Why Three-Terminal Feedthrough Capacitors Lower Inductance

This article explores the physical and electrical reasons why three-terminal feedthrough capacitors outperform standard two-terminal chip capacitors in high-frequency electronic filtering. By examining internal electrode structures, current loop geometry, and mutual inductance, we will cover how three-terminal designs drastically minimize parasitic Equivalent Series Inductance (ESL) to maintain signal integrity in modern electronic circuits.

The Problem with Standard Two-Terminal Capacitors

To understand the advantage of a three-terminal capacitor, it is necessary to look at the limitations of a standard two-terminal Multi-Layer Ceramic Capacitor (MLCC). In an ideal world, a capacitor acts purely as a capacitive element, dropping in impedance as signal frequency increases. In reality, all capacitors possess parasitic Equivalent Series Inductance (ESL).

In a two-terminal MLCC, high-frequency noise must enter through one terminal, travel across the internal electrodes, and exit through the opposite terminal to reach the circuit board's ground plane. Because this path covers the entire length of the component, it creates a relatively large current loop. Since inductance is directly proportional to the area of the current loop, this elongated path generates significant ESL. At high frequencies, this ESL dominates the capacitor's behavior, turning the component into an inductor and ruining its ability to filter out high-frequency noise.

Reshaping the Internal Geometry

Three-terminal feedthrough capacitors solve the current loop problem by fundamentally altering how the component connects to the circuit. Instead of two ends, these components have two pass-through signal terminals on opposite ends and a shared ground terminal wrapping around the middle of the capacitor body.

Internally, the signal electrode runs straight through the component, carrying the main DC current or signal. The ground electrodes are interleaved with the signal electrodes, perpendicular to the signal path, and connect directly to the central external ground band.

This changes the physical path the high-frequency noise must take:

• Shorter Path to Ground: Noise does not need to travel the entire length of the capacitor. Instead, it travels a much shorter, wider path from the center of the component directly out to the side ground terminals.
• Parallel Grounding: Because the ground connection exists on both sides of the capacitor's body, the noise currents split in two directions to reach the ground plane. Functioning like two inductors in parallel, this dual-path configuration instantly cuts the ground path inductance in half.

The T-Filter Configuration

A standard two-terminal capacitor is wired in a shunt configuration, meaning it branches off the main signal line. High-frequency noise can easily bypass a two-terminal capacitor if the parasitic inductance of the mounting pads and vias is too high.

A three-terminal feedthrough capacitor is wired directly inline with the signal trace, creating an electrical "T-filter." The signal is forced to pass directly through the component. Because the input and output terminals are part of the same continuous electrode structure, the series inductance located on the input and output branches becomes part of the transmission line rather than the ground path. This forces the high-frequency noise directly into the low-inductance central ground path, preventing the noise from bypassing the capacitor.

Mutual Inductance Cancellation

The geometric layout of a three-terminal capacitor also takes advantage of magnetic flux cancellation. As high-frequency currents flow through the internal electrodes and out through the opposing side ground terminals, they travel in opposite directions.

When two currents flow in opposite directions in close proximity, their magnetic fields oppose and cancel each other out. This phenomenon reduces the mutual inductance within the capacitor body, further driving down the overall ESL of the component.

Superior High-Frequency Performance

Because of the shorter ground path, parallel ground connections, inline T-filter structure, and magnetic flux cancellation, a three-terminal feedthrough capacitor achieves an ESL that is a fraction of a standard two-terminal MLCC.

A lower ESL directly translates to a much higher Self-Resonant Frequency (SRF). While a standard chip capacitor might become inductive and lose its filtering capability at a few hundred megahertz, a three-terminal feedthrough capacitor of the exact same capacitance value remains effective well into the gigahertz range. This allows engineers to use a single three-terminal capacitor to replace multiple standard capacitors, saving board space while achieving superior high-frequency decoupling and noise suppression.