Part of Analogue and filter calculators
RC High-Pass and AC-Coupling Calculator
Solve cutoff frequency, resistance, or capacitance for a first-order high-pass or AC-coupling network.
Inputs
Select which value to calculate from the first-order RC cutoff relationship.
Results
RC high-pass filtering attenuates frequencies below cutoff. For AC coupling, use the effective resistance seen by the capacitor, including bias and input resistances.
Use an RC high-pass filter when low-frequency content or DC must be rejected
A first-order RC high-pass network attenuates DC and low-frequency content while passing higher-frequency signal content. It is also the basic model behind many AC-coupling capacitor checks.
AC coupling
Choose a series capacitor and effective resistance so wanted low-frequency content is not excessively attenuated.
Bias removal
Block DC offsets before an amplifier, comparator, ADC, or measurement stage.
Transient recovery
Estimate how quickly a coupled node settles after startup, switching, or a large signal step.
Equations and model
The calculator uses the ideal first-order RC cutoff relationship. In a real AC-coupled input, the hard part is choosing the correct effective resistance seen by the capacitor.
Cutoff frequency
The -3 dB corner frequency for an ideal first-order RC high-pass filter.
Time constant
The same R and C also define the transient settling behaviour after a step or bias change.
Effective resistance
For AC coupling, use the resistance seen by the capacitor, including bias and input resistances.
fc - Cutoff frequency
Unit: hertz (Hz)
Frequency where the ideal output amplitude is about 0.707 of the high-frequency passband value.
R - Effective resistance
Unit: ohms (Ω)
The resistance seen by the capacitor that sets the high-pass corner.
C - Series capacitance
Unit: farads (F)
The capacitor that blocks DC and passes changing signal content.
τ - Time constant
Unit: seconds (s)
The related time-domain settling constant of the coupling or high-pass network.
Worked example
This example uses the same shared RC cutoff model as the low-pass calculator and test coverage.
Design question: An AC-coupled signal feeds an input with 100 kΩ effective resistance. What capacitor gives about a 16 Hz high-pass corner?
Inputs: R = 100 kΩ, target fc = 16 Hz.
Capacitance: C = 1 / (2π × 16 Hz × 100 kΩ) = 99.5 nF, so 100 nF is a practical first choice.
Time constant: τ = R × C ≈ 10 ms.
Next check: verify the effective resistance, capacitor tolerance, startup settling, leakage, bias current, and acceptable low-frequency droop.
General high-pass filtering versus AC coupling
The equation is the same, but the engineering concern is often different.
High-pass filter
- Main concern is frequency response and low-frequency attenuation.
- Source and load impedance still affect the real pole.
- May be part of a wider analogue filter chain.
AC coupling
- Main concern is blocking DC while preserving wanted signal content.
- Bias network and input resistance define the effective R.
- Startup settling, leakage, and signal swing must be checked separately.
Assumptions and limitations
First-order only
The model covers one ideal RC pole. It does not model higher-order or active high-pass filters.
Effective R can be non-obvious
Bias resistors, input resistance, source impedance, and terminations can all change the resistance seen by the capacitor.
Tolerance and leakage matter
Capacitor tolerance, dielectric behaviour, leakage, and bias currents can shift the low-frequency response and DC operating point.
Related calculators and next checks
Follow the next check based on whether the concern is coupling, time-domain settling, low-pass filtering, or an amplifier input.
RC time constant calculator
Use when the high-pass network also needs settling or recovery-time checks.
RC low-pass filter calculator
Use for the complementary first-order low-pass case.
Op-amp gain calculator
Use when the coupling capacitor feeds an amplifier input or bias network.
Analogue and filter hub
Follow related RC, filter, AC-coupling, and analogue design workflows.
Engineering conversion calculator
Convert Hz, kHz, nF, µF, and SI-prefixed component values.
FAQ
Is an AC-coupling capacitor the same as a high-pass filter?
Electrically, a coupling capacitor with a resistance to a bias point or input impedance forms a high-pass response. The practical design also needs bias point, startup settling, and signal swing checks.
Which resistance should I use for AC coupling?
Use the effective resistance seen by the capacitor. That may include input resistance, bias resistors, termination, and source resistance depending on the circuit topology.
What happens below the cutoff frequency?
A first-order high-pass filter increasingly attenuates lower-frequency content below the cutoff. At cutoff, the ideal amplitude is about -3 dB relative to the high-frequency passband.
Engineering reference
Equations, assumptions, and design guidance
Solves the ideal first-order RC high-pass cutoff relationship.
Equations and variables
fc = 1 / (2 * pi * R * C)tau = R * C- fc
- Cutoff frequency (Hz)
- R
- Resistance (ohm)
- C
- Capacitance (F)
Assumptions and limitations
Assumptions
- The source and load impedances do not shift the effective resistance.
- The capacitor is ideal across the frequency range of interest.
Limitations
- Component tolerance, capacitor ESR/ESL, op-amp input impedance, and source/load interaction are not included in this nominal solve.
Worked example and design use
1 kOhm and 100 nF filter
Inputs: R = 1 kOhm, C = 100 nF
Outputs: fc is about 1.59 kHz, tau = 100 us
Design guidance
- For AC coupling, use the effective resistance seen by the capacitor, including bias and input resistances.
- Check R and C tolerances when cutoff placement is critical.