Introduction of AC coupling circuit

As shown in the figure below, a capacitor is connected in series with the non-inverting input of the op amp for AC coupling, a simple way to isolate the DC component of the input voltage (VIN). This is especially useful in high gain applications where even small DC voltages at the op amp input limit the dynamic range and even cause output saturation. However, adding a capacitive coupling to the high-impedance input without providing a DC path for the current at the non-inverting input can cause problems.

In effect, the input bias current flows into the coupled capacitor and charges it until it exceeds the nominal value of the common-mode voltage of the amplifier input circuit or limits the output. Depending on the polarity of the input bias current, the capacitor will charge to the positive or negative voltage of the power supply. The closed loop DC gain of the amplifier amplifies the bias voltage.
This process can take a long time. For example, a field effect transistor (FET) input amplifier with a charge current I/C of 10–12/10–7=10 μV/s when a bias current of 1 pA is coupled to a 0.1 μF capacitor, or 600 μV minutes. If the gain is 100, the output drift is 0.06 V per minute. Therefore, general laboratory testing (using an AC-coupled oscilloscope) does not detect this problem, and the circuit does not experience problems until after a few hours. Obviously, it is very important to avoid this problem altogether. AC-Coupling Features: The use of capacitors to achieve LVDS data connections for AC coupling has many benefits, such as level shifting, removal of common-mode errors, and avoidance of input voltage faults. Coupler: In a microwave system, it is often necessary to divide a microwave power into several paths proportionally. This is the power distribution problem. The components that implement this function are called power distribution components, that is, couplers, and mainly include: directional couplers, power splitters, and various microwave branch devices. These components are generally linear multi-port reciprocal networks