What’s the Difference Between Analog and Digital Circuits in PCB Design?

This article is part of the TechXchange: PCB Tools and Technology

What you’ll learn:

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

Several products in the electronics industry require both analog and digital printed-circuit-board (PCB) designs. The design requirements for analog circuits and digital circuits vary and the PCB engineer must follow corresponding guidelines while designing the circuit board. The signal requirements and effects of interference are quite different in these circuits. It’s necessary to have a good understanding of the major differences between the two circuit designs while optimizing the PCB for better performance.

The signal value for a digital circuit is always binary, whereas the analog signal varies over a range of minimum to a maximum value. This provides a larger error margin in digital signal transmission, but the analog signals must be well-controlled during transmission and reception. Hence, designing an analog circuit may be comparatively difficult and needs a better understanding of the signal transmission.

Usually, the external signal received by a sensor is analog and it must be converted to a digital signal for processing. A typical application will be a hybrid circuit that involves both analog and digital circuitry. As a result, the layout design should be isolating the two sections to avoid interference.

PCB Design Guidelines for Analog Circuits

Component placement must be compact and analog signal routing should be short. Any noisy analog component should be placed in the center and not near the board edges. This strategy helps in absorbing the noise by the analog GND plane and avoids interference with the digital circuit.
It’s highly recommended to place and route analog and digital circuits separately and avoid any high-speed digital routing in the analog section.
There should be a dedicated analog GND plane. The return path of analog signals shouldn’t be interrupted by vias, cut-outs, or plane splits. If the return signal doesn’t have the shortest distance, then noise will be generated.

PCB Design Guidelines for Digital Circuits

Component placement of high-speed circuitry should be done strategically by considering the PCB assembly requirements as well. Enough spacing for automated placement and soldering should be considered during the component placement stage itself.
Choice of via size and shape must be based on the signal type and its electrical and mechanical requirements. Typically, power and ground vias will be larger compared to signal vias. High-speed signal vias can be blind or buried vias. Micro vias also are employed based on electrical requirements.
Trace routing is crucial for differential signals and high-speed clock signals. Factors like the layer thickness in PCB stackup, track width, and return path should be designed as per the standard recommendation.
Electromagnetic-interference (EMI) shielding, heat dissipation, and noise filtering depend on a good design of reference planes. The power and ground planes should be balanced and planned for the power and signal integrity of the PCB.

Differences Between Analog and Digital Circuit PCB Design

Digital circuits are more immune to noise interference as compared to analog circuits. The binary value in digital circuits can accept a wide range of data as valid input and will work satisfactorily, whereas analog circuits are highly susceptible to even slight noise interference. Also, the analog circuit involves a sequence of operations like modulation, amplification, transmission, reception, demodulation, etc., wherein noise interference can occur in any of these stages. Ensuring the required signal-to-noise ratio and reducing crosstalk adds complexity to an analog circuit.
The requirement of efficiency and accuracy makes the analog circuit design intricate and expensive. Digital circuits are comparatively easy to design, with many automated tools available in the market. This makes digital PCBs cost-effective and precise.
Since the analog circuit can directly interact with the physical world and doesn’t need any conversion, there’s no loss of information. But the digital circuit uses analog-to-digital converter (ADC) and digital-to-analog converter (DAC) circuitry, which may cause data loss and degradation of the information. Hence, the layout and routing of these signals should follow stringent guidelines.
In a mixed-signal circuit, the digital signals are the strongest source of interference for analog signals as they contain high harmonic frequency. High clock signals and switching power-supply circuits should be designed cautiously in mixed-signal PCBs to avoid noise interference in analog circuit operation.
Sensitive analog circuits require appropriate frequency terminations. To avoid space radiation, the analog circuit should be placed far from the digital circuit; a shield box can be used to guard against electromagnetic interference.
Ground plane design is critical in the analog circuit because the return signal path should be uninterrupted to avoid any interference. This isn’t a concern with digital circuits, though, as they’re tolerant to noise in the ground plane.
In the PCB design, parasitic components like parasitic capacitance and parasitic inductance can cause a problem when traces are routed close to each other. If the change in voltage is large enough in one trace, then 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’s recommended to reduce the inductive reactance of digital circuit traces and capacitive coupling of an analog circuit to avoid noise from electromagnetic sources.

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

There are several factors to consider while designing sensitive analog circuits and high-speed digital circuits in a PCB design. Many contract manufacturers (CMs) are experienced in supporting and building both analog and digital PCBs. They also will 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, since they’re aware of the different strategies required in analog and digital PCB design.