Three-Terminal Filter Capacitors

Key Takeaways:

• Three-terminal filter capacitors have a significantly lower equivalent series inductance (ESL) than two-terminal capacitors, improving high-frequency noise suppression.
• While more expensive, three-terminal capacitors deliver enhanced performance in specific broadband filtering applications, especially above 20 MHz.
• Using multiple two-terminal capacitors in parallel can partially mimic the effects of one three-terminal capacitor but often cannot fully replace their performance benefits.

When I first heard about three-terminal filter capacitors, I was curious about what practical advantages they have over the more typical two-terminal capacitors everyone uses. Manufacturers often claim these capacitors offer extremely low equivalent series inductance (ESL) because they provide two grounding points instead of one. But how does that translate into real-world circuit performance? I decided to dive deeper—and even took inspiration from an excellent side-by-side comparison by Mach One Design EMC that uses Vector Network Analyzer (VNA) testing to directly compare these capacitors.

Here’s the gist: the ESL in a capacitor limits its effectiveness at high frequencies. The lower the ESL, the better a capacitor can suppress high-frequency noise. The three-terminal capacitor is designed to reduce the ESL by splitting the grounding paths, effectively lowering the inductance involved. This difference is especially noticeable in frequencies above 20 MHz, making these capacitors very effective for broadband noise filtering [YouTube video](https://www.youtube.com/watch?v=H1Voe1HxdcQ).

What Exactly Is a Three-Terminal Capacitor?

Unlike two-terminal capacitors, which have just an input and output terminal, a three-terminal filter capacitor adds an extra grounded terminal. Internally, it has two power terminals and one return (ground) terminal. This unique construction allows the displacement current inside the capacitor to split its return path, effectively lowering the loop inductance by creating parallel current paths inside the device.

This internal layered structure with dual grounds helps significantly reduce parasitic inductance, a common culprit for poor high-frequency performance in two-terminal capacitors.

Testing the Capacitors: Real-World Comparison

Mach One Design EMC’s practical test compared a three-terminal capacitor from Murata’s NFM series (10 nF, 1206 size) with a traditional two-terminal MLCC capacitor of a similar value and rating. Using a specialized test board connected to a VNA, the performance of both capacitors was analyzed across a broad frequency range (up to 50 MHz and beyond).

Key Observations:

 

• Both capacitors showed nearly the same capacitance at lower frequencies.
• Starting around 20 MHz, the two-terminal capacitor’s performance degraded due to its higher ESL.
• The three-terminal capacitor maintained a low impedance, suppressing noise up to 100 MHz with about a 10 dB better reduction than the two-terminal capacitor.
• This validates the datasheet claims regarding the lower ESL of three-terminal capacitors and their superior filtering capabilities in high-frequency regions.

Can Two-Terminal Capacitors Compete?

Since three-terminal capacitors cost more and require special PCB footprints, the obvious question is: Can multiple two-terminal capacitors in parallel replace a single three-terminal capacitor?

Mach One Design EMC tested this by connecting two 10 nF two-terminal capacitors in parallel. This almost doubled the capacitance and lowered the effective ESL, improving performance significantly. However, while parallel capacitors closed much of the gap, they still couldn’t fully match the three-terminal capacitor’s performance in the 40-100 MHz range. Resonances and complex parasitic effects sometimes appeared with multiple capacitors in parallel.

In short:

• Using multiple two-terminal capacitors helps but doesn’t completely replicate the distinct advantage of the three-terminal design.
• For critical high-frequency applications where board space and noise reduction matters, three-terminal capacitors are often the better choice.

Some engineers prefer cleverly using a mix of capacitors for a balance of cost, size, and performance. But if you want one capacitor with the best broadband suppression at frequencies above 20 MHz, the three-terminal filter capacitor stands out.

Practical Applications and Benefits

The unique design means three-terminal capacitors are often used in:

• Power line filters for DC/DC converters, where switching noise is high-frequency and broad-spectrum.
• Radiated emission reduction in sensitive equipment, such as automotive or communication devices.
• Input/output line filtering on PCBs, helping reduce both conducted and radiated noise with fewer components than traditional multi-element pi-filters.

In fact, Murata’s application notes highlight how one three-terminal capacitor can achieve noise reduction similar to three two-terminal capacitors in a pi-filter arrangement, saving space and simplifying design without compromising performance.

Things to Watch Out For

• Cost: Three-terminal capacitors are generally more expensive and require a PCB footprint designed for three terminals.
• Size availability: May be limited compared to the widespread sizes of traditional two-terminal capacitors.
• Design complexity: Deciding when and where to use these requires understanding your noise environment and frequency requirements.

Still, the trade-offs often lean in favor of using three-terminal capacitors when dealing with modern high-frequency switching circuits. Their unique grounding structure delivers noise suppression that’s difficult to achieve otherwise.

Final Thoughts

My exploration into three-terminal filter capacitors shows they genuinely live up to their promise of lower parasitic inductance and better high-frequency noise suppression. While not always necessary, they’re an excellent tool in the toolkit for anyone dealing with EMI and high-speed electronics. If you’re struggling with noise issues at higher frequencies and can afford the extra cost and footprint, giving three-terminal capacitors a try can yield meaningful improvements.

For further learning, you might check out Murata's detailed application notes and explore other capacitor technology comparisons on trusted electronics forums and datasheet archives.

FAQ

Q: What is the main advantage of a three-terminal filter capacitor?
A: Its internal grounding structure reduces equivalent series inductance (ESL), improving high-frequency noise suppression compared to two-terminal capacitors.

Q: Can I replace a three-terminal capacitor with multiple two-terminal capacitors?
A: Using multiple two-terminal capacitors in parallel improves performance but often doesn't fully match the broadband suppression of a single three-terminal capacitor, especially above 40 MHz.

Q: Are three-terminal capacitors more expensive?
A: Yes, they tend to cost more and require specialized PCB footprints, but their improved performance can justify this expense in high-performance designs.

Q: Where are three-terminal capacitors commonly used?
A: They're primarily used in high-frequency noise filtering applications such as DC/DC converter power lines, automotive electronics, and EMI-sensitive circuits.