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The Partitioning and Layout of a Mixed-Signal PCB for Electromagnetic Compatibility

Key Takeaways

  • In mixed-signal PCBs, physically separating analog and digital signals in a process called partitioning is required. 

  • Partitioning and the proper layout of mixed-signal PCBs can reduce crosstalk and interference.

  • The basic rules for electromagnetic compatibility in a mixed-signal PCB are:

  1. Make the current paths more local to the source and as compact as possible, with the smallest possible loop area.

  2. Provide only one reference ground plane to a system, otherwise, you are deliberately creating a dipole antenna.

Mixed-signal PCB

Mixed-signal PCBs are more vulnerable to crosstalk

Consider a high-frequency converter circuit PCB, where the input voltage and currents are analog signals. The switches in the converter start switching with the application of digital clock signals. The power circuit in the converter is analog and the control circuit is digital. 

A converter PCB  is a mixed-signal PCB, as it handles both analog and digital signals together. In mixed-signal PCBs, physically separating the analog signals and digital signals in a process called partitioning is required. 

Designing mixed-signal PCBs can be a challenge, as the analog and digital components are of different current, voltage, and power ratings. Luckily, the partitioning and layout of mixed-signal PCBs can be simplified by following some basic design rules. 

Designers Must Partition and Carefully Layout Mixed-Signal PCBs

Achieving electromagnetic compatibility (EMC) is a serious concern to designers of mixed-signal PCBs. When analog and digital signals co-exist in a PCB without isolation, there is a fair chance that they will get mixed up and cause crosstalk and electromagnetic interference. For example, digital logic ground currents are capable of contaminating low-level analog signals on a mixed-signal PCB. This can result in errors in feedback loops, glitches in the control system, and harmonics in the output waveforms. In these cases, the electromagnetic compatibility of the mixed-signal PCB is compromised and leads to the poor performance of the product. Proper partitioning and a thoughtful layout keep digital and analog signals isolated and prevent them from interfering with each other, greatly reducing crosstalk

While partitioning and laying out mixed-signal PCBs, two rules must be followed:

  1. Make the current paths more local to the source and as compact as possible, with the smallest possible loop area.

  2. Provide only one reference ground plane to a system, otherwise, you are deliberately creating a dipole antenna.

These two rules are considered the basic foundation of partitioning and designing a layout for mixed-signal PCBs.  Read on for further analysis of these two golden rules. 

The Need For Local and Compact Current Paths

Every signal that flows in a PCB trace returns to the source via the ground plane. The signal trace and return trace form a current loop in the PCB. As per the first rule given above, the return traces should be laid so that they are adjacent to the source and form the smallest loop area. 

Why Is This Arrangement Recommended and How Can It Reduce EMI?

All the return currents prefer to flow through low impedance routes. The current loop is of the lowest impedance when the return trace is directly below the signal trace. When the loop formed by the signal and its return trace is large, it creates a high impedance path. This is due to the parasitic capacitance and inductance in the current loop. 

When the signal and return trace are farther from each other, the value of parasitic capacitance is high and it adds to the loop impedance. The loop inductance and the distance traveled by the electric charge are also related, which further increases the impedance of the path. When the loop is large, an electric charge leaving from the source travels a long distance to reach the ground. This increases the inductance of the current loop, which in turn increases the impedance.

When high-frequency analog signals flow through the large current loops of high impedance, they emit radiation and cause interference. Similarly, low-level analog signals are more susceptible to EMI when flowing through high impedance current loops. Also, the signal and return trace form a loop antenna which makes the problem of electromagnetic interference adverse. This is why it is desirable to keep the current loop short, local, and compact. 

graphic of a split ground plane and a partitioned plane

  1. Split ground plane with common power supply
  2. Partitioned plane

The Need For a Single Reference Ground Plane

Splitting the ground plane is one method of isolating digital and analog grounds. In this method, there is a split between the ground planes, and it is not possible to route the traces over the split. In such split ground PCBs, the two ground planes are connected only at the power supply, creating a large current loop which is not good for an EMI-friendly PCB design. In addition to this, the analog and digital ground planes are at different potentials and the long wires laid on these planes create a real-world dipole antenna emitting electromagnetic radiations. 

Considering all these factors, it is ideal to partition mixed-signal PCBs. The partitioning keeps the reference ground plane common. The analog signals are routed in the analog section and digital signals in the digital section, and thus, the signals remain in their proper sections. 

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