RC Filter Calculator

A single resistor and capacitor is often all you need to knock down noise on a sense line or set a corner frequency ahead of an ADC input. This calculator solves the first-order RC low-pass or high-pass corner frequency in either direction — pick R and C to hit a target cutoff, or check what corner an existing R/C pair on a board actually produces.

RC Filter

First-order low-pass / high-pass corner frequency

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Topology

Solve for

f₋₃dB

Corner frequency

1.59 kHz

Capacitance

100 nF

Time constant τ

100 µs

10–90% rise time

220 µs

Roll-off

−20 dB/dec above fc

Formula

f_{−3dB} = \frac{1}{2 π \cdot R \cdot C}

τ = R \cdot C

t_{rise} \approx 2.2 \cdot τ

Low-pass: passes frequencies below the corner and rolls off at 20 dB/decade above it. The corner formula is the same for both.

R = resistance (Ω)
C = capacitance (F)
f_−3dB = corner frequency, where the output is 3 dB down (Hz)
τ = time constant (s)
t_rise = 10–90% rise time for a step input (s)

For anti-alias filtering, place the corner at roughly 1/10 of the sample rate. For an ADC driver, τ should be under about 1/8 of the acquisition window so the sampling cap settles.