lm4924sdx National Semiconductor Corporation, lm4924sdx Datasheet - Page 14

• Manufacturer Part Number: lm4924sdx
• Description: 2 Cell Battery, 40mw Per Channel Output Capacitor-less Ocl Stereo Headphone Audio Amplifier
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Information
PCB LAYOUT GUIDELINES
This section provides practical guidelines for mixed signal
PCB layout that involves various digital/analog power and
ground traces. Designers should note that these are only
"rule-of-thumb" recommendations and the actual results will
depend heavily on the final layout.
Minimization of THD
PCB trace impedance on the power, ground, and all output
traces should be minimized to achieve optimal THD perfor-
mance. Therefore, use PCB traces that are as wide as
possible for these connections. As the gain of the amplifier is
increased, the trace impedance will have an ever increasing
adverse affect on THD performance. At unity-gain (0dB) the
parasitic trace impedance effect on THD performance is
reduced but still a negative factor in the THD performance of
the LM4924 in a given application.
GENERAL MIXED SIGNAL LAYOUT
RECOMMENDATION
Power and Ground Circuits
For two layer mixed signal design, it is important to isolate
the digital power and ground trace paths from the analog
power and ground trace paths. Star trace routing techniques
(bringing individual traces back to a central point rather than
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daisy chaining traces together in a serial manner) can
greatly enhance low level signal performance. Star trace
routing refers to using individual traces to feed power and
ground to each circuit or even device. This technique will
require a greater amount of design time but will not increase
the final price of the board. The only extra parts required may
be some jumpers.
Single-Point Power / Ground Connections
The analog power traces should be connected to the digital
traces through a single point (link). A "PI-filter" can be helpful
in minimizing high frequency noise coupling between the
analog and digital sections. Further, place digital and analog
power traces over the corresponding digital and analog
ground traces to minimize noise coupling.
Placement of Digital and Analog Components
All digital components and high-speed digital signal traces
should be located as far away as possible from analog
components and circuit traces.
Avoiding Typical Design / Layout Problems
Avoid ground loops or running digital and analog traces
parallel to each other (side-by-side) on the same PCB layer.
When traces must cross over each other do it at 90 degrees.
Running digital and analog traces at 90 degrees to each
other from the top to the bottom side as much as possible will
minimize capacitive noise coupling and cross talk.