Thank you for the suggestion.
All are already connected to the same socket.
I have solved the problem as far as measurement goes and would like to update. I would still appreciate any insight, as I don't understand the effect that I'm seeing.
The load cell amplifier outputs both negative and positive voltages (something like -7 to 7V). When I connected to the Arduino, I flipped the signal and ground, so measuring the ground in an analog input and connecting the signal to the ground. It worked fine before connecting the additional stage device.
I don't get the same effect when I connect ground from the load cell to the Arduino ground and the signal to the analog input.
As far as my practical problem goes, I simply connected the signal through a voltage divider thus mapping the [-7,7] to [0,5] (See attached diagram). So other than losing some resolution, I can perform my measurement.
I am still very curious as to why connecting the additional ground to the amplifier caused the output voltage to be close to zero. I'll try to research it too, but suggestions or explanations are more than welcome.
Regarding additional information - I'm using the Aurora scientific
403A deviceNewport
ESP301 stage with 1 motor connected to it.
Arduino Uno R3
Attached are the rough values I used for the voltage divider.
Thank you all again,
U