AC coupling capacitor sizing and high-pass corner frequency
An AC coupling capacitor is a high-pass network plus a biasing problem. The cutoff equation is simple, but the effective resistance is easy to get wrong.
How do you size an AC coupling capacitor?
Choose the lowest frequency that must pass, identify the effective resistance seen by the capacitor, then solve C = 1 / (2 * pi * fc * R). After that, check settling time, bias point, leakage, and signal swing.
Model summary
- High-pass corner: fc = 1 / (2 * pi * R * C).
- Capacitor sizing: C = 1 / (2 * pi * fc * R).
- R is the effective resistance seen by the capacitor, often involving bias resistors, input resistance, source resistance, or termination.
Worked example
For a 100 kOhm biased input and a 16 Hz target corner, C = 1 / (2 * pi * 16 * 100 kOhm) = 99.5 nF.
A 100 nF capacitor is a practical nominal first choice.
The time constant is about 10 ms, so startup and recovery behaviour should still be checked.
What to check after capacitance
Verify cutoff, tolerance, startup settling, bias headroom, and the receiving input common-mode range.
Common mistakes
- Using only the input resistance and forgetting parallel bias resistors.
- Choosing a huge capacitor without considering startup settling, leakage, dielectric type, or physical size.
- Forgetting that the coupled node still needs a valid DC bias point.
When the approximation breaks down
- The simple model assumes one dominant capacitor and one effective resistance.
- Large signal swings, bias currents, leakage, and nonlinear capacitor behaviour can require simulation or measurement.