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There is a 6-wire Wheatstone bridge type load-cell and one of the wire is called shield and it is electrically connected to the metallic case of the load cell as below(I marked that with yellow indicators):

enter image description here

Two identical load-cells are wired to two amplifier units and the amplifiers are powered with an isolated SMPS supply. All is shown below with my primitive drawing below:

enter image description here

LC1 and LC2 gray parts represents the metallic part of the load-cell and the yellow wire is the built in shied wire attached to it. 5 orange wires are the other wires between the load cell and the amplifier unit. The outputs go to a single-ended data acquisition board.

I was not using the shield wire before(yellow wire in the above illustration). I mean the shield wire was not wired to to the earth. We were sometimes having interference problems and I was suggested to earth the frame.

Is my way of earth grounding the shield in this case i.e. earthing the shields at a single point to the mains earth correct?

I really couldn't find an clear information on this.

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  • \$\begingroup\$ I would connect it to the PSU casing rather than the mains earth. These should be connected anyway. \$\endgroup\$ – Eugene Sh. Nov 10 '17 at 16:49
  • \$\begingroup\$ The thing is the PSU is plastic and I wired the shield wires to the PSU's earth terminal. Do you have an idea why this earth is recommended or is it recommended at all? I implemented this because of a suggestion but I cannot verify it. Wiring the shields(the metallic body of load cells) to the mains earth I mean. Is it a common good practice? \$\endgroup\$ – atmnt Nov 10 '17 at 16:56
  • \$\begingroup\$ Doesn't the PSU have protective earth connection/screw? Do you have it's name/photo? \$\endgroup\$ – Eugene Sh. Nov 10 '17 at 16:57
  • \$\begingroup\$ Yes it has earth screw on it I used that one. Here is the photo: uk.rs-online.com/web/p/din-rail-panel-mount-power-supplies/… \$\endgroup\$ – atmnt Nov 10 '17 at 16:59
  • \$\begingroup\$ Ideally the PE path should be daisy-chained mains-PSU-Amplifier-LCx \$\endgroup\$ – Eugene Sh. Nov 10 '17 at 17:09
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I would connect them like this:

enter image description here

Why?

Because if you don't, you could get capacitive coupling between the shield and the sensor that the shield is supposed to be shielding. A shield should be at the same potential as the thing it is shielding. Any electrical field on the sensor shield will flow down the shield to the AGND and be shunted away from your sensor. Make sure your AGND is low impedance to the other grounds in your system.

If the cable is attached to earth or chassis ground, there might be the possibility that you are shunting the currents from the shield to earth ground where they belong, but also picking up common mode noise from the earth ground.

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  • \$\begingroup\$ At the data acq. side 3 meters far away the AGND is already connected to the earth. So both the shield and the AGND is earth grounded in my picture. But AGND and the shield are isolated from each other so no ground loop should occur. Does that still make capacitive coupling? \$\endgroup\$ – atmnt Nov 11 '17 at 14:40
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    \$\begingroup\$ The capacitive coupling happens between the wires and the sensor. The idea is you want the shield to be the same potential as the AGND so there is no capacitive coupling within the cable or sensor housing. There are two problems with shields, capacitive coupling and inductive coupling. I've already described capacitive coupling. Inductive coupling happens from magnetic field effects within the cable, you want to 1) minimize the currents on the shield of the cable. 2) keep the voltage the same as your electronics. \$\endgroup\$ – laptop2d Nov 11 '17 at 17:51
  • \$\begingroup\$ The best place would probably be the AGND or the power supply ground, chassis ground would be the last place, however this is dependent on many factors so I would test it and see which location is best if you have problems. \$\endgroup\$ – laptop2d Nov 11 '17 at 17:54
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The purpose of the shield is to provide a low impedance path for common mode noise to some low impedance relative to the stray coupling impedance. Considering E and I fields can be several orders of magnitude above the signal, this impedance ratio is critical for radiated noise.

Meanwile conducted ground noise is a big unknown depending on the application and proximity of SMPS line filters and ground surges and environmental RF noise.


The problem with connecting shields at both ends depends on ground currents between the two grounds and thus the induced CM current along the shield is the amplitude of the ground current. This often only problematic near large AC-DC power supplies or systems that use Y caps to filter CM noise to earth ground or if there is a ground fault with leakage to ground. It can also be problematic with near RF AM transmitters.

There is no general solution. Here are some variations in shielding;

For EEG signals the shield is often driven with the signal common mode voltage called active shielding because a stray E-field of 100V/m is 140dB above a signal of 10uV.

If the noise spectrum is RF from SW transmitters and SMPS radiated noise, the shield can act as an antenna and pre-amp input non-linearities for CM signals can get rectified producing baseband modulation noise, so RF CM chokes are used for microphones and DC power charger cords, and earth grounding is only on the AC frame. For mobiles or laptops there is no earth ground unless connected to a VGA monitor or some earth grounded audio port.

For ethernet signals the Common mode impedance is usually coupled to earth ground by 1nF caps to avoid AC ground currents going between end-points on earth grounded PC's. These always also include CM chokes to raise the CM impedance so that the differential impedance can attenuate CM noise.

Where the noise can be largest near the sensor such as in an automotive petrol engine with spark plug noise or a difference in ground noise between end-points, the shield to frame is often only connected at the source and not at the received end to avoid frame ground noise conducting thru the shield.

enter image description here

Below showing that common mode noise can come from the SMPS so CM choke is needed. ( same is true for Laptop chargers but shield is only earth grounded at one end. ) N.B.

Recommnded:

  • connect yellow gnd drain wire as an option
    • if no ground current expected to reduce radiated noise
    • or if environmental radiated noise is too high.
    • but may increase conducted noise if SMPS is earth grounded on output if ground impedances are low and gnd noise current is high.

schematic

simulate this circuit – Schematic created using CircuitLab

In this case strain gauge connectors are earth grounded thru the connector body to chassis sensors, and the internal ground path ought to be short to the power cable so this provides a decent shield for the signals. The option to connect the yellow shield drain wire at the AC ground destination depends on the difference in ground noise between the source and destination. This is hard to determine without testing or knowing the stray noise in the installation. One has to know how to test and measure noise by simulating nearby noise with solenoids or relay coils pulsing on long wires nearby. Then you can assess radiated noise effectiveness. For conducted ground noise, other tests are used for susceptibility simulation.

So there is no general answer to what is best for your application, but at least you have the option to terminate the shield at both ends or add a large Ferrite core or torroid donut or ferrite clamshell depending on the amplitude and spectrum of local interference. This is a routine design consideration for EMC for both ingress in this case and egress for PWM bridge controlled steppers.

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