Low-inductance decoupling capacitors from X2Y

While at PCB West I can across a nice paper (pdf) from high-performance capacitor maker X2Y. They have some capacitors that might replace two or three capacitors in a decade arrangement, saving you space and cost. The key point was on page 14 that showed how a 1 uF X2Y capacitor can have better performance than 10 uF electrolytic or tantalum, and almost twice a good as a regular 1 uF ceramic capacitor. The whole paper is good and should convince you that no component on your board, as well as the board itself, is trivial in modern high-speed designs.

Decoupling high-speed circuits is still a bit of an art. When testing high-speed comparators or those silly digital circuits, it can help to have a decade pair of capacitances as close to the power and ground pins as possible. It is not just the decade in capacitance value you want— it is two different sizes, like and 0805 and 0402. This guaranties that the self-inductance of the capacitors will be different and the pair of capacitors can source current over a broader range of frequencies. Just making the decoupling capacitor bigger can backfire— a bigger capacitor may well have bigger series inductance and that is what prevents them from delivering fast-moving currents to chip power pins. You need to look at the impedance curves over frequency and decide what capacitor value and, just as important, what capacitor physical size is best, and some times use combinations as I just mentioned. Be sure to explain to purchasing that you really need a 0.1 uF 0603 NOT a 0.1 uF 0805, even if manufacturing has figured out how to solder it to the smaller footprint. The decoupling caps are a critical part of the performance of high-speed systems and you need to audition and qualify them just as you do an IC.