Page 16 - EngineerIT October 2022

ELECTRONICS

Basic guidelines for layout design

of mixed-signal PCBs

By May Anne Porley, applications engineer, and Kevin Chesser, product application engineer

his article will detail what to consider when designing the board, while auxiliary components such as decoupling
the layout of mixed-signal PCBs. It covers component capacitors and crystals must be placed as close as possible to
Tplacement, board layering and ground plane the mixed-signal device.
considerations. The guidelines discussed in this article provide
a practical approach to the layout design of mixed-signal boards Analogue and digital blocks partition
and should assist engineers of all backgrounds. To help minimise the common return path for analogue and
digital signals, analogue and digital block separation may be
Introduction considered, such that analogue signals will not mix with digital
A mixed-signal PCB design requires basic understanding of signals.
analogue and digital circuitry to minimise, if not prevent, signal
interference. Modern systems consist of components that
are operational with both digital and analogue domains, and
these must be carefully designed to ensure signal integrity all
throughout the system.
PCB layout, as an important part of the mixed-signal
development process, can be intimidating and component
placement is just the beginning. There are also other factors
that must be considered, including board layers and how to
properly manage these to minimise interferences caused by
parasitic capacitors that can be unintentionally created between Figure 1: Partition of analogue and digital circuits.
the interplane layers of the PCB.
Grounding is also an integral process in the PCB layout Figure 1 shows a good example of analogue and digital circuit
design of a mixed-signal system. While grounding is a frequently separation. Some considerations when separating analogue and
debated topic in the industry, constructing a standardised digital sections are:
approach may not always be the simplest task for any engineer. • It is recommended that sensitive analogue components such
For instance, a single issue of quality grounding can affect the as amplifiers and voltage references be placed within the
entire layout of a high performance, mixed-signal PCB design, analogue plane. Similarly, the noisy digital components such
and for this reason, this area should not be overlooked. as logic controls and timing blocks must be placed on the
other side/digital plane.
Component Placement • If a system contains one mixed-signal analogue-to-digital

Similar to building a house, it is essential to create a floor plan converter (ADC) or a digital-to-analogue converter (DAC) with
of the system before placing the circuit components. This step low digital currents, this can be treated similarly to analogue
will set the overall integrity of the system design and should components that are included in the analogue plane.
help avoid noisy signal interference. • For designs with more than one ADC and DAC with high
In developing a floor plan, it is advisable to follow the signal current, splitting the digital and analogue supplies is
path of the schematic, especially for high-speed circuits. The recommended. That is, AV CC must be tied to the analogue
location of components is also a critical aspect of the design. section while DV DD should be connected to the digital section.
The designer should be able to identify the important functional • Micro-processors and micro-controllers may take up space
block, signals, and the connection between the blocks to and will generate heat. These components must be placed in
identify the best fit location of each component in the system. the centre of the board for better thermal dissipation, and at
Connectors, for instance, are better placed on the edges of the same time, close to their related circuit blocks.

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