Hey! This may well be a tired/amateurish question, and if it is I'm sorry, but I've done a lot of research on this and I'm still a bit confused about what to do.
I have a project, a set of animatronic eyes, that requires 8 SG-90 servos. Until now, I've been using the barrel jack to power them directly through the Arduino (oops). I should mention, until now I've been using a third-party Uno Rev3.
After learning how horrible a person I was, my plan was to power the servos on their own and use the Arduino (I'll be using a Nano from now on) to provide them with a signal. The only thing is, I need to ground the servos and the nano for that to properly work. Here's my problem: I need to keep the nano plugged into USB, as it's receiving serial data from my computer so I can use an Xbox controller with it. My main concern is that the GND line coming in via serial could be different compared to the GND of the servos, which I imagine would cause a problem. Does anyone have an idea on what I could do?
Welcome to the forum
The whole point of joining the GND connection of the Nano and the GND of the servo power supply is to provide a common point of reference for both systems in respect of the data signal passing between them
There will be no problem keeping the Nano plugged into USB and you powering the servos with an external supply. What are you planning to use as that supply ?
I'm thinking about using a LiPo battery, but if that doesn't work, I'll just use a benchtop power supply. Thank you so much for clarifying; that's a huge relief. Apologies if this is in fact a commonly-asked question!
More commonly people either try to use the Arduino as a power supply as you did or use an external power supply with no common GND connection
I think you can connect the ground of an external battery supply to the USB ground with no problem. But I'm not sure about a bench power supply that is plugged into mains. I'm not saying it won't work, but I'm just not sure it would. If it were me, I would use my meter to see if there's any potential between the two UNconnected grounds, and only connect them if it's zero.
Alright, that's a good idea, thanks. Is there anything I can do if they're not the same?
If your meter shows a differential between the two grounds, then it means the two power systems are already connected in some way. I don't know what the solution would be in that case because large amounts of current could flow if there's a voltage potential. Maybe the engineers here can give us the answer.
The advantage of battery power is that it's not referenced to anything, so you can just connect its ground to the USB ground with no problem.
Hi, @finchymcfinch
Use a lab power supply to do your prototyping, the lab supply will have current limit on it.
LiPos do not.
Tom..... 
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