Comparator Hysteresis Calculator

Configuration

Direction changes, thresholds remain the same set.

Mode

Pick compute or reverse-design values.

Reference Voltage Vref (V) *

Often ground or mid-supply for single-supply systems.

Output High Voh (V) *

Use realistic saturation level, not ideal rails.

Output Low Vol (V) *

For single-supply comparators, this is near 0 V.

R1 (from output to threshold node) *

This resistor sets hysteresis strength.

R2 (from Vref to threshold node) *

This resistor weights the reference point.

Desired Upper Threshold VTH+ (V) *

Input level causing the opposite state transition.

Desired Lower Threshold VTH- (V) *

Keep a gap to reject noise around the edge.

Fix Which Resistor?

Choose which resistor you already selected.

Fixed Resistor Value *

Common values: 10 kΩ to 1 MΩ range.

Results appear above this form after calculation.

Formula Used

The switching node is a two-source resistive divider between the output and the reference. With R1 from the output to the threshold node and R2 from Vref to the same node:

VTH = (Vref·R2 + Vout·R1) / (R1 + R2)
VTH+ uses Vout = Voh and VTH- uses Vout = Vol
ΔV = VTH+ − VTH- = (Voh − Vol)·R1 / (R1 + R2)

How to Use This Calculator

Select the Schmitt trigger configuration that matches your wiring.
Enter Voh, Vol, and your chosen Vref.
Choose Compute thresholds and enter R1 and R2.
Or choose Design from desired thresholds to get resistor values.
Press Calculate to view thresholds and hysteresis metrics.
Use CSV or PDF buttons to export the calculated result set.

Example Data Table

Voh (V)	Vol (V)	Vref (V)	R1	R2	VTH+ (V)	VTH- (V)	ΔV (V)
5.0	0.0	2.5	10 kΩ	100 kΩ	≈ 2.727	≈ 2.273	≈ 0.455
3.3	0.1	1.65	22 kΩ	220 kΩ	≈ 1.792	≈ 1.508	≈ 0.284

Example values are illustrative and depend on comparator output swing.

Professional Article

1) Overview of Comparator Hysteresis

Comparator hysteresis adds intentional positive feedback to create two switching thresholds. The output changes state at one level, then requires a different level to switch back. This behavior is also called Schmitt triggering and is widely used in digital conditioning.

2) Why Hysteresis Improves Noise Immunity

Without hysteresis, small noise around the threshold can cause rapid toggling. A controlled hysteresis width sets a noise band that must be exceeded. For example, a 0.30 V band rejects ripple smaller than 0.30 V peak-to-peak.

3) Threshold Equations and Output Swing

This calculator models the threshold node as a divider between the output and Vref. When the output is high, the upper threshold is computed using Voh. When the output is low, the lower threshold is computed using Vol. The hysteresis width depends on the output swing Voh − Vol.

4) Typical Design Targets and Ranges

Many designs use resistor values from 10 kΩ to 1 MΩ to balance loading and noise. With Voh = 5 V, Vol = 0 V, Vref = 2.5 V, R1 = 10 kΩ, and R2 = 100 kΩ, the thresholds are about 2.727 V and 2.273 V, giving ΔV ≈ 0.455 V. Doubling both resistors keeps thresholds nearly unchanged but reduces current.

5) Choosing Vref for Single-Supply Systems

A mid-supply reference such as Vref = VCC/2 is common for centered switching. Ground reference is common for pulse shaping and zero-crossing style detection. If the input is biased, set Vref near the desired center level to maximize margin.

6) Resistor Selection and Loading Effects

The threshold network should not overload the reference source or input stage. A low resistor pair increases current and can shift Vref if it is weak. A very high pair increases sensitivity to input bias currents and leakage. A practical check is keeping divider current at least ten times bias current.

7) Dynamic Behavior and Practical Limits

Propagation delay and output saturation can slightly shift effective switching points. Fast comparators may switch in tens of nanoseconds, while slower parts can be microseconds. Output rise and fall levels also depend on load, pull-ups, and supply headroom. Use realistic Voh and Vol values when precision matters.

8) Verification, Documentation, and Exports

After computing thresholds, verify the input waveform crosses both levels cleanly. Record Voh, Vol, Vref, and resistor choices for repeatable builds. The CSV export is useful for design reviews and version tracking. The PDF export supports lab notes and project documentation today.

FAQs

1) What does comparator hysteresis mean?

It means the comparator uses two thresholds instead of one. The input must rise above VTH+ to switch one way, and fall below VTH− to switch back, improving stability.

2) How do R1 and R2 affect the hysteresis width?

The width is ΔV = (Voh − Vol)·R1/(R1 + R2). Larger R1 increases hysteresis, while larger R2 reduces it. The ratio matters more than the absolute values.

3) Do inverting and non-inverting configurations change the thresholds?

The divider math is the same for the threshold node. What changes is which input pin sees the signal, so the direction of switching versus rising input differs.

4) Why should I enter realistic Voh and Vol?

Many outputs do not reach the supply rails under load. If Voh is lower or Vol is higher than ideal, the thresholds and hysteresis width change accordingly.

5) Can I design resistors for specific VTH+ and VTH−?

Yes. Use the design mode to enter desired thresholds. The calculator derives the resistor ratio and a required Vref. Fix one resistor value to obtain practical R1 and R2.

6) What resistor range is usually a good starting point?

A common starting range is 10 kΩ to 220 kΩ. It limits current while keeping the node less sensitive to leakage. Increase values when power is critical and references are strong.

7) Why might a computed threshold look out of range?

If Vref is outside the expected window, or the output swing is limited, the divider can place thresholds above Voh or below Vol. Recheck Vref, Voh, Vol, and the resistor ratio.