Comparator Calculator

A Schmitt trigger comparator is a circuit that switches its output at two different voltage levels, creating a noise-resistant hysteresis band — and this Comparator Calculator handles the math for designing one. Enter your positive and negative supply voltages, then choose a calculation mode: provide a resistor ratio (R2/R1) and reference voltage to get your switching thresholds, or enter your desired high and low thresholds to find the required resistor ratio and reference voltage. Outputs include High Threshold (VTH), Low Threshold (VTL), hysteresis width, and the calculated R2/R1 ratio and Vref.

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Positive supply voltage must be greater than negative supply

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Ratio of feedback resistor to input resistor

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Upper threshold voltage

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Lower threshold voltage

Results

High Threshold Voltage (VTH)

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Low Threshold Voltage (VTL)

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Hysteresis Width

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Required Resistor Ratio (R2/R1)

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Required Reference Voltage

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Frequently Asked Questions

What is a comparator with hysteresis?

A comparator with hysteresis, also called a Schmitt trigger, is a circuit that compares input voltage to reference voltage with two different threshold levels. This prevents oscillation and provides noise immunity by having separate switching points for rising and falling inputs.

How does the resistor ratio affect hysteresis?

The resistor ratio R2/R1 determines the amount of positive feedback and controls the hysteresis width. A smaller ratio provides less hysteresis (narrower switching window), while a larger ratio creates wider hysteresis for better noise immunity.

What's the difference between inverting and non-inverting comparator configurations?

In a non-inverting configuration, the input signal goes to the positive terminal and reference to negative terminal. The output switches high when input exceeds the upper threshold. Inverting configuration reverses this, with input on negative terminal.

How do I choose the right reference voltage?

The reference voltage should be between your supply voltages and positioned appropriately relative to your desired threshold levels. It serves as the baseline around which the hysteresis window is created.

What happens if my resistor ratio is too large?

A very large resistor ratio creates excessive hysteresis, making the comparator less sensitive to input changes. This can cause delayed switching and reduce the effective input range of your circuit.

Can I use an op-amp as a comparator?

Yes, rail-to-rail op-amps like the LMV324 can function as comparators, though dedicated comparators like the LM339 are typically faster and more suitable for switching applications due to their open-collector outputs.

Why is hysteresis important in comparator circuits?

Hysteresis prevents unwanted oscillation when the input signal is near the threshold voltage, especially with noisy signals. It ensures clean switching by requiring the input to move significantly in the opposite direction before switching back.

How do supply voltages affect the threshold calculations?

Supply voltages determine the output swing of the comparator and affect the feedback voltage levels. The positive supply voltage is used in calculating the threshold voltages through the voltage divider formed by the feedback network.