A shielded enclosure requires power entry filtering to suppress conducted EMI and prevent internal circuits from radiating interference. An engineer specifies conventional capacitors for filtering the power line to ground at both the enclosure entry point and inside the enclosure, resulting in additional cost and PCB real-estate consumption. An alternative design uses a single feedthrough capacitor that delivers filtering right at the enclosure penetration point via its three-terminal configuration. This design difference is not arbitrary — the three-terminal architecture is fundamental to the superior performance that makes feedthrough capacitors the preferred choice for shielded penetration applications.
Understanding Feedthrough Capacitors
Basic Definition
A feedthrough capacitor is a specialized EMI filtering component specifically designed to suppress conducted emissions at the point where a power line or signal passes through a shielded enclosure wall. Unlike conventional two-terminal capacitors, feedthrough capacitors provide a direct, low-impedance connection to the shielding material, enabling superior filtering performance.
The three terminals are:
- Input Terminal:Connection to the power line (external to shield)
- Output Terminal:Connection to the power line (internal to shield)
- Ground/Shield Terminal:Direct connection to the shielding material and PCB ground plane
Why Three Terminals Are Necessary
The Principle: Symmetrical Filtering
The fundamental reason for three-terminal design is to achieve symmetrical filtering performance on both sides of the shielded enclosure wall, which justifies the additional structural complexity.
A conventional two-terminal capacitor, when installed between a power line and ground, provides capacitive coupling to ground at that single point. The filtering occurs at one location only.
Mounted right at the enclosure penetration boundary, the three-terminal feedthrough capacitor uses a single ceramic dielectric between its central conductor and shield shell. When the shell is firmly attached to the enclosure wall, this single capacitor delivers filtering effect for circuits on both the external and internal sides of the shield, together with:
- Direct low–impedance electrical connection to the shielding material
This dual-sided filtering is more effective than either side alone and eliminates the need for two separate capacitors.
Lead Inductance Elimination
Conventional capacitors mounted on PCBs introduce parasitic lead inductance — the inductance inherent to the wire leads connecting the component to the PCB traces. At EMI frequencies (high MHz to GHz range), this lead inductance can be 1-5 nanohenries or more, significantly reducing filtering effectiveness.
Feedthrough capacitors minimize ground-path lead inductance by mounting the outer shell directly against the shielding enclosure. The ground/shield terminal forms an integrated mechanical-electrical interface with the shield without intermediate wire leads. This creates a low-impedance high-frequency return path critical for good EMI suppression.
Ground Plane Integration
The third terminal enables direct interfacing with either the metallic enclosure shield or the PCB ground plane. In typical installations, the outer shell locks onto the enclosure wall. Where system requirements demand it, the shield terminal may also connect to the ground plane through low-impedance vias. This direct path provides a return-current route with minimal impedance at high frequencies.
| Aspect | Conventional Capacitor | Feedthrough Capacitor |
| Number of Terminals | 2 | 3 |
| Effective Filtering Boundary | Single-side only | Operates on both sides of shield wall |
| Ground Connection | Via trace and vias | Direct via shield |
| Lead Inductance | 1-5 nH typical | Minimized |
| Ground Plane Integration | Indirect | Direct |
| Common-Mode Filtering | Moderate | Superior |
| High-Frequency Performance | Limited by inductance | Limited by SRF only |
How Feedthrough Capacitor Filtering Works
Input Side Filtering
The external power line connects to the input terminal. Operating current passes straight through the central conductor toward the output terminal, while external high-frequency EMI noise couples through the capacitive dielectric and returns to ground via the shield/ground terminal (third terminal). This path filters external EMI before it enters the shielded enclosure.
Output Side Filtering
The internal power line connects to the output terminal. Operating current travels along the central conductor toward the external side. Conducted EMI noise generated by internal circuits couples through the capacitive dielectric and returns to ground via the shield/ground terminal. This path suppresses conducted emissions that would otherwise propagate outward and radiate from the enclosure.
Common-Mode Filtering
Both input and output sides reference the same shield/ground plane through the third terminal, providing effective common-mode filtering. Common-mode signals (signals present equally on all conductors relative to ground) are attenuated by the symmetrical connection to the shield.
Comparison with Conventional Solutions
Single Capacitor Approach
A single conventional capacitor installed between the power line and ground at the enclosure entry point provides one filtering path. Conducted EMI on the exit side is not addressed.
