Hi,
the confusion is almost perfect
.
I assume you are reffering to VBUS which is pin 1 in figure 2.5 (page 8) in [1]. This is a common practice which you will see as a recommendation in almost every datasheet. The capacitor has a twofold purpos, which are:
(i) Acts as LPF in combination with your prasitic trace inductance and trace resistance and here of course also with your ferrite bead. For this circuitry including the ferrite bead (FB), where the FB has a high impedance at high frequencies and the capacitor represents a low impedance at high frequencies, this combination effectively attenuates high frequency disturbances.
(ii) The capacitor acts as buffer/energy-reservoir which provides energy/current almost instantaneous when the IC demands it e.g. sourcing a high output current (fast output switching). For such fast current demands the parasitic trace impedance of your trace AND usb cable represents a high impedance. Consequently, the delivered current from e.g. a wall-plug usb charger is "delayed".
This two capacitors also contributes to a low pass filter, with the impedance (L + R) of your USB cable. The 4.7 µF is an electrolytic capacitor which typically has a lower self-resonsnt frequency (fs1), whereas the 100 nF capacitor is a ceramic one with a higher self-resonant (fs2) frequency. The electrolytic capacitor provides a good filter performance for lower frequencies, and the higher frequencies will be attenuated with the ceramic one (fs1 < fs2).
Note, a capacitor provides (as intended) a capacitive behaviour when operated up to ist self-resonant frequency, by means of its impedance. Starting from low frequencies to higher frequencies the impedance decreases. At frequencies above the self-resonant frequency the capacitor acts like an inductor, where its impedance increases with increasing frequency.
And also this two capacitors act as a buffer, as the USB cable might be long and thus resulting in a high inductance.
Further, the FB and the capacitors on both sides are creating a \[\prod]-filter, which also attenuates noise/ripple which is leaving the VBUS output. This helps to avoid the distribution of noise created within a certain IC across the supply line (VDD i.e. VBUS) and the disturbance of other ICs on the same supply line.
BR
[1]
https://www.ftdichip.com/Support/Do...B_Hardware_Design_Guidelines_for_FTDI_ICs.pdf