Supply Bypassing and Decoupling, Are They Really Necessary?

We see these bypass or decoupling capacitors on the supply pins, and maybe we wonder what if we just take them out so we can save a few components on the circuits. Or perhaps out of plain ignorance we forgot about it, and I have seen people do that and be faced with seemingly impossible problem to contend with, like outputs oscillating, or behaving weird even in the absence of inputs. What could be wrong? Only to find out at the end of the day that they missed something critically important in the circuit.

So what do they do really? They provide a low impedance path to the ground for noise and any ac components that might be present on the power supply lines. For example, a sudden current change in an IC can cause a large glitch on the supply. The capacitor will help prevent that by providing immediate current requirements. In this manner it helps maintain the supply node to be low impedance. Being low impedance means it can source a varying amount of load current and being able to maintain a constant voltage. This is ideally what we wanted our power supply to be.

Bypass Capacitor

So what’s in the characteristics of capacitors that can make them do the job? From a practical point of view, they store electrical charge. The charge that they hold can help with the transient current requirements of the components, which can otherwise ‘overburden’ the power supply. Also, having an impedance that decreases in frequency, they shunt noise components to ground instead of passing them to the component and messing up with its operation. These are the reasons why the bypass capacitors should be placed as close as possible to the supply pin of the component. It is preferable that the other terminal connects directly to a ground plane. If a ground plane is not present, it should connect directly to the ground pin of the component.

Recommended Readings and References:

Bypass Capacitors, No Black Magic Here, by Bonnie Baker
Decoupling Techniques by Analog Devices

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