Low Pass Filter Calculator

A low-pass filter allows signals with a frequency lower than a selected cutoff frequency to pass through while attenuating signals with higher frequencies. The underlying math depends on the specific circuit topology. For a standard passive resistor-capacitor (RC) network, the cutoff frequency (Fc) is calculated using the formula: Fc = 1 / (2 * pi * R * C), where R is resistance in ohms and C is capacitance in farads. For active inverting and non-inverting op-amp filters, feedback and input resistors additionally dictate the circuit's overall voltage gain, ensuring that the filtered output maintains the desired signal amplitude.

Suppose you are designing a passive RC low-pass filter for an audio processing application and need a cutoff frequency around 1,000 Hz (1 kHz). You select a standard capacitance value of 0.1 microfarads (0.0000001 F). Using the cutoff frequency formula Fc = 1 / (2 * pi * R * C), we can rearrange to solve for the required resistance: R = 1 / (2 * pi * Fc * C). Substituting our values gives R = 1 / (2 * 3.14159 * 1000 * 0.0000001), which equals approximately 1,591 ohms. Choosing a standard 1.6 k-ohm resistor will yield an actual cutoff frequency of roughly 995 Hz, providing ideal attenuation for high-frequency hiss.

When selecting components, always account for component tolerance; standard carbon-film resistors typically have a 5% tolerance, while standard ceramic capacitors can vary by 10% to 20%, shifting your actual cutoff frequency. For active op-amp filters, ensure your operational amplifier has a gain-bandwidth product significantly higher than your target operating frequency to prevent signal distortion. Finally, always keep source and load impedances in mind, as connecting a low-impedance load to a passive RC filter can unintentionally load down the output voltage and alter the filter response curve.