We like small MLCCs for low inductance and low price. Low inductance is desirable for HF decoupling. However,
- Inductance does not depend on cap value, only on package and mounting.
- Value determines price
Let's consider a 4-layer (or more) board with a ground plane in one of the top layers, usually layer 2. Now, the power supply you want to decouple can either be:
- a local supply with a local regulator for one sensitive analog chip with one or a few power pins
Unless it is BGA, the inductance of your chip's VCC pin and the track leading to it will usually be higher than the ESL of a properly mounted MLCC. Therefore, paralleling MLCCs will usually not improve inductance, but it will worsen resonance. Use the highest value MLCC in X7R that will fit in say, 0603 or 0805. Add bulk capacitance if needed to make regulator happy. If regulator datasheet boasts "stable with 1µF MLCC"... check output impedance with network analyzer, if it looks ugly then add some bulk capacitance...
A 10nF 0805 cap will have the same inductance as a 1µF 0805 cap. But the 0805 cap will store 100x more energy. Therefore, I'd use 1µF... and no 10nF in parallel!
Note: a precision opamp will not meet its settling time specs if its power supplies are polluted with large HF spikes due to caps resonating. HF PSRR of opamps isn't good.
- traces feeding many chips
You will need one cap per chip at least, but those traces add inductance, and worsen resonance. Paralleling MLCCs with tracks is a bad idea. This will also inject noise into your GND as the caps resonate. Depending on circumstances, adding ESR can be beneficial. Think about 0R1 chip resistors. Simulate the network. If you use ferrite beads, remember they are inductors, you are making a LC network. Check the bead spice model, and adjust bulk cap ESR for damping.
- (1) a power plane which feeds many chips
A plane will allow you to parallel many caps without (too much) resonance problems, provided they are properly mounted, values properly selected, etc.
Decoupling a plane is where you use those 10nF capacitors, in numbers, to reduce inductance.
Note about fancy low-ESR polymer caps. If you parallel one of those with a MLCC without proper care they'll resonate...