speed applications, use MAX942 (80ns) or LT1016 (10ns) per Linear Technology selection guide

Frequently Asked Questions

Prevent output chatter (rapid oscillation) when input signal crosses threshold slowly or with superimposed noise. Per TI application report SLAA068, hysteresis > 2× noise amplitude eliminates false triggers. For 10mVpp noise, use > 20mV hysteresis.
Positive feedback from output to non-inverting input: R_fb connects output to V+ input. When output is high, V+ rises (raising V_TH-); when output is low, V+ falls (lowering V_TH+). The difference creates hysteresis window. ICs with built-in hysteresis include TLV3201 (6mV) and MAX9117 (user-programmable).
Yes — Schmitt triggers (74HC14, 40106) provide digital-level hysteresis for logic signals; comparators with feedback resistors provide analog-level hysteresis for sensor interfaces. ADCs with internal comparators (TI ADS1115) include programmable threshold hysteresis for window comparator functions.
Rule of thumb: V_hyst > 2× noise + 2× comparator offset voltage + 2× expected threshold drift. For a sensor signal with 50mVpp noise and LM339 (5mV max offset): V_hyst > 100mV + 10mV = 110mV minimum. Use 150mV for margin. Excessive hysteresis causes delayed detection — balance noise immunity against response time.

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