Low Pass Active Filter Calculator
Example Data Table
These examples show common first-order active low pass filter settings.
| Use Case | R | C | Gain | Approx Cutoff | Note |
|---|---|---|---|---|---|
| Audio tone smoothing | 10 kΩ | 10 nF | 2.0 | 1.59 kHz | Useful for soft high-frequency reduction. |
| Sensor noise cleanup | 33 kΩ | 100 nF | 1.5 | 48.2 Hz | Good for slow analog signals. |
| Control loop smoothing | 4.7 kΩ | 1 µF | 1.0 | 33.9 Hz | Use a stable buffer stage. |
| ADC input conditioning | 1 kΩ | 100 nF | 1.0 | 1.59 kHz | Keeps source impedance modest. |
Formula Used
The calculator uses a common non-inverting active low pass model. The cutoff frequency depends on the filter resistor and capacitor.
fc = 1 / (2πRC)
Av = 1 + (Rf / Rg)
|Vout / Vin| = Av / √(1 + (f / fc)²)
Phase = -tan⁻¹(f / fc)
For cascaded identical stages, the magnitude is raised by the selected order. The rolloff becomes -20 × order dB per decade. The tolerance range is estimated by shifting the cutoff by the selected component tolerance.
How to Use This Calculator
- Enter the filter resistor and capacitor values.
- Enter feedback resistor values for non-inverting gain.
- Add the frequency where response should be checked.
- Enter input voltage, tolerance, slew rate, and supply voltage.
- Press the calculate button to view cutoff, gain, phase, and output.
- Use CSV or PDF buttons to save the report.
Active Low Pass Filter Design Guide
What the Calculator Does
A low pass active filter passes low frequencies and reduces high frequencies. It uses an op amp with resistors and capacitors. The op amp adds buffering and gain. This makes the circuit stronger than a simple passive RC filter. The calculator estimates cutoff frequency, gain, phase shift, and output level. It also checks useful design limits.
Why Cutoff Frequency Matters
Cutoff frequency is the main design point. At this point, a first-order filter drops by about three decibels. Signals below cutoff pass with little loss. Signals above cutoff become weaker. The resistor and capacitor set this frequency. A larger resistor or capacitor gives a lower cutoff. A smaller value gives a higher cutoff.
Using Gain in Active Filters
Active filters can also amplify the signal. The calculator uses a non-inverting gain equation. This gain depends on Rf and Rg. A unity gain design can work as a buffer. A higher gain design can boost weak sensor or audio signals. Always check output headroom before using high gain.
Phase and Rolloff
Phase shift shows timing change through the filter. It increases as frequency rises. The slope also becomes stronger with higher order stages. A first-order stage rolls off at about twenty decibels per decade. Two identical stages double that slope. This is useful when sharper filtering is needed.
Practical Design Checks
Real parts have tolerance. The true cutoff can move above or below the calculated value. The op amp must also handle the output speed. Slew rate becomes important at high frequency and high voltage. Supply voltage can limit maximum output swing. Use conservative values when building production circuits. Test the final circuit with real components.
FAQs
1. What is a low pass active filter?
It is a filter that passes lower frequencies and reduces higher frequencies. It uses an op amp, so it can add gain, buffering, and better load isolation.
2. What sets the cutoff frequency?
The cutoff frequency is mainly set by the resistor and capacitor. The formula is one divided by two times pi times resistance times capacitance.
3. What does gain mean here?
Gain shows how much the op amp increases the passband signal. In this calculator, gain follows the non-inverting equation using Rf and Rg.
4. Why does phase shift matter?
Phase shift shows how much the output signal timing changes. It matters in audio systems, feedback loops, sensor circuits, and control applications.
5. What is filter order?
Filter order shows how many stages act together. A higher order gives a steeper rolloff, but it may add more phase shift and design complexity.
6. What is slew rate checking?
Slew rate checking estimates whether the op amp can change output voltage fast enough. High frequency and high output voltage need more slew rate.
7. Can this calculator design audio filters?
Yes. It can estimate useful audio cutoff points, gain, phase shift, and output level. Final audio designs should still be tested with real parts.
8. Why include component tolerance?
Tolerance shows the likely cutoff shift caused by real resistor and capacitor variation. It helps you choose safer values for practical circuits.