# Ground, common, and Earth ground confusion

I am learning on transmitters and input modules nowadays. But the usage of word 'ground' is confusing me. What does 'ground' mean there (earth or power supply common)?

If it is earth ground, why we need to do that? Just wiring to power supply common won't be enough? (Isn't it a real reference point of signal?) You can say it is to prevent having a floating signal, but what happens if power supply common is directly earth grounded? Also isn't it the nature of differential signal measuring to have a floating signal? If the input signal is differential and we make it earth grounded, won't it be like a single-ended signal?

If it should be earth grounded what happens if we don't wire it?

I also attached one example. What happens if I don't wire '-' of input signal to earth there?

## 2 Answers

Earth grounding is primarily used for safety and EMI reasons. There is no requirement for earth ground for a circuit to function properly. For example, cell phones are not earth grounded. Ground is a reference conductor that is used as a return path for signals. It is often thought of as a perfect conductor with zero resistance, but the reality is it has resistance and inductance that can cause the voltage to vary at different locations. Differential transmitters and receivers themselves are not usually floating, they are biased at some DC voltage relative to ground. The two ends of a differential channel can be at different ground potentials if the signal remains within the common mode range of the receiver, or DC isolation is provided, for example with capacitors or a transformer.

• Thanks for your answer. "Differential transmitters and receivers themselves are not usually floating, they are biased at some DC voltage relative to ground" Ground you mentioned above is power supply common(not earth) ? Commented Feb 12, 2019 at 20:41
• It is whatever ground the differential transmitter or receiver is connected to. Commented Feb 12, 2019 at 23:58

It's the most common mistake, almost an universal one in almost every schematic, to draw the reversed pine tree for GND or COM while it should be reserved for EARTH. Normaly for GND/COM they should draw a solid triangle, preferably with the mention "0V". In this context of electronic communication this symbol means GND.

Things get funny when you add TVS diodes or GDT's which do have to connect to the real earth. Then people should somehow think that something was wrong with their symbol... You will also see some fork shaped ground symbol, which means chassi ground. Neither earth nor GND technicaly but often refering to GND too.