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Cellphone board layout for enhanced audio

Posted: 16 Nov 2007 ?? ?Print Version ?Bookmark and Share

Keywords:cellphone board design? enhanced audio quality? audio circuitry?

For PCB layout engineers, cellphones provide the ultimate challenge. Each subsystem has conflicting requirements, and modern cellphones include nearly every subsystem to be found in a portable device. A well-designed board must maximize the performance of each device connected to it and prevent the various systems from interfering with one another. Inevitably, the conflicting subsystem requirements result in compromise.

Although the audio functionality found in cellphones is increasing, the audio circuitry is often given the least consideration during board design. The recommendations here help ensure a well laid-out board that does not sacrifice audio quality.

Do

?Give careful consideration to the floorplan. The ideal floorplan will partition each type of circuitry into a different area.

? Use differential signals whenever possible. Audio devices with differential inputs reject noise.

? Keep ground currents separate to prevent digital currents from adding noise to analog circuits.

? Use star grounding for analog circuits. Audio power amps tend to draw large currents that can adversely affect both their own and other reference grounds.

? Flood unused board area with ground. By running ground flood near signal traces, unwanted high-frequency energy in the signal lines can be shunted to ground through the capacitive coupling.

Don't

?Mix circuitry on the board. Although the RF section of a cellphone is typically considered analog, noise coupled from the RF section into the audio circuitry can be demodulated into audible noise.

? Route analog audio signals long distances across the board. Long analog audio traces may pick up noise from digital and RF circuitry.

? Forget the importance of the ground return. A poorly grounded system will likely have high distortion, noise and crosstalk along with low RF immunity.

? Interrupt the natural return path of digital currents. This path yields the smallest loop area, minimizing antenna effects and inductance.

? Neglect the need to locate bypass capacitors as close as possible to the supply pin they are bypassing.

- Adrian Rolufs
Strategic Applications Engineer
Audio Products, Maxim Integrated Products Inc.




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