I ruined another perfectly good USB cable in the name of science. It came with my PicoScope. It was the one with the lowest drop. The cable says 24 Awg (data) and 20 awg power, having opened it and exposed its veins and soldering my own power wires to them I can asure it was good quality
So I ended with a Frankenstein cable, which even heat shrinks could not make less ugly, but functional it is.
I used my bench power supply to bring the AWG to life. Then I plugged it into the DSO. It got recognized normally. With output turned on it draws about 366 mA from the power supply.
The measured voltage drop at the scoop was still there, but lower: 7,2 mV. This when there’s a connection. Unconnected it went from 37 mV (good cable) to 14 mV.
So one might think my reasoning was wrong. But that’s not entirely true, there is still a shared wire (or trace) in the AWG. If one measures the GND voltage between what comes in the AWG and what comes out. That where the 14 mV is lost. This looks like wrong design to me. Why not split as soon as possible, there’s a metal case which could bring gnd from back to front
The moral of the story is also it does not matter how fat the USB cable will be, those 14 mV are lost any way. And it is probably due to 365mA (thus 0.038 ohm), so not easily “short circuited”.
But shorting the gnd’s of the BNC connectors seems the only way to get it down to less than 0,5 mV.
I think a new Frankenstein project is about to arise. 2 T splitters and some jumper wire will do just fine.