Filters and Signals
Model class: Exact ideal relationship
Sallen-Key low-pass designer
Calculate ideal equal-component Sallen-Key low-pass natural frequency and the Q set by the non-inverting gain.
Interactive engine
Start with the stated conditions.
Values stay in this browser. Choose a representative scenario, then calculate deliberately.
Example ready
Calculate to inspect the result.
The result will identify the direct answer, assumptions, and any warning that changes the next decision.
How much does each input matter?
Each row holds every other input fixed at its current value and moves one input by ±10%, using the same model as the main result. It identifies which input is the binding lever, not a full tolerance stack.
Assumptions to check
- The entered Sallen-Key-filter values represent the stated operating condition.
- This exact ideal relationship is evaluated in the declared lumped or first-pass model.
- A Sallen-Key-filter 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 the Sallen-Key-filter result as a guaranteed operating limit rather than a first-pass exact ideal relationship estimate.
- Mixing a data-sheet value measured under one condition with this Sallen-Key-filter calculation performed at another.
- Selecting a component before checking the boundary this calculation names: 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.
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
Is this Sallen-Key-filter result sufficient to approve a design?
No. It applies exact ideal relationship reasoning to the entered Sallen-Key-filter values and names the checks that still need selected-part data, a higher-fidelity model, or measurement. 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.
What does this Sallen-Key-filter calculator assume that could make the result wrong?
Natural frequency is one divided by two pi RC for equal components; resulting Q is one divided by three minus the non-inverting gain. If the entered values do not match the real operating condition, the result no longer describes the actual circuit.
Where should this Sallen-Key-filter result go next?
Compare this Sallen-Key-filter result with Selecting an op-amp from the signal envelope, then use the stated next decision below the calculator to move from this first-pass number toward an implementation.