ADS1147: why ferrite bead or inductor are not recommend for power rail of ADC?

Hi Sunny,

Ferrite beads and inductors are not recommended on the supplies because they can "choke" any large instantaneous current required by the ADC. The ADC's average current is shown in the datasheet, but there are certain times where the ADC needs significantly more current for a very short duration. For example, the ADC might need to draw 10's of mA for several microseconds when the IDACs turn on, when the reference turns, during digital communication, etc. This lasts for a very short duration (again, maybe microseconds depending on the actual action that draws the current), but since an inductor cannot respond quickly to this current demand, the supply voltage starts to droop. This can cause the ADC to not function properly, enter brownout, initiate a power-on reset (POR), etc.

Instead, it is recommended to use clean supplies via LDOs. If necessary, a very small value resistor can be added in series to create an RC filter with the decoupling capacitor, though again there will be a voltage drop across this resistor (hence the suggestion to keep the value very small)

-Bryan