Calculator Inputs
Example Data Table
| Topology | R1 | R2 | C1 | C2 | Stage Gain | Expected Use |
|---|---|---|---|---|---|---|
| First Order Active | 15.9 kΩ | N/A | 10 nF | N/A | 1.00 | Simple audio noise reduction |
| Second Order Sallen Key | 10 kΩ | 10 kΩ | 10 nF | 10 nF | 1.586 | Butterworth style response |
| Second Order Sallen Key | 22 kΩ | 22 kΩ | 4.7 nF | 4.7 nF | 1.20 | Sensor signal smoothing |
Formula Used
First order cutoff: fc = 1 / (2πRC)
First order magnitude: |H(f)| = K / √(1 + (f / fc)²)
Non-inverting gain: K = 1 + Rf / Rg
Sallen Key natural frequency: f0 = 1 / (2π√(R1R2C1C2))
Sallen Key Q: Q = √(R1R2C1C2) / (R1C1 + R2C1 + R1C2(1 - K))
Second order magnitude: |H(jω)| = K / √((1 - (ω / ω0)²)² + (ω / (Qω0))²)
Slew rate: SR = 2πfVpeak
Tolerance cutoff range: fc minimum and maximum are estimated by applying resistor and capacitor tolerance together.
How To Use This Calculator
- Select a first order or Sallen Key filter topology.
- Enter resistor and capacitor values with matching units.
- Choose direct gain or enter feedback resistor values.
- Add test frequency, input voltage, supply values, and load resistance.
- Press the calculate button to show results above the form.
- Review cutoff, gain, Q, phase, output swing, and warnings.
- Download the CSV or PDF report for records.
Active Low Pass Filter Design Guide
Understanding Active Low Pass Filters
An active low pass filter passes slow signal changes and reduces faster noise. It uses an amplifier with resistors and capacitors. The amplifier can add gain. It can also buffer the next circuit. This helps sensors, audio paths, control loops, and data systems.
Why Cutoff Frequency Matters
Cutoff frequency marks the point where response starts falling. A first order filter is three decibels down at cutoff. A second order Sallen Key stage can fall faster. Its shape depends on Q and gain. A low Q gives a smooth rolloff. A high Q can create peaking near cutoff. This calculator shows both choices, so a design can be compared before parts are bought.
Using Real Component Values
Real resistors and capacitors are never perfect. Tolerance changes the final cutoff. Small parts also carry noise and parasitic effects. Very large resistors can increase noise. Very small resistors can load the source. Capacitors may change with voltage, temperature, and aging. The calculator includes tolerance estimates to show a likely cutoff range.
Gain, Output, and Op Amp Limits
Active filters depend on the selected amplifier. The gain setting changes signal level. It can also affect Sallen Key stability. The output must stay inside the available supply swing. Large signals at high frequency need enough slew rate. A slow amplifier can distort even when the math looks correct. Gain bandwidth should also exceed the filter needs by a healthy margin.
Good Design Workflow
Start with a target cutoff and expected input signal. Choose a topology that matches the needed slope. Enter practical component values from standard series. Check cutoff, Q, phase, output level, noise, and limit warnings. Then adjust values until the result fits the circuit. Export the report for review. Build and test the final design with real instruments.
Interpreting The Result
Use the magnitude figure to predict output amplitude. Use phase to judge timing shift. Use order and attenuation to compare noise removal. Results are estimates, not replacements for simulation. Breadboards, op amp models, capacitor type, layout, and load impedance can change behavior. Treat warnings as design checks before moving to a schematic or prototype. Final testing should confirm cutoff, gain, noise, phase, and signal headroom on hardware.
FAQs
What is an active low pass filter?
It is a filter that passes lower frequencies and reduces higher frequencies. It uses an amplifier, so it can add gain and buffer nearby circuit stages.
Which filter types are included?
The calculator includes a first order active filter and a second order Sallen Key filter. It also supports multiple identical cascaded stages.
What does cutoff frequency mean?
Cutoff frequency is the main transition point of the filter. For a first order stage, response is about three decibels lower there.
Why is Q factor important?
Q controls the shape of a second order response. Low Q is smooth. Higher Q can make the filter sharper, but it may create peaking.
Why check gain bandwidth?
The amplifier must be fast enough for the selected frequency and gain. Low gain bandwidth can shift cutoff, change phase, and reduce accuracy.
Can I export the result?
Yes. After calculation, use the CSV button for spreadsheet data or the PDF button for a simple printable report.
Does this replace circuit simulation?
No. It gives fast design estimates. Use simulation and bench testing for final approval, especially at high frequency or high gain.
How should I choose component values?
Start with practical resistor values, then select capacitors that meet the desired cutoff. Avoid extreme values that increase noise or load the source.