Change I/O discharge protection policy
At this moment, we have decided that the shield should be connected to a global, unique, ground. This ground is shared between I/Os and the rest of the board.
Currently, the protection policy included in projects is to place a protection device such as a TVS, clamp the discharge voltage and direct the discharge current through the TVS, the protection device. However, the path is completed in the GND plane of the board. There, this discharge current will follow the minimum impedance return path to the backplane GND (which means different geographical return current distribution depending upon spectral components of the return current). This is a potential risk to the integrity of the components in the board and an increase of noise in the board.
In cases in which high discharge currents are flowing through the GND board plane, due to the impedance of this one, high voltage differences can be experienced in a component. Subsequently, irreversible damage can be produced.
A different solution is proposed. Two different ground domains can be used: one for I/Os and a different one for the rest of the board. The I/O ground should be connected to the chasis and the other one will be connected as usual (to the backplane). Then, discharge currents will not affect board circuitry.