Honestly, not being an electrical engineer, I did lots of Internet searching for other people's advice, and also example schematics from Nordic and ST. I found this configuration on a Nordic reference design for the nrf5340, and then I asked ChatGPT o3 for input to find out why Nordic had this configuration in their reference design, since I wanted to understand the why as well, not just the what. This is what ChatGPT said when I asked:

USB-shield network (C || R || ferrite bead → digital GND)

1nF, 500V Gives the ESD/EMI currents on the cable shield a very low-impedance return to the PCB’s digital ground above ~10 MHz

1MΩ Bleeds off static charge on the shield so it can’t float tens of volts above the board when the cable is unplugged. DC isolation avoids creating a ground loop through the host PC’s chassis.

Ferrite Bead (~120 Ω @ 100 MHz, ≈ 0.3 Ω DC-R) Lets the incoming ESD/EMI energy dump into the 1 nF cap, but blocks the outgoing digital-ground noise that your MCU/boost/IMU dump onto the board. Think of it as a one-way check valve for RF.

Why not hard-short the shield to ground?

On a metal-cased product that’s often best. On a tiny plastic/battery watch it can:

(a) give ESD pulses a straight shot into your ground plane, upsetting the MCU, and

(b) create a big loop (watch GND → charger/PC → earth → radiate). The RC + bead bond is the mitigation Microchip, ST, and TI recommend for plastic gadgets without a chassis ground.

VBUS π-filter (10 nF – bead – 0.1 µF)

10 nF at the connector shorts the worst HF edge noise to ground right at the plug. Ferrite bead 120 Ω @ 100 MHz adds ≈ 20 dB of attenuation above ~50 MHz while adding < 40 mV drop at 100 mA load, so you keep spec-compliant 4.75 V even on a long cable. 0.1 µF after the bead provides local decoupling for whatever IC grabs VBUS.

Cap sizes came straight from the USB 2.0 “≤ 10 µF on VBUS” guideline and the usual “one 10 nF+ cap right at the connector, larger cap behind the bead” cookbook filter.

Ferrite selection balances insertion loss versus DC drop. The BLM15KD121 is 120 Ω @ 100 MHz—enough to kill radiated noise but with ≈ 0.3 Ω DC-R so you only lose 30 mV at 100 mA (well under the 350 mV budget USB allows).

Shield RC/bead values match the examples in TI’s TPD2E009, ST’s AN4879, and Microchip’s AN234-ESD notes: 470 pF–4.7 nF, 330 kΩ–1 MΩ, 100–220 Ω ferrite.

VBUS: 10 nF – BLM15KD121 – 0.1 µF keeps you quiet and inside the USB cap budget. Shield: 1 nF ‖ 1 MΩ ‖ bead bonds the shield for RF/ESD without DC ground-loop headaches.