Calculator Input
Example Data Table
| Use case | R1 | R2 | C1 | C2 | Rf | Rg | Approx result |
|---|---|---|---|---|---|---|---|
| Audio low pass | 10 k ohm | 10 k ohm | 10 nF | 10 nF | 10 k ohm | 10 k ohm | 1.59 kHz |
| Sensor smoothing | 22 k ohm | 22 k ohm | 100 nF | 100 nF | 4.7 k ohm | 10 k ohm | 72.34 Hz |
| Band shaping | 4.7 k ohm | 4.7 k ohm | 4.7 nF | 4.7 nF | 5.6 k ohm | 10 k ohm | 7.20 kHz |
Formula Used
Second order cutoff or center frequency:
f0 = 1 / 2π√(R1 × R2 × C1 × C2)
First order cutoff frequency:
fc = 1 / 2πRC
Non-inverting gain:
K = 1 + Rf / Rg
Simple Sallen Key equal-component Q estimate:
Q = 1 / (3 - K)
Gain in decibels:
Gain dB = 20 log10(Vout / Vin)
Slew rate requirement:
SR = 2π × f × Vpeak
How to Use This Calculator
Choose the filter response first. Select low pass, high pass, band pass, or notch. Pick the active topology. Enter resistor values in ohms. Enter capacitor values in nanofarads. Add Rf and Rg for gain. Set the test frequency for response checking. Enter the expected input peak voltage. Add component tolerance and available gain bandwidth. Press calculate. The result appears above the form and below the header.
Use CSV for spreadsheets. Use PDF for quick reports. Check Q, gain bandwidth margin, and slew rate before final hardware selection.
Op Amp Filter Design Guide
What This Calculator Does
An op amp filter shapes signals before amplification, conversion, or control. It can reject noise, limit bandwidth, remove offsets, or isolate useful signal bands. This calculator helps estimate core values for active filter work. It supports first order active stages and second order Sallen Key style stages.
Why Cutoff Matters
Cutoff frequency defines the point where the filter begins strong attenuation. In a low pass circuit, signals below this point pass more easily. In a high pass circuit, signals above this point pass more easily. Band pass and notch filters use a center frequency instead.
Gain and Q
Gain sets the passband level. The non-inverting gain is based on Rf and Rg. Q describes selectivity. A higher Q gives a sharper response. It may also create peaking near resonance. A lower Q gives a smoother response. It may reduce ringing and overshoot.
Component Choice
Real capacitors and resistors have tolerance. Their actual values can move cutoff frequency. This tool estimates a simple tolerance range. Use precision parts for tight filters. Use stable capacitors when temperature drift matters. Film capacitors are common in audio filters. C0G ceramic parts work well at smaller values.
Op Amp Limits
The op amp must have enough gain bandwidth. It also needs enough slew rate. A weak op amp can shift cutoff, add phase error, or distort output. The calculator gives a suggested bandwidth margin. It also estimates minimum slew rate from output peak voltage and test frequency.
Practical Design Notes
Start with standard resistor values. Then choose capacitor values that are easy to buy. Keep resistor values moderate. Very high resistance adds noise. Very low resistance increases loading. Simulate the final circuit before production. Then measure the real board with expected source and load conditions.
Best Use
This calculator is useful for early design, lab checks, education, and report preparation. It does not replace full circuit simulation. It gives a fast estimate for values, gain, Q, bandwidth, phase, and op amp requirements.
FAQs
What is an op amp filter?
It is an active filter using an operational amplifier. It shapes signal frequency response while also offering gain, buffering, or impedance control.
What does cutoff frequency mean?
Cutoff frequency is the transition point of the filter. It often marks the frequency where output falls by about three decibels.
What is Q in filter design?
Q means quality factor. It shows how selective or sharp the filter response is near the cutoff or center frequency.
Why does gain affect a Sallen Key filter?
Gain changes feedback behavior. In equal-component Sallen Key stages, higher gain can raise Q and create more response peaking.
Can this calculator design a band pass filter?
Yes. It estimates center frequency, bandwidth, Q, gain, phase, and cutoff limits for a second order band pass response.
Why is op amp gain bandwidth important?
The op amp must respond faster than the filter signal. Low gain bandwidth can shift response and reduce accuracy.
What happens if slew rate is too low?
The output cannot follow fast voltage changes. This may cause distortion, rounded waveforms, and wrong amplitude readings.
Should I simulate the final design?
Yes. Use this calculator for quick estimates. Then confirm performance with simulation, tolerance checks, and real measurements.