Floating power supply for Arduino - Is it still connected to the USB ground?

Other Hardware General Electronics
1

I am trying to power my Arduino Mega 2560 from a benchtop power supply. I plan to essentially feed in a 9V supply directly on the Vin pin and connect the GND pin to the floating ground pin on the power supply. I noticed on the schematic that the UGND is connected to GND through this jumper. On the Arduino that I have (Mega2560 Rev3), these two pins are tied together.

Does this mean that even with the external power supply the Arduino will be connected to the UGND, and in effect the protective earth?

What is the best way to float the Arduino, i.e. power it such that it is isolated from PE? Would it be ok to remove the jumper connection between UGND and GND? Or should I try something like a linear USB isolator like this?

As a final question, if I remove the UGND-GND jumper and try to power the Arduino from the USB, will the GND be floating? I am a bit confused by this because without the UGND connection, the GND does not seem to be connected to any power supply pins. In other words, if I remove the UGND-GND jumper connection, and then power the Arduino through the USB, the Vcc that goes into the microcontroller doesn't have a good ground reference. Is this correct? I could be way off on this.

2

The USB port won't work without a "common ground". If you are using a laptop running from it's battery there will be no earth ground (assuming no other earth-grounds to the computer or anything else).

The USB port (and almost anything else that's digital) can be optically isolated if necessary so it's possible to make the USB work without any physical/electrical connections.

3

DVDdoug:
The USB port won't work without a "common ground".

I don't follow this. The D+, D- and UGND, as per the Mega2560s schematic, only goes to ATMEGA16U2. Isn't the UGND coming from the laptop or the computer that I connect to? Also isn't this UGND used only for the differential USB communication? I thought the USB ground could be different from the power ground. Am I wrong?

4

I thought the USB ground could be different from the power ground. Am I wrong?

Yes.

GND? Or should I try something like a linear USB isolator like this?

Yes.