Ground connections play a very crucial role in the EMI/EMC performance of the system that we have designed. There will be two major grounds, chassis ground, which is the box in which PCB will ultimately be fixed or the circuit ground, which is basically the signal ground which is the return path for the signals. Designer has to remember that most of the EMI/EMC issues arise out of improper techniques followed for grounding during the PCB layout. Designers follow different methods while grounding the circuits, but the below are the major points that need to be remembered how to connect the chassis ground and signal ground.
1. The signal and chassis ground should be connected through a low impedance path. Generally, the PCB connects to the chassis near the mounting holes.
2. The most followed technique to achieve low impedance is to have a capacitor between the signal path and mounting hole and mounting hole connects to chassis directly.
3. If there are any I/O connectors in your board, the grounding to chassis can happen through the mounting holes of the connector.
4. Let us assume there are 4 connectors in sequence at one edge of the board, in this case, the left most connector mounting hole connects to mounting hole of PCB and rightmost hole of connector connects to the left most mounting hole of the next connector and
the chain continues till the last connector. The right most hole of last connector connects to the mounting hole of PCB. This way a cage is formed on one corner of the PCB.
5. When there are no connectors, the mounting holes of the PCB are shorted through a thicker trace so that all the mounting holes are shorted. This forms a Faraday cage around the PCB.
6. In the case of bigger chassis, we always see that a separate ground line is connected to the chassis. This ground line is that of the power supply line and is always preferred to connect this way. The reason for this is that, any potential on the chassis should be grounded as this is not safe and lead to shocks. so, just connecting circuit ground to chassis ground is just not
enough, the chassis must further be connected to ground leg (Earth ground). This helps during the circuit fault conditions also.
7. If there are two or more systems interconnected, it is the external cable (cable shield as ground) that helps to connected the chassis grounds of two systems, other than the signal ground that is present in the signalling path.
Whatever may be the method, noise generated within the system and noise that is present outside the system should never disturb the system functionality. Also, the noise within the system should be restricted so that other systems in the noisy system environment don't get affected.
]]>
0 Comments