What is an RC filter calculator?
RC filters shape how signals pass through a circuit based on frequency. A low-pass filter allows low-frequency signals to pass while attenuating higher frequencies. A high-pass filter performs the opposite: it blocks low frequencies and passes higher ones.
This calculator helps you design both types of filters by solving for:
- Cutoff frequency (fc)
- Capacitor value C
- Resistor value R
- Frequency response characteristics
These filters are widely used in sensor conditioning, audio electronics, PLC analog inputs, power supplies, and noise suppression networks.
Low-pass vs high-pass filters
The behavior of RC filters can be summarized as follows:
| Filter Type | Behavior | Formula | Common Use |
|---|---|---|---|
| Low-pass | Passes low frequencies, attenuates high frequencies | fc = 1 / (2πRC) | Sensor smoothing, anti-aliasing, removing high-frequency noise |
| High-pass | Passes high frequencies, attenuates low frequencies | fc = 1 / (2πRC) | AC coupling, blocking DC offsets, removing drift/low-frequency noise |
Understanding the cutoff frequency
The cutoff frequency (fc) is the point where the output signal falls to 70.7% of the input (−3 dB). Above or below this frequency, the filter increasingly attenuates the signal.
For a simple RC network:
fc = 1 / (2πR C)
Magnitude response of RC filters
RC filters have a smooth and predictable response:
- Each RC stage provides a 20 dB/decade slope
- Filters are first-order, phase-shifting signals gradually
- Increasing R or C lowers the cutoff frequency
Example applications
- Noise suppression: Low-pass filters remove switching noise or high-frequency interference from sensor and PLC inputs.
- Blocking DC offsets: High-pass filters remove DC drift while preserving AC content in audio, instrumentation, and communication circuits.
- Anti-aliasing: Low-pass filters are used before ADC sampling to prevent high-frequency folding.
- Coupling capacitors: High-pass filters are formed when a capacitor connects two stages—passing AC while blocking DC.
Design considerations for accurate filtering
- Component tolerances: Real-world R and C values may vary by ±5% to ±20%, shifting the cutoff frequency.
- Load impedance: The next stage (e.g., ADC, op-amp, PLC input) affects the effective R value and must be considered.
- Signal amplitude: Large DC offsets or high-voltage swings may require protective elements or buffering.
- Noise environment: For industrial systems, differential filtering, shielding, or multi-stage RC networks may be required.
This calculator provides a fast, accurate starting point for designing analog filters in sensors, power electronics, audio processing, and industrial control systems.
