Hello community, I've run into a problem with ground loops that I can't find

Hello community, I’ve run into a problem with ground loops that I can’t find a way to solve. I’m hoping someone here has experienced something similar to this and can offer a solution.

I’m building a system that consists of multiple display units (SK6812), potentially quite far from both the controller hub and from each other. I’m running RS485 signals over twisted pairs to each unit to allow the distance from the hub to a unit. The signal is not isolated at any point, so I also have a ground reference alongside it. RS485 requires a ground reference, this needs to connect to the receiver ground, which in turn must be connected to the LEDs (supply ground) so that the signal levels are correct, and this is where the issue starts.

Because the distance between units may vary a lot, there’s a possibility of some of them being close enough to share a power source, but the cable will still be run to every unit individually to reduce cable thickness among other things. This means, if there is a significant difference in current draw along the cable, there will be a small difference in voltage between the “ground” line at each unit. This difference would cause a significant current through the signal grounds, which are not set up to carry much current at all.

It’s important to note that RS485 can handle signal offsets of several volts each way, so signal levels aren’t an issue here.

Here are some solutions that I’ve considered so far:

  1. At the unit, join signal ground to supply ground through a resistor of several K ohms, so the signal ground of the network will be the average of all the supply grounds and barely any power will be dissipated. I’ve heard that this will ruin the noise immunity of the RS485 signal though (can anyone verify this?).

  2. Place a switch on the back of each unit that disconnects the signal ground. Rely on the wide input voltage range of the receivers to deal with the resulting offset. This again may cause issues with noise immunity, and it’s also relying on users to configure it correctly (I plan to turn this project into either a product or an online guide).

  3. Throw away the RS485 receivers I already bought and redesign around something isolated (are there optoisolators fast enough?)

Here’s a (very rough) diagram of the problematic layout.missing/deleted image from Google+