What are ferrite beads?
Inductors.
What they are used for?
to remove high frequency "noise" (EMI) from low frequency signals, e.g. DC signals (block RF travelling on such signals in or out).
Is there a Voltage Rating?
I do not think so. Why should the voltage above a ferrite bead become 600 to 700 Volts, if the system runs with 3V3?
What are the most important values of a ferrite bead?
a) the maximum current. Yes, you are right to design with 20% DC current through the bead, e.g. rated for 100mA and drawing just 20mA through the bead. If you saturate the inductor - it will not filter RF anymore.
b) the filter characteristic (the attenuation of RF and at which frequency)
Most important parameters for a ferrite bead are: where is the maximum attenuation - at which frequency, to block high frequency "noise"?
You can optimize for largest "attenuation" (filtering) and at lowest (but desired) frequency.
c) the Ohms Resistance for DC, e.g. in X milli-Ohm: the value the inductor would act like a resistor for DC, creating a voltage drop. The lower this DC resistance, the lower the voltage drop. If your signal is a DC signal, e.g. VDDA, you might want to go with a low DC resistance value as the most important value.
The frequency characteristic of a bead to select depends more on the fact, which high frequency components, e.g. a radio, a WiFi/BT chip, switching regulators, ... you have on the board. They could generate EMI, "cross-talk" into DC signals, so that you want to remove the RF coupling (EMI).
It means also: place the ferrite bead (often accompanied by a cap, like a RC/LC low pass filter), close to the VDDA pin (with a very short trace from bead to VDDA pin). Otherwise, if the bead is very far away - you lose the capability to remove RF noise (still EMI after bead).
BTW:
If you use a ferrite bead not in the DC power supply connections for the entire chip, just powering the VDDA part, the current might be pretty small. You could go with a smaller bead (for current rating). The voltage should not matter (because you do not have high voltages or a switched power regulators after it, which could generate such a (backwards) high voltage).
Select based on:
- current rating - good to go with 20% of max. current rating
- frequency characteristics (attenuation and frequency)
- DC resistance
