Optimal Grounding for Loadcell Shields: Logic Ground or Earth Ground?

[j777]

Hello,

When is it OK to connect the shield wire of a loadcell to the 0V rail vs chassis ground? I have seen it connected to the 0V rail but other sources seem to indicate that it should be connected to chassis ground. Any thoughts/advise?

Thanks

 

[berkeman]

What's a loadcell? Can you point us to a datasheet?

 

[j777]

Sorry for not being clear. A load cell is a form of strain gauge commonly used in scales. Below is a link to a datasheet for a load cell.

http://www.flintec.com/loadcells/bk2.pdf"

 

[berkeman]

Ah, thanks. For environments where there can be EM noise that the shield is picking up, it should not be connected to the ground of your circuit. If it were, shield currents could then flow through your circuit ground, which will introduce noise into your measurement. Also, ESD and other transients would be introduced into your circuit by making that connection.

 

[j777]

How is this connection made in a system such as a laptop computer where the only "ground" is the 0V rail?

 

[berkeman]

How is this connection made in a system such as a laptop computer where the only "ground" is the 0V rail?

A laptop would be a bad choice for a data aquisition controller for this situation. Because laptops have no Earth ground connection (a 3rd prong in a power cord to the AC mains), and because they have switching power supplies inside them, they are very noisy with respect to the outside environment.

With a grounded device, like a desktop PC, the switching power supplies are designed to dump common-mode capacitive switching noise into chassis ground, instead of driving the logic ground with this noise. Laptops, with their floating 2-wire connections to the outside world (or even with no connection running off batteries) dump this common-mode switching noise into logic ground, which goes right into your precision analog measurement circuitry. I've had aweful difficulties trying to do precision analog stuff with laptop computers -- ack!

It's best to use chassis/Earth ground for dumping noise, and keep your measurement circuitry floating with respect to chassis/Earth ground, at least in the frequencies of measurement interest.

 

[j777]

Thanks, that's exactly the information I needed. I'm not using a laptop but I have the choice of connecting the shield wires to logic ground or Earth ground and because connecting them to Earth ground is a little more work I wanted to make sure it is the best thing to do.

Once again thanks for taking the time to help me out.