Why star ground????

[Muzzer]

CANbus lines only wobble up and down by a volt - so although higher signal levels can appear to make the job easier, they aren't necessary. And of course, they can become the cause of noise problems themselves.

In SMPS and VFD design (my background), you may need to be switching hundreds of amps and hundreds of volts simultaneously, with transition times in the 10s of nanoseconds and switching frequencies of 100kHz or more. In the middle of all this you may need to implement accurate analogue voltage conditioning and measurement circuits and operate microcontrollers, whilst also meeting stringent EMC standards. Some of the noise problems we come across in cabinet wiring are relatively simple in comparison but can still make life pretty difficult.

As pointed out, there is often no such thing as a hard ground, due to real world inductances. Even a ground plane within a circuit board has inductance - I've seen many examples of noise problems where the designer has assumed that a large ground plane / copper fill under all the circuits will provide some form of magical ground. However, induced voltages are easy to generate between points in a ground plane and can cause unpredictable behaviour, particularly in ICs if you take the internal substrate even a few hundred mV above ground. The solution is to bring the 0V connections for the various analogue, digital, power etc nodes together at a star point - they are often essential at circuit board level, too.