Active Filter Designer (Sallen-Key / MFB)
From f₀ and Q to real resistors and capacitors — rounded to the E-series, error included.
Picking between the two topologies
Sallen-Key is non-inverting, needs only unity-gain stability from the op-amp and is forgiving at moderate Q — but its high-frequency rejection is limited by the signal feeding through C1 into the op-amp's rising output impedance, so a fast op-amp matters more than people expect. MFB (multiple feedback) inverts, holds its stopband better and tolerates component spread at higher Q; it's the default for anti-aliasing in front of an ADC. In both cases the design comes down to satisfying
and the real engineering is what this tool automates: the capacitor ratio must exceed (times for MFB) for real resistor solutions to exist, and after rounding to the E-series your f₀ and Q move — the table shows by how much. Cascade stages using the Q values from the filter designer to build full Butterworth or Chebyshev responses in hardware.