Proper Decoupling Capacitors

If you’ve been building circuits for any length of time, you probably know you need decoupling capacitors to keep your circuits stable. But even though it’s a favorite technique of ours, just scattering some around your PCB and hoping for the best isn’t necessarily the best approach. If you want to dig deeper into the why and how of decoupling, check out [Stephen Fleeman’s] post on the topic.

It is easy to think of capacitors as open circuits at DC and short circuits at high frequencies, shunting noise to ground. But the truth is more complex than that. Stray resistance and inductance mean that your simple decoupling capacitor will have a resonant frequency. This limits the high frequency protection so you often see multiple values used in parallel to respond to different frequencies.

Because the stray resistance and inductance plays a part, you may want to use fatter traces — less resistance — and shorter runs for less inductance. Of course, you can also use power and ground planes on the PCB as a form of decoupling. At the end of the post, [Stephen] talks a little about the importance of digital and analog ground that interact in a specific way.

If you want to do some empirical testing, you can build a test rig and do the work. Or check with [Bil Herd] about PCB inductance.

A bench setup with a spectrum analyzer and a PCB under test

Clever Test Rig Clarifies Capacitor Rules-of-Thumb

If you’ve done any amount of electronic design work, you’ll be familiar with the need for decoupling capacitors. Sometimes a chip’s datasheet will tell you exactly what kind of caps to place where, but quite often you’ll have to rely on experience and rules of thumb. For example, you might have heard that you should put 100 µF across the power supply pins and 100 nF close to each chip. But how close is “close”? And can that bigger cap really sit anywhere? [James Wilson] has been doing research to get some firm answers to those questions, and wrote down his findings in a fascinating blog post.

A PCB used to measure the effect of capacitor placement
The test board has two-layer and four-layer sections. The inter-layer capacitance greatly affects the PDN’s performance in each case.

[James] designed a set of circuit boards that enabled him to place different types of capacitors at various distances along a set of PCB traces. By measuring the impedance of such a power distribution network (PDN) across frequency, he could then calculate its performance under different circumstances.

The ideal tool for those measurements would have been a vector network analyzer (VNA), but because [James] didn’t have such an instrument, he made a slightly simpler setup using a spectrum analyzer with a tracking generator. This can only measure the impedance’s magnitude, without any phase information, but that should be good enough for basic PDN characterization.

The results of [James]’s tests are pretty interesting, if not too surprising. For example, those 100 nF capacitors really ought to be placed within 10 mm of your chip if it’s operating at 100 MHz, but you can get away with even 10 cm if no signals go much above 1 MHz. A bulk 100 µF cap can be placed at 10 cm without much penalty in either case. Combining several capacitors of increasing size to get a low impedance across frequency is a good idea in principle, but you need to design the network carefully to avoid resonances between the various components. This is where a not-too-low equivalent series resistance (ESR) is actually a good thing, because it helps to dampen those resonances.

Overall, [James]’s blog post is a good primer on the topic, and gives a bit of much-needed context to those rules of thumb. If you want to dive deeper into the details of PDN design or the inductance of PCB traces, our own [Bil Herd] has made some excellent videos on those topics.

Parts: 0.1uF Decoupling Capacitors

Most ICs need to be decoupled from their power supply, usually with a 0.1uF capacitor between each power pin and ground. Decoupling is usually used to remove noise and to smooth power fluctuations. Every project will need a few decoupling capacitors; our mini web server project has three ICs that require a total of 11. This can be an expensive part to buy in singles, so it’s crucial to stock up online. Read more about our favorite bulk through-hole and surface mount decoupling capacitors after the break. Continue reading “Parts: 0.1uF Decoupling Capacitors”