====== Decoupling Capacitors: The Most Important Noise-Fighting Component ======

If you've designed PCBs before, you've probably been told to place a 0.01 µF to 0.1 µF capacitor near every IC power pin.

Most engineers follow this rule, but many never stop to ask why.

The answer is simple: noise.

Whenever a chip operates, its current consumption changes rapidly. Even a short PCB trace has some resistance and inductance, which means the supply voltage at the chip can momentarily dip or fluctuate as the chip switches on and off internally.

{{ :noise_book:decap2.png?nolink |}}

Without proper decoupling, these small voltage fluctuations can become a source of noise and may affect the operation of the chip itself or nearby circuits.

A decoupling capacitor acts like a small local energy reservoir. When the chip suddenly needs more current, the capacitor can supply it immediately. When demand decreases, the capacitor recharges from the power supply.

A useful way to think about this is to imagine a dam.

When there is too much water, the dam stores it. When there is not enough water, the dam releases it. A decoupling capacitor does the same thing with electrical energy, helping keep the power supply stable even when demand changes rapidly.

So where should the capacitor be placed?

The answer is simple: as close to the IC power pins as possible.

The connection between the capacitor and the chip should be short and direct. Long traces reduce the effectiveness of the capacitor and allow more noise to develop.

{{ :noise_book:decap.png?nolink |}}

If there is only one PCB noise-reduction technique you remember from this book, make it this one: place a decoupling capacitor next to every IC power pin.

[[index|←Noise Is My Friend]]