The basic rules for EMC of mixed-signal PCBs are.
- Make the current path closer to the source and as compact as possible so that the loop area is as small as possible.
- Provide only one reference ground plane for a system; otherwise, it is equivalent to intentionally creating a dipole antenna.
Imagine a high frequency converter circuit PCB where the input voltage and current are analog signals. The switches in the converter start switching with the application of a digital clock signal. The power supply circuit in the converter is analog and the control circuit is digital.
The converter PCB is a mixed-signal PCB because it handles both analog and digital signals. In a mixed-signal PCB, the analog and digital signals need to be physically separated in a process known as "partitioning".
Designing a mixed-signal PCB is a challenge because analog and digital components have different currents, voltages, and power ratings. However, following some basic design rules will help simplify the partitioning and layout of mixed-signal PCBs.
Designers must partition mixed-signal PCBs and design layouts carefully
Achieving electromagnetic compatibility (EMC) is an issue of great concern to mixed-signal PCB designers. When analog and digital signals coexist on a PCB without isolation, they are likely to be mixed together, causing crosstalk and electromagnetic interference. For example, digital logic ground currents can contaminate low-level analog signals on a mixed-signal PCB. This can lead to feedback loop errors, control system failures, and harmonics in the output waveform. In these cases, the electromagnetic compatibility of the mixed-signal PCB is compromised, resulting in poor product performance. Proper partitioning and proper layout help keep digital and analog signals isolated and prevent them from interfering with each other, thereby greatly reducing crosstalk.
When partitioning and layouting mixed-signal PCBs, two rules must be followed.
1. Keep the current path closer to the source and as compact as possible so that the loop area is as small as possible.
2. Provide only one reference ground plane for a system; otherwise, it is equivalent to intentionally creating a dipole antenna.
These two rules are considered to be the basic principles of mixed-signal PCB partitioning and layout design. Here, let's learn more about these two golden rules.
Rule 1. Keep the current path localized and compact
Every signal that flows on the PCB returns to the source through the ground plane. The signal lines and return lines form a current loop on the PCB. According to the first rule mentioned above, the return lines should be laid so that they are adjacent to the signal source, creating a minimum loop area.
Q: Why is this setup recommended and why does this reduce electromagnetic interference?
All return currents prefer to flow through low impedance routes. When the return line is located directly below the signal line, the current loop has the lowest impedance. When the signal line and its return line form a large loop, a high impedance path is formed. This is caused by the parasitic capacitance and inductance in the current loop.
When the signal and return lines are far apart, the parasitic capacitance has a high value, increasing the impedance of the loop. There is also a relationship between the loop inductance and the distance the charge travels, which further increases the impedance of the path. When the loop is large, the charge leaving from the source has to travel a long distance to reach ground. This increases the inductance of the current loop, which in turn increases the impedance.
When high-frequency analog signals flow through large current loops with high impedance, they emit radiation and cause interference. Similarly, low-level analog signals are more susceptible to electromagnetic interference when flowing through high-impedance current loops. In addition, the signal and return lines form a loop antenna that exacerbates the EMI problem. Therefore we want to make the current loop short, localized and compact.
Rule 2. Need for a single reference ground plane
Split ground plane is a way to isolate digital and analog grounding. In this method, the ground planes are separated from each other, and it is not possible to wire at the split. In such PCBs where the ground planes are separated from each other, the two ground planes are connected only near the power supply, forming a large current loop, which is not favorable for PCB designs with non-sensitive electromagnetic interference. In addition, the analog and digital ground planes are at different potentials, and the long wires laid on these planes form a veritable dipole antenna that emits electromagnetic radiation.
With all these factors in mind, partitioning the mixed-signal PCB is the ideal approach. Partitioning keeps the reference ground plane common. Analog signals are routed in the analog section and digital signals are routed in the digital section, so that the signals are in their own place.
For EMI simulation of entire systems for large and complex very large scale, automotive, mobile and aerospace systems, Clarity™ 3D Transient Solver software can be used.
Clarity 3D Transient Solver increases the simulation speed of electromagnetic interference (EMI) designs by up to 10 times over traditional 3D field solvers, and has nearly unlimited simulation capabilities. Based on Cadence's massively parallel matrix solver technology, Clarity 3D Transient Solver enables time-consuming and expensive anechoic labs to perform prototype EMC testing, simulating large designs previously considered impractical or unsolvable, shortening design cycles and accelerating time.
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