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I'm trying to shield an ip camera from EMI generated image noise for low light photography. It's enclosed within a metal box that is then grounded. The subject is also inside the box. The camera needs a power supply at 5V and an Ethernet cable connection. These obviously have to pass through the box via small holes.

How can I ensure that the shielding is as effective as possible? Do I have to take special measures to prevent EMI propagating along the cables into the box? Or, doesn't it matter?

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  • \$\begingroup\$ I believe (hence a comment) that you want the metal box to be at ground potential, share its ground with the camera. Try to find an STP cable for your Ethernet (shielded twisted pair) and also connect its shield to shared ground. \$\endgroup\$ – jippie Oct 19 '14 at 5:32
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First: Don't try to solve a problem that is not there to begin with :) I know this is obvious, but attempted solutions to imagined problems often cause other problems.

Second: If you are worried about EMI noise visible on the low light images, I believe the problem should be addressed at the CCD chip design level. Maybe outside the scope of your question.

With that said, you have a standard EMI question and I can provide my standard EMI advice that have worked for many many products:

  • A Faraday cage (metal box) is almost always required if you have multiple boards or things sticking out from one board. In your case we haven't seen the camera, but it's likely that you need a Faraday cage. You may have a problem with the opening for the lens, depending on how that is engineered. A Faraday cage does not need to be solid, but can be a net. Do some testing to see what is sufficient if you want to optimize this.

  • All wires should preferably go in and out in one side of the box and preferably the same side where you make the connection between the internal ground and the Faraday cage.

  • All shielded cables exiting the Faraday cage should have the shield connected to the Faraday cage (look at the shield as an extension of the Faraday cage). Notice that getting the parasitic inductance really low is essential for high frequency operation. This is a good option for your Ethernet connection.

  • All unshielded cables exiting the Faraday cage should have a low pass, low inductance filter between each wire and the Faraday cage where the cable exits the Faraday cage. This would work well for your DC-connection.

With the information given, this would be my best advice for you. Feel free to ask more questions.

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Yes. Ferrite beads can help reduce the noise on the cables entering your shielded area. Use cables that are shielded so they do not pick up outside noise. Also use the shortest runs possible. It definitely matters.

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