Ground Questions: Power Supply & PCBs

[Corneo]

I got a question regarding ground in terms of a power supply and another one with regards to PCBs.

Usually you see people put a jumper cable between the negative terminal of a power supply to common ground. Why do people do this? Is it for the user to reference the power supply to actual Earth ground? Isn't the chassis tied to Earth ground using the third prong?

Whats the difference between digital ground and analog ground in terms of a PCB? How are things actually wired in terms of a layout file?

 

[berkeman]

The outputs of many lab power supplies are floating with respect to Earth ground, even if the power supply box uses a 3-prong power cord. This is handy if you want to stack power supplies in series, for example, to make a higher overall voltage.

Separate analog and digital grounds are used generally to keep digital noise out of the analog circuitry. You want to keep the grounds separate, and put the power supply in the middle, to avoid sharing any ground impedance between the distribution of power to the digital and analog sections.

See my discussion of star grounding in this thread, for example, and in the paper that I linked to in my post:

https://www.physicsforums.com/showthread.php?t=213955