Calculator Inputs
Use positive component values. For first order filters, R1 and C1 set the cutoff.
Example Data Table
| Use Case | Topology | R Values | C Values | Expected Result |
|---|---|---|---|---|
| Audio noise smoothing | First order non-inverting | R1 = 15.9 kΩ | C1 = 10 nF | About 1 kHz cutoff |
| Sensor signal cleanup | Second order Sallen-Key | R1 = 10 kΩ, R2 = 10 kΩ | C1 = 10 nF, C2 = 10 nF | About 1.59 kHz cutoff |
| PWM ripple reduction | Second order Sallen-Key | R1 = 33 kΩ, R2 = 33 kΩ | C1 = 100 nF, C2 = 100 nF | About 48 Hz cutoff |
Formula Used
First Order Active Low Pass
fc = 1 / (2πRC)
|H(f)| = |K| / √(1 + (f / fc)²)
Phase = -tan⁻¹(f / fc)
Non-Inverting Gain
K = 1 + Rf / Rg
Inverting Gain
K = -Rf / Rin
Sallen-Key Second Order Low Pass
fc = 1 / (2π√(R1R2C1C2))
Q = √(R1R2C1C2) / [C2(R1 + R2) + C1R1(1 - K)]
|H(f)| = |K| / √((1 - x²)² + (x / Q)²), where x = f / fc.
Slew Rate
SR = 2πfVpeak / 1,000,000
How To Use This Calculator
Choose the active low pass topology first. Enter resistor and capacitor values with matching units. Add the op amp gain resistors. Enter an input peak voltage and a test frequency. Press calculate. The result area appears below the header and above the form. Review cutoff, gain, phase, Q, attenuation, output level, slew rate, and headroom. Use the graph to study the frequency response. Download CSV or PDF files for reports.
Why Active Low Pass Filters Matter
An active low pass filter passes low frequencies and reduces high frequencies. It uses an op amp, resistors, and capacitors. The op amp can add gain, buffering, and isolation. This helps sensor, audio, and control circuits.
Cutoff And Gain Planning
The cutoff frequency is the main design point. At this frequency, an ideal first order filter is down by 3 dB. A second order stage changes more quickly after cutoff. Its Q value controls peaking and damping. A Butterworth response is usually smooth and flat. Higher Q can boost signals near cutoff. Lower Q gives more damping.
Gain must also fit the op amp. A non-inverting stage keeps the signal polarity. An inverting stage flips polarity. A Sallen-Key stage can provide gain and a steeper response. High gain needs enough gain bandwidth. The calculator estimates this need.
Component Choice
Real parts have tolerances. A one percent resistor is usually better than a five percent part. Film capacitors are stable in many signal filters. Ceramic capacitors can work well, but their value may change with voltage. Choose values that are easy to buy. Very high resistances can add noise. Very low resistances can load the previous stage.
Reading The Results
The result panel gives cutoff, gain, Q, attenuation, output level, phase, and slew rate demand. It also shows a plot. Use the test frequency to inspect one important point. The graph helps you see roll-off. Export the data when you need notes for reports.
Practical Design Tips
Start with the required cutoff. Pick a topology next. Use first order filters for simple smoothing. Use second order filters for stronger high frequency rejection. Keep output swing below the supply limits. Check the op amp data sheet before building. Breadboard the circuit when frequency is low. Use a clean layout for higher frequency work.
Safety And Verification
Always confirm polarity, supply rails, and maximum input range. Simulate the stage when the filter is critical. Measure the real cutoff with a signal generator if possible. Small differences are normal. Temperature, tolerance, and board capacitance can move the final response slightly. Record final values for future maintenance and revisions.
FAQs
What is an active low pass filter?
It is a circuit that passes lower frequencies and reduces higher frequencies. It uses an op amp with resistors and capacitors. The op amp can add gain, buffering, and better load isolation than a passive RC filter.
What does cutoff frequency mean?
Cutoff frequency is the point where the response starts dropping strongly. For a simple first order low pass filter, the signal is about 3 dB below the passband level at cutoff.
Why use a Sallen-Key filter?
A Sallen-Key filter gives a second order response with few parts. It can provide a steeper roll-off than a first order filter. It is common in audio, sensor, and control circuits.
What is Q factor?
Q factor describes damping in a second order filter. A higher Q can create peaking near cutoff. A lower Q gives a more damped response. Many smooth designs use a Q near 0.707.
Why does op amp gain bandwidth matter?
The op amp must have enough open loop performance at the filter frequency. If gain bandwidth is too low, the real circuit may lose accuracy, phase margin, and expected roll-off shape.
Can I use any capacitor value?
You can calculate with many values, but real designs need practical parts. Very small capacitors may be affected by stray capacitance. Very large capacitors may have tolerance, leakage, or size issues.
What does slew rate demand show?
Slew rate demand estimates how fast the op amp output must move. Higher frequency and higher output voltage need more slew rate. Choose an op amp with margin above this value.
Why does the output clip?
Clipping happens when the required output swing exceeds the op amp supply range. The calculator checks an estimated safe swing. Real limits depend on the selected op amp and load current.