You already know passive RC low-pass filters. Their limitation: every stage you cascade loads the previous one, and you lose signal amplitude. The Sallen-Key topology solves this by wrapping an op-amp around a second-order RC network, giving you a sharp 2-pole rolloff (−40 dB/decade) with unity gain and low output impedance — all in one stage.

The classic unity-gain Sallen-Key low-pass uses two resistors (R1, R2) and two capacitors (C1, C2) arranged in front of a voltage-follower op-amp. The signal feeds through R1 into a node connecting C1 to ground and R2. R2 then feeds the op-amp non-inverting input, with C2 from that input to ground. The op-amp output feeds back to the junction between R1 and C1.

Design procedure for equal-component values (R1 = R2 = R, C1 = C2 = C):

• Choose your cutoff frequency f_c.
• Pick a standard capacitor value C (ceramics in the 1 nF–100 nF range work well).
• Calculate R = 1 / (2π · f_c · C).
• This equal-component version gives you a Butterworth response (Q = 0.707), meaning maximally flat passband with no peaking.

Concrete example: You're building an anti-aliasing filter for a 10 kHz ADC sampling audio. You want f_c = 3.4 kHz. Pick C = 10 nF. Then R = 1 / (2π × 3400 × 10×10⁻⁹) = 4.68 kΩ. Use 4.7 kΩ standard resistors. Done — you now have a second-order Butterworth LPF that's −3 dB at 3.4 kHz and −40 dB/decade beyond that.

Why this matters over a passive filter: The op-amp buffer means you can feed this directly into your ADC's high-impedance input without worrying about loading. You can also cascade two Sallen-Key stages for a 4th-order filter (−80 dB/decade) without interaction between stages.

Practical tips:

• Use an op-amp with a gain-bandwidth product at least 10× your cutoff frequency. For a 3.4 kHz filter, almost any op-amp works — a TL072 or MCP6002 is fine.
• Use C0G/NP0 ceramic capacitors for precision, or at minimum X7R. Avoid Y5V — their capacitance shifts with voltage and temperature will move your cutoff.
• If you need a steeper initial rolloff (Chebyshev response), make C1 ≠ C2. Higher Q gives sharper knee but introduces passband ripple.
• Keep the filter components physically close to the op-amp on your PCB to minimize parasitic pickup.

Rule of thumb: For every additional Sallen-Key stage you cascade, you add another −40 dB/decade of rolloff. Two stages give you −80 dB/decade — enough to clean up most signals before an ADC.