ACkoppling

ACkoppling, or AC coupling, is a method used in electronics to pass an alternating current (AC) signal while blocking its direct current (DC) component. It is typically implemented with a capacitor in series with the signal path, followed by the input impedance of the next stage. The combination of the capacitor and the load forms a high-pass filter. The cutoff frequency is approximately f_c = 1/(2πRC), where R is the impedance seen by the capacitor (the input resistance of the following stage). At frequencies well above f_c, the capacitor presents little impedance and the signal passes with minimal attenuation; at lower frequencies, the signal is attenuated and the DC level is not transferred.

The primary purpose of ACkoppling is to prevent DC biases from one stage from affecting subsequent stages, while allowing the AC signal to be processed. This is important in amplifiers, measurement systems, and data acquisition where different stages require independent biasing. It is also used in measurement equipment, such as oscilloscopes, in AC coupling mode to remove large DC offsets from the signal under test.

Design considerations include selecting C to set f_c below the lowest frequency of interest, ensuring a stable input impedance, and accounting for phase shift near the cutoff. The technique introduces frequency-dependent attenuation and phase shift, and a transient charging behavior when a signal starts, or the DC level changes.

Variants exist, including active AC coupling with op-amps and differential configurations, but the basic concept remains a series capacitor that blocks DC while passing AC.