WM8994 Datasheet, PDF (356/359 Pages) Wolfson Microelectronics plc – Multi-Channel Audio Hub CODEC for Smartphones

WM8994
Production Data
PCB LAYOUT CONSIDERATIONS
Poor PCB layout will degrade the performance and be a contributory factor in EMI, ground bounce
and resistive voltage losses. All external components should be placed as close to the WM8994
device as possible, with current loop areas kept as small as possible. Specific factors relating to
Class D loudspeaker connection are detailed below.
CLASS D LOUDSPEAKER CONNECTION
Long, exposed PCB tracks or connection wires will emit EMI. The distance between the WM8994 and
the loudspeaker should therefore be kept as short as possible. Where speakers are connected to the
PCB via a cable form, it is recommended that a shielded twisted pair cable is used. The shield should
be connected to the main system, with care taken to ensure ground loops are avoided.
Further reduction in EMI can be achieved using PCB ground (or VDD) planes and also by using
passive LC components to filter the Class D switching waveform. When passive filtering is used, low
ESR components should be chosen in order to minimise the series resistance between the WM8994
and the speaker, maximising the power efficiency.
LC passive filtering will usually be effective at reducing EMI at frequencies up to around 30MHz. To
reduce emissions at higher frequencies, ferrite beads can also be used. These should be positioned
as close to the device as possible.
These techniques for EMI reduction are illustrated in Figure 100.
Figure 100 EMI Reduction Techniques
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PD, April 2012, Rev 4.4
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