emcshielding
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?
You may or may not be over thinking this, it depends on what your application is, and if you have to pass any regulatory inspections for a product. The general idea is to shunt the ESD to ground through the chassis and away from your electronics. This depends on if your enclosure is insulated or not. Another thing to keep in mind is you can also have RF running through the shield, and RF should be considered if you need to build a product to pass emissions testing (cables make great antennas, and can even help induct lightning RF onto your board). For now I'll talk about ESD. A good reference for all things ESD and RF is this book Electromagnetic Compatibility Engineering by Henry W. Ott. I'll quote it
So where should cable shields, transient voltage protectors, and I/O filters be connected when the product is in a plastic enclosure? There are three possibilities as follows:
In your case if none of the cables go on the outside of a vehicle I wouldn't worry too much about RF. If the cables aren't going to be in contact with people (buried in the dash) I wouldn't worry too much there either. If there going through the middle next to people I would worry. You can think of your vehicle ground like earth ground. The car may collect a charge but it also functions like a faraday cage so everything on the inside will be near 0 (except for something like a seat cover that has been charged up, any metal connected to the chassis will be near 0V (0v being the voltage with respect to the car and not "Earth" ground)).
I've also included an image to suppress ESD in a metal case from the same book:
There are many options to deal with chassis and GND. Here is what i do:
Best Answer
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.