I'm working on a Raspberry Compute Module 4 carrier PCB with an USB 3.0 controller.

This controller (TUSB7320) needs 3.3V supply and 3.3V analog supply.

Is it necessary to add an analog ground? This would be annoying because the place of the inputs.

Schematic supply

Supply inputs and decoupling capacitors

  • 2
    \$\begingroup\$ Generally, don't split grounds unless you are doing exclusively low-freqency stuff and have no high frequency or digital parts. There is much more to it, but there are many threads on SE about ground planes. One thing I want to add right here is: don't leave floating islands of plane around. \$\endgroup\$
    – tobalt
    Apr 27 at 15:10
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    \$\begingroup\$ When datasheet reference an analog ground, what they usually mean is that you should take care that the analog return current doesn't flow through something noisy, not that you have to electrically disconnect the grounds (which is almost always a bad idea). \$\endgroup\$ Apr 27 at 17:11
  • 1
    \$\begingroup\$ Does the chip have analog ground pins? Also you should get a spice model for that ferrite bead and check it doesn't have a huge ringing spike with your caps. \$\endgroup\$
    – bobflux
    Apr 27 at 20:07
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    \$\begingroup\$ One thing I learned working with ADC chips is that the reason for separate analog and digital grounds is often not to encourage separate PCB ground planes: it's because the internal bond wires between the IC pins and the die have a relatively high resistance so it's best if the analog and digital currents don't go through the same pin. But the pins are meant to be connected together externally. \$\endgroup\$
    – DamienD
    Apr 27 at 20:23

Probably not, the only time you should split planes is if you have currents that would generate voltages large enough to be seen by your ADC.

0.5oz copper is about 1mΩ/in^2 so it would take a large amount of current (50mA) to generate anything that a 16bit ADC could see (to change even 1 bit). (currents run from the source back to ground on the ground layer of the PCB)

Even with a large current, the layout can be change to redirect the current, so don't split the plane.


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