I have a situation where I need to connect 10 flat cables (20 way, 0.625mm pitch) to special PCI-E card in my computer. They go from PCI-E card on back to drive bays in front of computer case.
Those flat cables are around 50cm length and come to PCI-E card in one group, from one direction. They look very neat, stacked one to another and taped to stick togeather. I've read that stacking flat cables largely increases EMI, maybe I should consider distancing them somehow?
I have experienced some problems with communication between nodes connected with flat cable and PCI-E card, which was realized to be EMI problem.
First, flat cables didn't have any kind of EMI protection. Then, I installed ferrite cores on both ends of the cables. That helped a bit, but there are still problems.
I have some ideas:
- Putting aluminum tape around cables and grounding it on metal
case - Separating and twisting flat cables as much as possible (tho that
would look ugly and messy so I would like to avoid that) - Distancing them with one from another with aluminum tape and foam
tape
Are those good points?
What else can I do to get better EMI protection?
Best Answer
Ideally, put a Faraday cage (a grounded metal box) around everything from the back of the computer to wherever they are going and solidly connect it to the computer earth.
Ribbon cables are not really designed for use in the open.
There are special twisted pair ribbon cables, but that only works if the pinout is right, and it would not generally be helpful if it was not designed in.
As you've found, common mode chokes (the ferrites) help a bit.
The above are just general suggestions for cables carrying logic-level signals. It might be a bit different if the cables were carrying floating signals or had other unusual characteristics.
Differential signalling is good because the return path for the current can be twisted with the other wire so the net field is very low.