Shielded Cable

[A_neves]

Hello all,

I have a question.
When we need to use a shielded cable for digital signals, should the metal net tied to GND to properly shield the signals?

Thank You!

[Paul van Avesaath]

it depends on you EMC spec, but in 99% it is connected to GND.

[robertferanec]

In most of my boards (processor boards), I connect it directly to digital GND, but there are situations where you may need to connect it differently ...

For example in other cases it can be connected to Enclosure GND or sometimes there may be additional circuit ... the connection may depend if you are connecting two devices which are powered through different main power plugs, if devices are long way from each other (e.g. currents may flow then between devices through this GND) .. or it can depends on if one of the devices is powered from battery or if you are using plastic / metal enclosure or if you are (not) using transformer in power adapter ... there is no one answer.

If you are not sure, provide more options to Fit / Unfit different combination of components and during testing you will find the proper connections for your specific system.

[A_neves]

Hello all,

Thanks for your answers.
If I left unconnected, is metal net of shielded cable doing nothing?

Thank you!

[robertferanec]

I am not sure if I would leave it unconnected. I would provide the combination of footprints on PCB to connect it different ways (as I mentioned above). Initially you can connect it directly to board GND (or shield / earth GND) and then during testing you will see if to use a capacitor / bead / 0R resitor or something different. Use bigger footprints as you may want to be able to fit there higher voltage components (e.g 1nF / 2kV) and components fitted there will need to pass ESD tests.

[A_neves]

robertferanec I understand. But my doubt is when I need to crimp the wires for a molex connector like this: https://www.digikey.pt/product-detai...2903-ND/115035
How can I crimp the net shield of the cable in to the connector?

[robertferanec]

That headers do not have shield (that headers are only used for simple wire connections).

So you would need to:
- change the header on the board for a connector with a shield,
- or do not use shield,
- or you may need to find a way how to connect cable shield to your board ground (not a standard solution)

[A_neves]

Ok, thanks for your answers.
So if I left the cable shield net unconnected, the shield net do nothing?

Thank you!

[robertferanec]

So if I left the cable shield net unconnected, the shield net do nothing?

- Hmm ... maybe .. maybe not. There may be some currents generated in the shield from the signals inside of the cable and that could make the shield to generate some noise or emissions. But that may be just my imagination Cables what are just hanging around and are not properly grounded often cause problems during EMC measurements.