Preliminary Technical Data WM8788
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PTD, December 2011, Rev 2.3
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Coupling also occurs in a lower frequency form when ripple is present on the power supply rail
caused by changes in the load current or by limitations of the power supply regulation method. In
audio components such as the WM8788, these variations can alter the performance of the signal
path, leading to degradation in signal quality. A decoupling (or ‘bypass’) capacitor can be used to filter
these effects, by presenting the ripple voltage with a low impedance path that does not affect the
circuit to be decoupled.
These coupling effects are addressed by placing a capacitor between the supply rail and the
corresponding ground reference. In the case of systems comprising multiple power supply rails,
decoupling should be provided on each rail.
The recommended power supply decoupling capacitors for WM8788 are listed below in Table 5.
POWER SUPPLY DECOUPLING CAPACITOR
AVDD 4.7F ceramic
REFVDD 4.7F ceramic
VMIDC 4.7F ceramic
Table 5 Power Supply Decoupling Capacitors
All decoupling capacitors should be placed as close as possible to the WM8788 device.
The VMIDC capacitor is not, technically, a decoupling capacitor. However, it does serve a similar
purpose in filtering noise on the VMID reference. The connection between GND, the VMID decoupling
capacitor and the main system ground should be made at a single point as close as possible to the
GND pin of the WM8788.
Due to the wide tolerance of many types of ceramic capacitors, care must be taken to ensure that the
selected components provide the required capacitance across the required temperature and voltage
ranges in the intended application. For most application the use of ceramic capacitors with capacitor
dielectric X5R is recommended.
See Wolfson Applications Note WAN_0176 for further guidance on the choice of capacitor types.
RECOMMENDED EXTERNAL COMPONENTS DIAGRAM
Figure 15 provides a summary of recommended external components for WM8788. Note that the
actual requirements may differ according to the specific target application.
Figure 15 WM8788 Recommended External Components Diagram
