Sallen-Key filter cutoff calculator
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Topology
Component values — equal-R design
Filter response
C2 (computed)
C2 = 4·Q²·C1 — the ratio that sets the chosen Q
Cutoff frequency (fc)
fc = 1 ÷ (2π·√(R1·R2·C1·C2))
Quality factor (Q)
Q = 0.7071 (Butterworth)
Damping ratio (ζ)
ζ = 1 ÷ (2Q)
Roll-off
Second-order low-pass
About the Sallen-Key stage
The unity-gain Sallen-Key is the classic two-pole active filter: one op-amp, two resistors and two capacitors. The cutoff is set by the R·C products and the Q — the ratio C2/C1 (equal-R) or R1/R2 (equal-C) — sets the response shape. Butterworth gives the flattest passband, Bessel the most linear phase (best for square waves), and Chebyshev a steeper transition at the cost of passband ripple.
A single RC section at /tools/rc-filter is first-order (−20 dB/decade); this stage gives −40 dB/decade for one op-amp. Pick the nearest E24 resistor / standard capacitor values, then re-check fc and Q in Analyze mode.