Sallen-Key low-pass designer

Assumptions to check

• The entered Sallen-Key-filter values represent the stated operating condition.
• Values are evaluated in the declared lumped or first-pass model.
• A nominal calculation is not a component qualification or safety approval.

What this Sallen-Key-filter calculation establishes

Calculate ideal equal-component Sallen-Key low-pass natural frequency and the Q set by the non-inverting gain. The useful result is the stated electrical quantity and the decision it supports, not an unstated claim about a finished product. This engine keeps the governing relationship visible so an input, unit, condition, or model boundary can be reviewed before a value becomes a component or layout choice.

Natural frequency is one divided by two pi RC for equal components; resulting Q is one divided by three minus the non-inverting gain. Treat the number as a first-pass result for the declared operating point. When a source, load, temperature, frequency, waveform, component tolerance, or measurement condition changes, repeat the calculation at the relevant corner rather than assuming the nominal answer persists.

Worked decision context

Equal R = 10 kΩ and C = 10 nF set f0 = 1591.55 Hz; a non-inverting gain of 1.586 gives Q of about 0.707. That example verifies the equation and illustrates the scale of the result, but it does not select a part by itself. Compare the result with available values, ratings, tolerance bands, and the receiving circuit or physical environment before implementation.

Use the primary output to identify the binding constraint. If it leaves little margin, document which input dominates and use selected-part data, a higher-fidelity model, simulation, or measurement. This is especially important when a small numerical difference changes a thermal, timing, noise, or reliability decision.

Limits and validation handoff

It uses the equal-component coefficient set only. Op-amp GBW, slew rate, output headroom, and component tolerance change the achievable Q and passband accuracy. The calculation does not silently include omitted parasitics, installation conditions, manufacturing variation, or product policy. Those conditions can be decisive even when the arithmetic is exact for the selected model.

Record inputs, units, model assumptions, and the intended decision with the result. Verify the leading risk against the selected component data sheet and a representative measurement when the circuit has consequential energy, high voltage, safety, compliance, or reliability requirements.

Common mistakes

• Treating a nominal result as a guaranteed operating limit.
• Mixing a data-sheet value from one condition with a calculation at another.
• Selecting a component before checking rating, tolerance, and the physical implementation.

Model limit and handoff

Keep the entered Sallen-Key-filter conditions with the calculation, then validate the binding limit using the selected component, physical implementation, and representative operating corner.

FAQs