WM8959
Pre-Production
The distance between the WM8959 and the speakers should be kept to a minimum to reduce series
resistance, and also to reduce EMI. Further reductions in EMI can be achieved by additional passive
filtering and/or shielding as shown in Figure 99. When additional passive filtering is used, low ESR
components should be chosen to minimise series resistance between the WM8959 and the speaker,
maximising 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 placed as close to the device as possible will
be more effective.
SPKP
SPKN
SPKP
SPKN
EMI
WM8959
WM8959
WM8959
WM8959
WM8959
Short connection reduces EMI
Long, exposed tracks emit more EMI
LOW ESR
LOW ESR
SPKP
SPKP
SPKN
SPKN
LC Filtering
reduces EMI
Shielding using PCB ground
plane (or Vdd) reduces EMI
SPKP
SPKN
LC filtering is more effective at removing EMI at
frequencies below ~30MHz
Ferrite beads are more effective at removing
EMI at frequencies above ~30MHz
Ferrite beads reduce EMI
Figure 99 EMI Reduction Techniques
PP, May 2008, Rev 3.1
152
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
