I'm using this amplifier with this load cell.

There is a section in the amplifier's manual which states:

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First of all above terminal 5 in the amplifier's screw terminal which is called Screen/Ground is not internally connected to supply GND or output signal GND ect.

The load cell on the other hand has many wires to be connected to this amplifier One of the wire is connected to the load celløs metalic surface. Lets cal this wire load cell screen LC_S.

When I connect the load cell to this amplifier properly I have the follwing observation:

If I connect the load cell's screen wire LC_S to terminal 5 of the amplifier, the noise is typical.

But if I do not connect the load cell's screen wire LC_S to terminal 5 of the amplifier, there is 50Hz noise added(observed at the amplifier's output) which is bigger than the typical noise.

Why does this 50Hz noise disappears when I connect the metallic surface of the load cell to the amplifier's screen terminal? I don't get this because this connection do not have any eventual connection to the earth. The load cell is floating and the terminal 5 is not wired to anything(I checked with continuity test). Im really curios what makes this noise to disappear.

  • \$\begingroup\$ One shielding possibility is that the 'excitation' voltage source is AC from a center-tapped transformer. This may crosstalk to the sense wires, but not if you shield with that center-tap. \$\endgroup\$ – Whit3rd Nov 8 '17 at 6:46

It could be a Faraday shielding. In a car, the mobile signal is reduced even with big openings/windows. The car is insulated from earth via tires. The shielding of the load sensor and the shielding of the sensor cable can reduce the noise like in a Faraday cage, even without any connection to earth. In the contrary, sometimes the noise gets stronger if this shielding is connected to earth, since there are still more connections to earth resulting in a loop that acts like an antenna for noise. So sometimes it's important for shielding to have only one connection to earth at one single point.

  • \$\begingroup\$ But there is no cage. The sensor load cell is floating and its shield is connected to the terminal 5 spared for shield in the amplifier. I dont know if this should be connected to earth. If I connect one side to earth do you mean the earth pin of the AC power outlet? \$\endgroup\$ – user1999 Nov 8 '17 at 0:17
  • \$\begingroup\$ The various metal pieces form the cage. Go read the answer on IR receiver metal shields. electronics.stackexchange.com/questions/295629/… \$\endgroup\$ – analogsystemsrf Nov 8 '17 at 4:43

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