Editorial Note: Today’s design article comes from the book “Signal Integrity Issues and Printed Circuit Board Design” by Douglas Brooks. More information about the book can be found in this posting. The first half of chapter 9 - Electromagnetic Interference (EMI) was published a few weeks ago and can be found here. It covered the general background, talked about some basic truths, signal coupling and return loop area. Today we continue looking at other forms of loop area. The first half of chapter 10 - Reflections and Transmission Lines is available, and the second half will be made available in the future.
Unrelated Planes: Many boards have separate, well-defined areas on them for different power systems. There may be both 3-volt and 5-volt power supplies, for example, or perhaps separate analog and digital power supplies (or even transmitter and receiver power supplies, etc.). Often, in such cases, we define planar areas for these supplies.
Figure 9-11 illustrates a situation in which there are separate analog and digital power supplies and planes. Consider a trace that must be routed between the digital ICs 1 and 2. Ideally, the trace would be routed exclusively over the digital power plane.
Figure 9-11: Never route a trace over an unrelated plane.
What if we took the shorter path and routed it over the analog plane? The question then is, “Where is the return signal?” There are two possible answers to that question, both of which are bad.
One possible answer is that the return signal stays on the digital plane and flows around the analog plane. But this creates a wider loop area and therefore potential EMI problems. Another possibility is that the return signal somehow finds a path onto the analog plane and continues on under the trace. We may not have an EMI problem in this case, but what we do have now is a digital signal on the analog plane. This digital signal might interfere with other analog signals in the area and create a crosstalk or ground noise problem. After all, the whole reason we have separate power systems is to keep the analog and digital signals separated in the first place. Either of these possibilities is bad. So the right answer is: Don’t route traces over unrelated planes.
Stripline: We intuitively know that a shielded wire (e.g., a coaxial cable) performs well from an EMI standpoint. We probably are aware that another defense against EMI is a shielded enclosure. So we can understand that traces shielded between planes probably perform well, too. This is true, within limits.
The single most important rule for EMI control is to control loop area. Even in a stripline environment (where traces are shielded between two planes) a large loop area can cause EMI problems both externally (EMI radiation) and internally (crosstalk). But all other things being equal, a stripline environment adds an extra measure of EMI control. So, even though microstrip traces can be designed to perform fully satisfactorily from an EMI standpoint, designers sometimes opt to place all critical signal traces in a stripline environment for an added measure of safety.