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 you 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.
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