I'm working on a PCB that has shielded RJ45 (ethernet), RS232, and USB connectors, and is powered by a 12V AC/DC brick power adapter (I do the 5V and 3.3V step down on board). The entire design is enclosed in a metal chassis.
The shields of the I/O connectors are connected to a CHASSIS_GND plane on the periphery of the PCB and also make contact with the front panel of the metal chassis. The CHASSIS_GND is isolated from digital GND by a moat (void).
Here's the question: Should the CHASSIS_GND be tied to the digital GND plane in any way? I've read countless app notes and layout guides, but it seems that everybody has differing (and sometimes seemingly contradictory) advice about how these two planes should be coupled together.
So far I've seen:
- Tie them together at a single point with a 0 Ohm resistor near the power supply
- Tie them together with a single 0.01uF/2kV capacitor at near the power supply
- Tie them together with a 1M resistor and a 0.1uF capacitor in parallel
- Short them together with a 0 Ohm resistor and a 0.1uF capacitor in parallel
- Tie them together with multiple 0.01uF capacitors in parallel near the I/O
- Short them together directly via the mounting holes on the PCB
- Tie them together with capacitors between digital GND and the mounting holes
- Tie them together via multiple low inductance connections near the I/O connectors
- Leave them totally isolated (not connected together anywhere)
I found this article by Henry Ott (http://www.hottconsultants.com/questions/chassis_to_circuit_ground_connection.html) which states:
First I will tell you what you should not do, that is to make a single point connection between the circuit ground and the chassis ground at the power supply...circuit ground should be connected to the chassis with a low inductance connection in the I/O area of the board
Anybody able to explain practically what a "low inductance connection" looks like on a board like this?
It seems that there are many EMI and ESD reasons for shorting or decoupling these planes to/from each other, and they are sometimes at odds with each other. Does anybody have a good source of understanding how to tie these planes together?