What is the difference between analog and digital circuits in PCB design?

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What you will learn:

  • Analog circuit design guidelines.
  • Digital circuit design guidelines.
  • Differences in guidelines between the two.

Several products in the electronics industry require both analog and digital printed circuit board (PCB) designs. Design requirements for analog circuits and digital circuits vary and the PCB engineer should follow the corresponding guidelines when designing the printed circuit board. The signal requirements and the effects of interference are quite different in these circuits. It is necessary to fully understand the key differences between the two circuit designs while optimizing the PCB for better performance.

The signal value for a digital circuit is always binary, while the analog signal varies over a range from a minimum value to a maximum value. This provides a greater margin of error in digital signal transmission, but analog signals must be well controlled during transmission and reception. Therefore, designing an analog circuit can be relatively difficult and requires a better understanding of signal transmission.

Usually, the external signal received by a sensor is analog and it needs to be converted to a digital signal for processing. A typical application will be a hybrid circuit that involves both analog and digital circuits. Therefore, the network design should isolate the two sections to avoid interference.

PCB Design Guidelines for Analog Circuits

  • Component placement should be compact and analog signal routing should be short. Any noisy analog component should be placed in the center and not near the edges of the board. This strategy helps absorb noise through the analog GND plane and avoids interference with the digital circuit.
  • It is strongly recommended to place and route the analog and digital circuits separately and avoid high speed digital routing in the analog section.
  • There should be a dedicated analog GND plane. The return path of analog signals must not be interrupted by vias, cutouts or plane separations. If the return signal does not have the shortest distance, noise will be generated.

PCB Design Guidelines for Digital Circuits

  • The placement of high-speed circuit components should be done strategically taking into account the PCB assembly requirements as well. Sufficient spacing for automated placement and soldering must be considered during the component placement step itself.
  • The choice of via size and shape should be based on the type of signal and its electrical and mechanical requirements. Typically, the power and ground vias will be larger than the signal vias. High-speed signal vias can be blind or buried vias. Micro-vias are also used depending on electrical requirements.
  • Trace routing is crucial for differential signals and high-speed clock signals. Factors such as layer thickness in PCB stackup, track width and return path should be designed according to the standard recommendation.
  • EMI shielding, heat dissipation, and noise filtering depend on good reference plane design. Power and ground planes must be balanced and planned for PCB power and signal integrity.

Differences Between Analog and Digital PCB Design

  • Digital circuits are more immune to noise interference than analog circuits. The binary value in digital circuits can accept a wide range of data as a valid input and will work satisfactorily, while analog circuits are very susceptible to even slight noise interference. Moreover, the analog circuit involves a sequence of operations such as modulation, amplification, transmission, reception, demodulation, etc., in which noise interference may occur at any of these steps. . Ensuring the required signal-to-noise ratio and reducing crosstalk adds complexity to an analog circuit.
  • The requirement for efficiency and precision makes analog circuit design complex and expensive. Digital circuits are relatively easy to design, with many automated tools available on the market. This makes digital PCBs cost effective and accurate.
  • Since the analog circuit can interact directly with the physical world and requires no conversion, there is no loss of information. But the digital circuit uses analog-to-digital converter (ADC) and digital-to-analog converter (DAC) circuits, which may cause data loss and information degradation. Therefore, the layout and routing of these signals must follow strict guidelines.
  • In a mixed-signal circuit, digital signals are the strongest source of interference for analog signals because they contain high harmonic frequency. High clock signals and switching power supply circuits must be carefully designed into mixed-signal PCBs to avoid noise interference in the operation of the analog circuit.
  • Sensitive analog circuits require appropriate frequency terminations. To avoid spatial radiation, the analog circuit must be placed far from the digital circuit; a shielding box can be used to guard against electromagnetic interference.
  • Ground plane design is critical in analog circuitry because the return signal path must be uninterrupted to avoid interference. This is not a problem with digital circuits, as they tolerate noise in the ground plane.
  • In PCB design, parasitic components such as parasitic capacitance and parasitic inductance can be a problem when traces are routed close to each other. If the voltage change is large enough in one trace, the interference will reduce the voltage tolerance in the other trace, leading to errors. Such scenarios are common in digital circuits, which have a large transient switching current. It is recommended to reduce the inductive reactance of digital circuit traces and the capacitive coupling of an analog circuit to avoid noise from electromagnetic sources.

Although there are some similarities when designing analog and digital circuits, it is also necessary to understand the differences. This will allow for an effective layout strategy in PCB design. The latest EDA tools have built-in simulation support to assess possible noise issues in the design. This becomes a great advantage when designing mixed-signal PCBs, especially when it comes to reducing board re-spins.

Several factors must be considered when designing sensitive analog circuits and high-speed digital circuits in a PCB design. Many subcontractors (CM) are experienced in supporting and building analog and digital circuit boards. They will also address EMI issues and include the regulatory compliance required for successful PCB prototyping or assembly. Collaborating with such experienced CMs will be of great value, as they know the different strategies required in analog and digital PCB design.

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