What does filtered analog power mean in datasheet?

Question

I am making a USB hub and found a controller chip on Digikey. In the datasheet, it says some of the pins need 3.3V filtered analog power. I then searched the datasheet and found nothing else giving more information. Can you guys give me some guidance on this?

Answer 1

Some chips are purely digital in function, some are purely analog. Chips like MCUs and USB hub chips can contain a lot of sensitive analogue circuitry like PLLs for their internal operation, in addition to the digital circuitry.

This means that on some chips the digital and analog parts can have separate power supply pins so that the pins can have separate external bypass capacitors to prevent large digital current spikes from affecting the sensitive analog parts. Sometimes they can use the same supply voltage directly, but it also allows to add ferrite beads or resistors for extra filtering for the analog supplies. In extreme cases a separate low noise linear voltage regulator can be used for the analog parts, even if digital parts are powered from a noisy switch mode regulator.

Answer 2

This sounds like a bad description of what the designers probably intended. Usually when datasheets state something like this it means the device needs a power bypass capacitor. A power bypass capacitor gives local energy storage when the load changes (in this case the USB controller). For most digital devices (100mA to 10mA) a 0.1uf capacitor is sufficient, or for higher power parallel a 10uf with the 0.1uf cap.

Answer 3

"All digital signals are at least 50% analog." (TS)

If it says "analog", I would expect a full datasheet or App Note to define the level of ripple.

For example from Diodes Inc. the Analog 3.3V is used for PCIE where noise can increase the chance of errors.

    Power supply ripple noise(Analog 3.3V)
        f     min nom max
    < 1.2MHz  -10  0  10mV
    > 1.2MHz  -50  0  50mV  (similar to logic supply)

How you generate this low ripple depends on you and your design.

1. Your supply loaded Vpp ripple into 50 Ohms AC . This is the proper way to measure supply ripple, terminated at scope.
2. Your choice of Cs, ESR and close proximity to designated supply and Vss pins. These are the most common solutions if the supply is well under 10mV AC coupled to 50 Ohm dummy load at scope. Step Loads will make this worse.
3. The step load variation % and stability of supply.
4. The impedance between supply and caps. A tiny ferrite bead has a similar effect to a 100pF shunt. This depends on cable impedance, and length, shared current step-load impedance affects load regulation error, inline filters, ground-shift noise, transformer feed thru noise pulses and other effects.

Typically 10 uF, 1uF 0.1uF low ESR are selected.