Analog and Digital Supply

[zhenning]

For the circuit that needs analog and digital supply with the same voltage, do we separate them and use 2 different dc-dc converter for each analog and digital domain as isolation? As I understand the fast switching of the transistors in the digital domain will induce additional unwanted noises in the analog domain. From a TUSB1310 design guide, it also suggest to add Pi filters for each supply.

1.1-V, 1.8-V and 3.3-V Analog Supplies
Since circuit noise on the analog power terminals must be minimized, a Pi filter is recommended for each
supply.
Analog power terminals should have a 0.1-μF bypass capacitor connected to VSSA (ground) in order for
proper operation. Place the capacitor as close as possible to the terminal on the device and keep trace
length to a minimum. Smaller value capacitors like 0.01-μF are also recommended on the analog supply
terminals.

[robertferanec]

Hello zhenning, we often use filters based on Ferrite BEADS + capacitors. This works oki for standard CPU boards (like CPU + Audio) to separate digital from analog. However, if I would need a really clean analog power, I would use an LDO to create it.

Have a look at some of our schematics, e.g. OpenRex Schematic:

[zhenning]

I see. Thanks Robert! How about for the GND? I came across some recommendation to split the ground too by connecting the VSSA as a trace to the terminal directly. Some other suggestion is to connect the digital and analog GND through a point contact.

[robertferanec]

In our boards (CPU boards) I found out, that the best way is to use one solid GND for everything. However, I do place analog circuits into a "quiet" corner of the PCB and I do not mix digital / analog tracks & components together. Of course, this is not a general rule and for analog sensitive circuits you may need to take a different approach - the best is to try to follow chip manufacturer recommendations (they may recommend to use separate analog and digital ground).