Calculadora de Filtro Passa-Baixa / Passa-Alta

Frequência de corte -3dB para filtros RC e RL

Operating Mode

Target Variable

Required Parameters

Resistance

Ohm

Capacitance

Waiting for input data...

Ad Placement

Sidebar Adaptive Ad Slot

Quick Answer

The -3dB cutoff frequency (fc) is where the signal power drops by half. For RC filters: fc = 1 / (2πRC). For RL filters: fc = R / (2πL).

Documentation

High Pass Filter Calculator

Design a first-order passive high-pass filter and compute the -3 dB cutoff frequency from resistor and capacitor (or inductor) values — or enter any two of R, C, L, fc to solve for the third.

Quick start: Select RC High Pass, enter R and C → instant fc. 10 kΩ and 0.1 µF → ≈ 159 Hz. Switch Target Variable to find C for a desired cutoff.

RC high pass filter formula

fc = 1 / (2π × R × C)

Symbol	Meaning
fc	Cutoff frequency (Hz) — -3 dB point
R	Resistance (Ω)
C	Capacitance (F)

Rearranged:

C = 1 / (2π fc R) — pick C when you know target fc and R
R = 1 / (2π fc C) — pick R when fc and C are fixed

Time constant: τ = RC = 1 / (2π fc)

A series capacitor and shunt resistor is the usual RC high-pass. DC is blocked; frequencies above fc pass.

RL high pass filter formula

fc = R / (2π × L)

Use RL when an inductor is already in the path (power or RF). Capacitors dominate most signal-level designs because they are smaller and cheaper.

What a high pass filter does

A high-pass filter passes frequencies above fc and attenuates those below, including DC. Below fc, a first-order high-pass rises at +20 dB per decade. Applications:

DC blocking / AC coupling into an amplifier or ADC
Audio rumble / thump removal
Sensor high-side conditioning (kill offset and 1/f)
Break frequency design on a Bode plot — another name for fc

Worked example — 1 kHz RC high pass

Target fc = 1 kHz, choose R = 3.3 kΩ:

C = 1 / (2π × 1000 × 3300) ≈ 48.2 nF
Nearest E12: 47 nF → actual fc ≈ 1.03 kHz
Use C0G/NP0 ceramics; Y5V parts shift badly with bias

Use Target Variable → Find capacitance (C) with R = 3.3 kΩ and fc = 1 kHz.

High-pass vs low-pass

Type	Passes	Blocks	Slope (1st order)
High-pass	Above fc	Below fc (incl. DC)	+20 dB/decade below fc
Low-pass	Below fc	Above fc	−20 dB/decade above fc

Same fc = 1/(2πRC) formula — only the capacitor/resistor placement changes. Need the other topology? Use the low pass filter calculator.

Common mistakes

Reading fc as a brick wall — first-order filters roll off gently; expect ±20 dB/decade, not instant blocking.
Ignoring source impedance — if the driver already has 10 kΩ output Z, your effective R is higher than the printed resistor.
Y5V/Z5U capacitors — capacitance can collapse under DC bias; cutoff shifts badly. Prefer C0G/NP0.
Wrong topology — this page is passive first-order RC/RL high-pass. Active / Sallen-Key stages use different equations.

Related tools

Low Pass Filter Calculator — RC & RL low-pass
RC Filter Calculator — RC low-pass and high-pass only
RC/LC Filter Cutoff Calculator — all four RC/RL modes
Reactance Calculator — Xc and XL at any frequency
RC Time Constant Calculator — τ = RC step response

Design Notes

First-order passive filters only roll off at -20dB per decade. They are great for simple noise filtering, anti-aliasing before a slow ADC, or debounce circuits, but inadequate for separating close frequencies. When designing RC filters, avoid using extremely large resistors (e.g., >1MΩ) because the input bias current of the next stage (like an Op-Amp or ADC) will cause a DC voltage error. Conversely, avoiding extremely small resistors (<100Ω) will save power and prevent overloading the driving source.

Common Mistakes

1
Expecting a sharp cutoff from a simple RC filter. It's a gentle curve, not a brick wall.
2
Not balancing the source impedance. If your signal source already has 10kΩ output impedance, and you use a 1kΩ series resistor for your filter, your effective R is 11kΩ.
3
Ignoring capacitor tolerances. Cheap Y5V or Z5U ceramic capacitors can vary by +20%/-80% with temperature and DC bias, completely ruining your cutoff frequency.

Engineering Handbox

1. Identify variables: R = 10,000, C = 0.0000001 2. Multiply R × C = 0.001 seconds (Time Constant τ) 3. Multiply by 2π: 0.001 × 6.283 = 0.006283 4. Divide 1 by result: 1 / 0.006283 = 159.15 Hz

VerificationThe cutoff frequency is approximately 159.15 Hz.

Knowledge Base

O que é a frequência de corte -3dB?

O ponto onde a potência cai pela metade (tensão a 70,7%). Definição padrão universal em eletrônica.

Fórmula do filtro RC passa-baixa?

fc = 1/(2π×R×C). Exemplo: R=10kΩ, C=100nF → fc = 159,15 Hz.

Diferença entre passa-baixa e passa-alta?

Passa-baixa: frequências baixas passam. Passa-alta: frequências altas passam. Mesma fórmula de corte.

Inclinação do filtro de primeira ordem?

-20 dB/década ou -6 dB/oitava. Para inclinação mais íngreme, cascateie múltiplos estágios.

Que tipo de capacitor para filtros?

C0G/NP0 para precisão. X7R para uso geral. Evite Y5V/Z5U — valor muda drasticamente.

Related Engineering Tools

capacitors filters