Shielded cable grounding 1

[Roach]

Can someone explain why if I ground both ends of a shielded cable carrying DC signals in a wireway with AC current carrying wires, I get noise, but if I ground just one end, I don't?


Thanks

 

[jwerthman]

Very small differences in ground potential between the ends of your shielded cable can result in current flow in the shield if it is grounded at both ends. The current in the shield will induce a voltage in your signal circuits inside the shield. Low level signal circuits should generally be grounded only at one end to prevent the flow of shield current.

 

[Roach]

Thanks for the answer. This is common sense, although not so common in my case. Thanks again.

 

[peebee]

Here's an analog to the termination resistor using water waves:

When water waves hit the sides of a pool or a bulkhead, they reflect back into the water.

When waves crash onto the beach, the energy is dissipated and there is little or no reflection.

An unterminated cable looks like the hard pool wall. The resistor acts like the beach and dissipates the energy.

You get maximum energy transfer into the resistor when the cable impedance matches the resistor impedance.

Such resistor installations are recommended with most electrical waveguides. They are commonly installed residentially on cable TV systems -- those little blank caps are actually resistors. Sometimes, if you don't use the resistors on cable TV wiring, the reflectances can cause ghost images in the TV picture.

 

[jbartos]

Suggestion to the first posting. The grounding both ends of the shield actually created a turn of transformer like device that has an AC current flowing in it. This induces AC voltage and current in the DC conductor that are superimposed on the DC current and voltage thus creating a noise.