RC Low-Pass and High-Pass Filter Guide
RC filters use a resistor and capacitor to selectively pass or block signals based on their frequency. They are the simplest and most widely used filter circuits, appearing in audio systems, sensor conditioning, power supplies, and communication systems.
Open RC Filter SimulatorRC Filter Topologies
The Cutoff Frequency
Every RC filter has a cutoff frequency (f_c) — the frequency at which the filter begins to have significant effect. At f_c, the output is 3dB below (approximately 70.7%) of the input level.
Example: R = 10kΩ, C = 100nF → f_c = 1 / (2π × 10,000 × 0.0000001) = 159 Hz.
Low-Pass Filter
Passes low frequencies, blocks high frequencies. Place the resistor in series with the signal path, and the capacitor from the output to ground.
At frequencies well below f_c, the capacitor has high impedance and most voltage appears at the output. At frequencies well above f_c, the capacitor has low impedance and shorts the output to ground.
Common uses: Removing high-frequency noise from sensor signals, anti-aliasing before an ADC, audio bass extraction, power supply smoothing.
Simulate Low-Pass FilterHigh-Pass Filter
Passes high frequencies, blocks low frequencies (and DC). Place the capacitor in series with the signal path, and the resistor from the output to ground.
At low frequencies, the capacitor has high impedance and blocks the signal. At high frequencies, the capacitor is nearly a short circuit and the signal passes through to the resistor output.
Common uses: AC coupling (blocking DC offset between stages), removing low-frequency interference (50/60Hz hum), audio treble extraction, differentiating circuits.
Simulate High-Pass FilterFilter Rolloff
A single RC filter has a rolloff of −20dB per decade (−6dB per octave). This means: for every 10× increase in frequency above f_c (for low-pass), output falls by 10×. This is a gentle slope — a first-order filter.
For steeper rolloff, cascade multiple RC stages (second-order = −40dB/decade, third-order = −60dB/decade) or use active filters (op-amp based) which provide sharper cutoff without signal attenuation in the passband.
RC Filter vs Active Filter
| Feature | RC (Passive) | Active (Op-Amp) |
|---|---|---|
| Components | R + C only | R, C + op-amp |
| Power required | No | Yes (op-amp supply) |
| Signal gain | Attenuation only | Can amplify |
| Loading sensitivity | High | Low (buffered output) |
| Rolloff (1st order) | -20dB/decade | -20dB/decade (but sharper possible) |
Common Beginner Mistakes
Loading the Filter Output
Connecting a low-impedance load to an RC filter lowers the effective resistance of the bottom component, shifting the cutoff frequency. Always buffer the filter output with an op-amp voltage follower if driving a low-impedance circuit.
Wrong Component Positions
Low-pass = resistor in series, capacitor to ground. High-pass = capacitor in series, resistor to ground. Swapping these creates the opposite filter from what you intended.
Expecting Sharp Cutoff
A single RC filter is first-order with -20dB/decade rolloff. Signals don't stop abruptly at the cutoff frequency. For a sharper response, cascade multiple RC stages or use an active filter with a Sallen-Key topology.
Cutoff Frequency Calculation Errors
f_c = 1/(2π×R×C) requires R in ohms and C in farads. A 10kΩ resistor is 10,000Ω. A 100nF capacitor is 0.0000001F. Forgetting to convert unit prefixes is the most common source of calculation errors.