Datasheet

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6 PCB Design and Performance
6.1 PCB Layout
PCB Design and Performance
This section covers the layout design of the PCB, describing the physical and mechanical characteristics
of the EVM. This section also shows the resulting performance of the EVM which can be compared to the
device specification listed in the data sheet. The list of components used on the module is also included in
this section.
The DAC7631 EVM is designed to demonstrate the performance quality of the installed DAC device under
test, as specified in the data sheet. Careful analysis of the EVM’s physical restrictions and factors that
contributes to the EVM’s performance degradation is the key to a successful design implementation. The
obvious attributes that contributes to the poor performance of the EVM can be avoided during the
schematic design phase by properly selecting the right components and designing the circuit correctly.
The circuit should include adequate bypassing, identifying and managing the analog and digital signals,
and knowing or understanding the components mechanical attributes.
The obscure part of the design lies particularly in the layout process. The main concern 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 should be properly separated from each
other. The power and ground plane is important and should be 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 contributes 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 use the
biggest possible trace width allowable in the design. These design practices can be seen in the following
figures.
The DAC7631 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 43,1800 mm (1.7000 inch) X 82,5500 mm
(3.2000 inch), and the board thickness is 1,5748 mm (0.0620 inch). Figure 3 through Figure 6 show the
individual artwork layers.
Figure 2. Top Silkscreen
DAC7631EVMSLAU148 – January 2005 5