PCB Layout
2-2
2.1
PCB Layout
The DAC7731 EVM is designed to preserve the performance quality of the
DAC under test, as specified in the data sheet. Carefully analyzing the EVM
physical restrictions and knowing the elements that contribute to the EVM
performance degradation are keys to a successful design implementation.
These obvious EVM performance degraders can be alleviated during the
schematic design phase by properly selecting the right components and
building the circuit correctly. The circuit should include adequate bypassing,
identifying and managing the analog and digital signals, and knowing and
understanding the components mechanical attributes.
The obscure part of the design is the layout process, where lack of knowledge
and inexperience can present a problem. The main concern here is primarily
with the placement of components and the proper routing of signals. The
bypass capacitors should be placed as close as possible to the pins and the
analog and digital signals must be properly separated from each other. The
power and ground plane is very important and are carefully considered in the
layout process. A solid plane is ideally preferred but sometimes impractical,
so when solid planes are not possible, a split plane does the job as well. When
considering a split-plane design, analyze the component placement and
carefully split the board into its analog and digital sections, starting from the
device under test. The ground plane plays an important role in controlling the
noise and other effects that otherwise contribute to the error of the DAC output.
To ensure that the return currents are handled properly, route the appropriate
signals only in their respective sections—meaning the analog traces should
only lay directly above or below the analog section and the digital traces in the
digital section. Minimize the length of the traces, but maximize trace widths
where possible in the design. This can be seen in the layout figures presented
on the following pages.
The DAC7731 EVM board is constructed on a four-layer printed-circuit board
using a copper-clad FR-4 laminate material. The printed-circuit board has a
dimension of 42,8625 mm (1.6875 inch)
×
80,9625 mm (3.1875 inch), and the
board thickness is 1,57 mm (0.062 inch). Figure 2 - 1 through Figure 2 - 5 show
the individual artwork layers.