Sunday, September 7, 2008

Planes and Split Planes

A fact often neglected by digital designers is that current travels in loops. For example, consider a single-ended signal being transmitted between two gates as shown below. The current travels in a loop from gate A to gate B and then back to gate A through the ground connection.

There are two potential problems here:
1. The grounds need to be connected by a low-impedance path If the grounds are connected with a high-impedance path, then there is a voltage drop between the ground pins in Figure 3. This causes disruptions to all of the devices referenced to this ground and can degrade input noise margins.

2. The loop area formed by the current loop needs to be as small as possible Loops act as antennas. Generally speaking, a greater loop area will increase the chances of the loop radiating and conducting. It should be a goal of every PCB designer that the return current is able to travel directly underneath the signal trace, thus minimizing loop area.

Using a solid plane for ground solves both problems simultaneously. A solid plane provides a low-impedance between all the ground points whilst allowing return currents to travel directly underneath their respective signal traces.

A common mistake made by PCB designers is to use holes and slots in the ground plane as shown below. The current flow when a signal trace is routed over a ground plane slot. The return current is forced around the slot, thus creating a large loop area. Picture below shows the current flow without a slot in the ground plane. Notice how the loop area is kept to a minimum.

Generally speaking, slots shouldn’t be used in ground. However, there are occasions when slots cannot be avoided. When this happens, the PCB designer must ensure that no signal traces are routed over the slot.

The same rules apply for mixed-signal PCBs except that they often use multiple grounds. This is typical of a high-performance ADC that may utilise separate grounds for analog, digital and clock circuits. When using separate grounds, ground plane slots are imperative but again the PCB designer must be prudent in ensuring that no signals are routed over the slots.

For exactly the same reasons, the identical philosophy of using planes applies to power supplies with one minor difference: Occasionally, a PCB designed with power and ground planes will be found to radiate around the edge of the board. The electromagnetic energy emitted from the edge can disrupt adjacent cards. The situation is shown in below. The solution, shown in picture B below, is to shrink the power plane so that the ground plane overlaps by a certain distance. This will reduce the amount of electromagnetic energy emitted outside of the board’s immediate area and will reduce any impact on adjacent boards.

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