Why do some capacitors have 4 terminals?

Capacitors with four terminals, often referred to as "four-terminal capacitors" or "feedthrough capacitors," are designed to provide improved performance in certain applications. Here are some reasons why these capacitors have four terminals:

1. Improved High-Frequency Performance:

   - Four-terminal capacitors are designed to reduce the effects of parasitic inductance and resistance that can degrade performance at high frequencies. By separating the input and output connections, these capacitors can maintain better impedance characteristics and lower equivalent series inductance (ESL), making them more effective in high-frequency applications.

2. Better Filtering and Decoupling:

   - These capacitors are commonly used in filtering and decoupling applications, particularly in power supply lines and signal lines that require effective noise suppression. The four-terminal design helps in providing a cleaner path for the desired signal while shunting unwanted high-frequency noise to ground more effectively.

3. Feedthrough Design:

   - The four-terminal configuration is often used in feedthrough capacitors, which are designed to pass a conductor through a bulkhead while providing high-frequency noise suppression. This design is particularly useful in electromagnetic interference (EMI) suppression, ensuring that signals pass through while noise is filtered out.

4. Minimized Ground Impedance:

   - By having separate input and output terminals for both the signal and ground, four-terminal capacitors can minimize ground impedance, improving overall performance in sensitive electronic circuits.

5. Reduced Crosstalk and Improved Isolation:

   - The separation of terminals helps reduce crosstalk and improve isolation between different parts of the circuit, which is crucial in high-speed digital and RF circuits.

In summary, four-terminal capacitors are designed to provide enhanced performance, particularly in high-frequency and noise-sensitive applications, by addressing the limitations of traditional two-terminal capacitors in terms of inductance, resistance, and overall impedance.