Decoupling placement: close and short
A decoupling capacitor supplies the current a chip pulls in an instant. Position and vias decide the effect more than the value does.
When an MCU pulls hundreds of mA for an instant on every clock, the distant regulator cannot supply it straight away because of trace inductance. The capacitor right next to the chip covers that instant.
Placement principles
- 100 nF for every power pin, within 2 mm of the pin
- A short, thick trace from capacitor to pin
- From the capacitor's ground pad, straight through a via to the ground plane
- Current passes in the order supply → capacitor → chip pin
Combination of values
| Value | Role |
|---|---|
| 100 nF (each pin) | Noise from a few to tens of MHz |
| 1 to 10 µF (per chip) | The hundreds-of-kHz band, load changes |
| Tens to hundreds of µF (board input) | Low-frequency bulk |
For small capacitances a small package (0402) has less parasitic inductance and is better at high frequency.
Common mistakes
- The capacitor is close but its ground via is far away → the loop grows and the effect is halved
- Several pins share one capacitor
- Decoupling or a filter bead left off VDDA (analog supply) → ADC noise