The article argues against blindly using 100nF decoupling capacitors, advocating for a more nuanced approach based on the specific circuit's needs. It explains that decoupling capacitors counteract the inductance of power supply traces, providing a local reservoir of charge for instantaneous current demands. The optimal capacitance value depends on the frequency and magnitude of these demands. While 100nF might be adequate for lower-frequency circuits, higher-speed designs often require a combination of capacitor values targeting different frequency ranges. The article emphasizes using a variety of capacitor sizes, including smaller, high-frequency capacitors placed close to the power pins of integrated circuits to effectively suppress high-frequency noise and ensure stable operation. Ultimately, effective decoupling requires understanding the circuit's characteristics and choosing capacitor values accordingly, rather than relying on a "one-size-fits-all" solution.
The blog post "Proper Decoupling Capacitor Practices, and Why You Should Leave 100nF Behind" argues that while the traditional practice of using 100nF (0.1µF) capacitors for decoupling integrated circuits (ICs) has been a long-standing rule of thumb, it's no longer the optimal solution in many modern digital circuit designs. The author contends that a deeper understanding of the underlying principles of decoupling and the specific needs of contemporary ICs calls for a more nuanced approach.
The core function of a decoupling capacitor is to provide a localized source of current for the IC, mitigating voltage fluctuations caused by rapid changes in current demand during switching operations. These fluctuations, often referred to as noise or ripple, can disrupt the proper functioning of the IC and other components in the circuit. Traditionally, 100nF capacitors were considered adequate for this task. However, with the advent of faster switching speeds and higher clock frequencies in modern digital circuits, the frequency content of this noise has shifted upwards. The author explains that the impedance of a capacitor decreases with increasing frequency. Therefore, a capacitor that presents a low impedance at lower frequencies (like a 100nF capacitor) may exhibit a significantly higher impedance at the higher frequencies characteristic of today's high-speed digital systems. This higher impedance renders the capacitor less effective at filtering out high-frequency noise.
The article emphasizes that the optimal decoupling strategy involves using a combination of capacitors with different values, effectively creating a "ladder" of capacitance. This approach leverages the unique impedance characteristics of different capacitor types across a wider frequency range. Specifically, the author advocates for including smaller value capacitors, such as 10pF or 100pF, placed physically closer to the IC's power pins. These smaller capacitors, while having lower overall capacitance, exhibit lower impedance at higher frequencies, effectively suppressing the high-frequency noise that the larger 100nF capacitor might miss. Larger capacitors, like the traditional 100nF and even larger values in the microfarad range, remain important for handling lower frequency noise and providing bulk capacitance for transient current demands.
The author further delves into the practical considerations of capacitor selection, such as Equivalent Series Resistance (ESR) and Equivalent Series Inductance (ESL). These parasitic elements can significantly impact the capacitor's performance at higher frequencies. Lower ESR and ESL values are generally desirable for effective decoupling. The physical placement of the capacitors is also highlighted as crucial, emphasizing the importance of minimizing the inductance of the connection between the capacitor and the IC's power pins. Short and wide traces are recommended to minimize this inductance.
In conclusion, the post advocates for a shift away from the blind application of the 100nF decoupling capacitor rule. Instead, it encourages a more deliberate and informed approach based on understanding the frequency characteristics of the noise being suppressed and the impedance properties of different capacitor values. The author champions a multi-capacitor decoupling strategy utilizing a range of capacitor values, including smaller capacitors placed very close to the IC's power pins, to effectively address the broader spectrum of noise present in modern high-speed digital circuits. This, they argue, ensures more robust and reliable circuit operation.
Summary of Comments ( 68 )
https://news.ycombinator.com/item?id=42830948
Hacker News users discussing the article about decoupling capacitors generally agree with the author's premise that blindly using 100nF capacitors is insufficient. Several commenters share their own experiences and best practices, emphasizing the importance of considering the specific frequency range of noise and choosing capacitors accordingly. Some suggest using a combination of capacitor values to target different frequency bands, while others recommend simulating the circuit to determine the optimal values. There's also discussion around the importance of capacitor placement and the use of ferrite beads for additional filtering. Several users highlight the practical limitations of ideal circuit design and the need to balance performance with cost and complexity. Finally, some commenters point out the article's minor inaccuracies or offer alternative explanations for certain phenomena.
The Hacker News thread linked has several comments discussing the original blog post about decoupling capacitors. Many of the comments delve into the nuances and practical considerations not fully explored in the blog post itself.
A significant point of discussion revolves around the practicality of the author's advice. One commenter points out that while striving for ideal decoupling is theoretically sound, real-world constraints often necessitate compromises. They mention that factors like board space, component availability, and cost can influence decisions, sometimes making the "perfect" solution impractical. This leads to a discussion about the "good enough" approach, where engineers aim for adequate decoupling rather than striving for theoretical perfection.
Another commenter raises the issue of the author's focus on digital circuits. They argue that the recommendations might not be universally applicable, particularly for analog circuits where noise considerations are significantly different. They suggest that applying the author's advice indiscriminately could lead to suboptimal results in analog designs.
Several commenters discuss the importance of considering the entire signal path. They emphasize that decoupling capacitors are only one part of the puzzle and that proper layout, grounding, and component selection all play crucial roles in minimizing noise and ensuring signal integrity. One comment specifically highlights the importance of minimizing the inductance between the decoupling capacitor and the component it's serving, emphasizing the importance of placing the capacitor as close as possible.
There's a thread discussing the use of ferrite beads in conjunction with decoupling capacitors. Commenters debate the effectiveness of ferrite beads, with some arguing for their use in specific situations while others express skepticism about their benefits. This leads to a discussion about the importance of understanding the impedance characteristics of different components and how they interact within a circuit.
Finally, some commenters offer additional resources and further reading on the topic of decoupling capacitors, providing links to application notes, articles, and textbooks. This suggests a general interest in the topic and a willingness to explore it in more depth. The shared resources indicate that the discussion sparked a desire for continued learning and a deeper understanding of the complexities of decoupling in electronic circuits.