In SigmaDSP datasheets, the pin description table often has a statement like the following:
This pin should be decoupled to DGND with a 100nF capacitor.
However, if you look at the application circuit, you will see 10uF and 0.1uF for this pin. What is the reason for this?
There are two (or more) capacitors connected to each supply. One (or more) is a "High Frequency Decoupling" capacitor (100 nF = 0.1 uF), and the other is a "Current Reservoir" capacitor (10 uF).
The datasheet refers to the "High Frequency Decoupling" capacitor and it typically uses nanofarads as a unit. This capacitor is used to act as a return path for high frequency current spikes due to dynamic switching inside the IC. Since this capacitor is specific to the pin, it usually appears in the pin description on the datasheet. The capacitor should be located as close as possible to the IC, with a direct connection (no vias) to the supply and ground pins. Connections to the supply and ground should be made by vias on the far side of the capacitor.
The 10 uF "Current Reservoir" capacitor is more of an application-level component. It is used as a "buffer" in the supply to make sure there are enough electrons available when the supply is under demand from the IC. This ensures that the supply line does not sag. Only one of these capacitors is required per supply (AVDD, PVDD, DVDD, etc.), unlike the decoupling capacitors, which require one per pin. So, since the current reservoir capacitor is not pin-specific, it is typically not listed in the pin description. The location of this capacitor is not so important; it can be located further away from the IC, maybe close to the board's regulator for that supply.
So, even though only the decoupling capacitor is referenced in the pin description, both capacitors are necessary! In the event that there are two or more pins for a given supply, you would need two (or more) 100 nF capacitors (one per pin) and still only one 10 uF capacitor for the supply itself.
Here is an example where the decoupling cap and the current reservoir cap are both located next to the IC:
Here is another example showing only the decoupling caps (the current reservoirs are located elsewhere on the PCB):
This FAQ was generated from the following discussion: Power supplies, capacitors, and application schematics