WM9082

Production Data

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PD, August 2012, Rev 4.1

20

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 WM9082

device as possible, with current loop areas kept as small as possible.

CLASS D LOUDSPEAKER CONNECTION

Long, exposed PCB tracks or connection wires will emit EMI. The distance between the WM9082 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 WM9082

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 8.

EMI

LOW ESR

LOW ESR

Long, exposed tracks emit EMI

Short connection wires will reduce EMI emission

Shielding using PCB ground (or VDD) planes

will reduce EMI emission

LC filtering will reduce EMI emission

up to around 30MHz

Ferrite beads will reduce EMI emission

at frequencies above 30MHz.

WM9082

WM9082

WM9082

WM9082

WM9082

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Figure 8 EMI Reduction Techniques