Two-Capacitor Approach
Two conventional capacitors (one external, one internal) provide filtering at both points but require:
- Double the component cost
- Double the PCB real estate
- Two separate ground connections
- Potential impedance mismatches if not carefully designed
Feedthrough Approach
A single feedthrough capacitor provides symmetrical filtering at both sides with:
- Single component cost
- Minimal PCB real estate
- Direct shield integration
- Optimized impedance matching through symmetric design
Performance Characteristics
Insertion Loss
For feedthrough capacitors, two insertion-loss metrics should be distinguished. The through-path insertion loss for the desired signal remains very low (0.5-2 dB) across the operating frequency band, varying with component design and frequency. Meanwhile, EMI insertion loss represents noise attenuation; well-designed feedthrough capacitors can achieve tens of decibels of EMI suppression for conducted interference.
Return Loss and VSWR
Return loss describes impedance-matching performance of the signal through-path. Properly designed feedthrough capacitors exhibit return loss >15 dB (VSWR <1.5:1) across the operating frequency range, indicating good impedance matching and minimal signal reflections.
Frequency Response
EMI noise attenuation from a feedthrough capacitor rises as frequency increases up to its self-resonance frequency (SRF), and drops off above SRF. Typical SRF ranges from 100-500 MHz depending on capacitance value and mechanical design. The through-path signal transmission maintains flat performance within the operating band.
Typical Applications
Shielded Enclosure Power Entry: Filtered penetration of power supply lines through shielded cabinet walls, particularly for military/aerospace and high-reliability systems.
Multi-Rail Filtered Connectors: Filtered connector interfaces carrying multiple power rails or signal lines, each filtered through individual feedthrough capacitors sharing the common shield connection.
RF System Power Distribution: Providing clean power to RF systems and preventing internal noise from radiating, particularly where high-frequency filtering is required.
Medical Equipment: EMC-compliant power entry for medical devices requiring high common-mode rejection and reliable filtering.
Installation and PCB Design Considerations
Mounting
The feedthrough capacitor outer shell (ground/shield terminal) must mount flush against the shielding material with solid mechanical and electrical contact. Depending on component type, low-impedance contact can be realized by torque-tightened mechanical mounting, soldering, or crimp termination.
Ground Plane Continuity
The ground plane should remain continuous under and immediately around the feedthrough mounting location. Where the shield terminal needs to interface with the PCB ground plane, place vias close to the enclosure penetration point for a direct, low-impedance connection.
Trace Routing
Input and output traces should be routed away from the shielded enclosure penetration point to minimize coupling of external EMI to internal circuits. Keep trace lengths short to minimize parasitic inductance.
Conclusion
Feedthrough capacitors are three-terminal devices because the three-terminal architecture enables symmetrical filtering on both sides of a shielded enclosure penetration. The third terminal (ground/shield connection) is not passive — it is electrically active and essential to filtering performance. The direct connection to the shield eliminates parasitic lead inductance, providing superior high-frequency filtering compared to conventional two-terminal capacitors. Proper installation with ground plane integration is necessary to realize the performance advantages.
Frequently Asked Questions
Q: Why can’t I just use a regular capacitor on both sides of the shield? You can, but two conventional capacitors are less effective, more costly, and consume more PCB space. Feedthrough’s three-terminal design provides symmetrical filtering with a single component, lower insertion loss, and better high-frequency performance due to eliminated lead inductance.
Q: How important is ground plane continuity? Essential. If the ground plane is broken or disconnected near the feedthrough penetration, the low-impedance return path is lost. The filtering effectiveness can degrade significantly. Continuous ground plane with vias placed close to the shield penetration ensures optimal performance.
Q: Can I use a feedthrough capacitor outside of shielded enclosures? Yes, but the three-terminal advantage is not fully utilized. Feedthrough capacitors still provide excellent high-frequency filtering due to low parasitic inductance. However, in non-shielded applications, conventional capacitors may be more cost-effective if the specific three-terminal advantage is not required.
Q: What voltage and capacitance values are typically available? Feedthrough capacitors are available in a range of capacitance values (typically 0.01 µF to 10 µF) and voltage ratings (typically 50V to 10,000V). Specific availability depends on the manufacturer. Check datasheets for your application requirements.
Q: How do I ensure I get the full benefit of a feedthrough capacitor? Install with solid electrical and mechanical connection of the shield terminal to the shielding material. Ensure ground plane continuity and proper via placement near the penetration. Verify proper routing of input and output traces. Follow the manufacturer’s application notes for optimal performance.
Next Steps
For designing shielded enclosure power entry filtering using feedthrough capacitors, contact LCA’s EMI filtering specialists for selection methodology, PCB layout guidelines, installation procedures, and design examples.
Technical guidance in this article reflects general feedthrough capacitor design principles and EMI filtering best practices. Specific requirements depend on your frequency range, filtering target, shielding design, and system architecture. Always verify component performance and installation procedures against manufacturer specifications before design commitment.
