what is common (point)?
When a circuit is connected to other circuits the various voltages around the system are all measured relative to one chosen single stable point, called the common (point). This common point can also be connected to the universal common, earth ground, via a water pipe for example.
When signal commons are connected (no conductor is perfect;they all have resistance and inductance), two physically separate commons or earth grounds are sometimes not at the same voltage. Why?
these differences in potential between the common points that cause troublesome "ground loops" which will cause errors.
At high frequencies, say >1Mhz, it is said that single point systems suffer limitations because long conductors have large inductive reactance to common...What does that it really mean that a conductor has a large inductive reactance to common and why?
Digital circuits at high frequencies use ground planes. But low frequencies should not use multipoint commons...Why?
also, what is a floating point?
G O O G L E
W I K I P E D I A
These sources have well written, long, detailed descriptions of these exact topics.
Give them a try, so one of us doesn't have to 're-invent the wheel' and restate something that has already been very adequately explained for a laypersons understanding.
If then, you find you are still confused on some SPECIFIC aspect of these topics, then, by all means, come back and ask us about that specific detail.
I'm too old.
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