Comparator Threshold Calculator
Calculate upper/lower switching thresholds for a comparator with positive feedback (Schmitt trigger behavior). This tool assumes a threshold node connected to Vref through Rref and to Vout through Rfb.
What is a hysteresis comparator?
A hysteresis comparator adds positive feedback so it uses two different trip points instead of one. This means the input voltage must cross one threshold to switch high and a different threshold to switch low. The gap between these thresholds is called the hysteresis window.
That window is incredibly useful in noisy systems. Without hysteresis, noise near the threshold can cause output chatter. With hysteresis, the circuit becomes much more stable and predictable.
How this calculator models the circuit
The calculator assumes a threshold node formed by two resistors:
- Rfb from comparator output to threshold node
- Rref from Vref to threshold node
The threshold-node voltage is:
Since output can be at either VOH or VOL, there are two threshold-node values:
VTH(output low) = (Rfb × Vref + Rref × VOL) / (Rfb + Rref)
The hysteresis width is:
Understanding inverting vs non-inverting selection
Inverting signal input (Vin on -)
Rising input usually trips against the threshold corresponding to output-high state, and falling input trips against the threshold corresponding to output-low state.
Non-inverting signal input (Vin on +)
The mapping of rising/falling trip points is reversed in this simplified model. The calculator reports both “threshold with output high/low” and the interpreted rising/falling trigger points based on your selection.
Practical design tips
- Choose resistor values high enough to limit current, but not so high that leakage/bias currents dominate.
- Set hysteresis wider than expected input noise plus margin.
- Use realistic output swing values for your actual comparator and load, not ideal rail values.
- If you use open-collector/open-drain outputs, include pull-up behavior in VOH estimation.
- For battery systems, verify thresholds at min and max supply voltage.
Worked example
Suppose VOH = 5 V, VOL = 0 V, Vref = 2.5 V, and Rfb = Rref = 100 kΩ. Then the threshold node becomes:
- VTH(output high) = 3.75 V
- VTH(output low) = 1.25 V
- Hysteresis width = 2.50 V
That means your circuit resists small fluctuations around a single threshold and only switches when the signal moves decisively.
Limitations of this quick calculator
This is an ideal first-pass design tool. It does not include input bias currents, propagation delay, output saturation recovery, source impedance interaction, or dynamic effects. For precision or high-speed work, validate with SPICE simulation and bench measurements.