WM8788 Preliminary Technical Data
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PTD, December 2011, Rev 2.3
18
APPLICATIONS INFORMATION
RECOMMENDED EXTERNAL COMPONENTS
AUDIO INPUT PATHS
The WM8788 provides 2 analogue audio inputs. The maximum analogue input signal level varies with
AVDD and with the input circuit configuration, as described in the following equation:
It is recommended that the resistors R
IN
and R
FB
should not exceed 10k.
For 1Vrms (0dBV) maximum input signal level when AVDD = 3.3V, the recommended input
components are R
IN
= 2k, R
FB
= 2k.
For 2Vrms (6dBV) maximum input signal level when AVDD = 3.3V, the recommended input
components are R
IN
= 2k, R
FB
= 1k.
The input pins INL and INR are referenced to the internal DC reference, VMID. A DC blocking
capacitor is required for each input. The choice of capacitor is determined by the filter that is formed
between that capacitor and the input resistor, R
IN
. The circuit is illustrated in Figure 14.
Figure 14 Input Signal Path External Components
It is recommended that an input capacitor is selected such that the Fc cut-off frequency is less than
20Hz. It is recommended that a 4.7F capacitance is used for all WM8788 input connections.
Tantalum electrolytic capacitors are particularly suitable as they offer high stability in a small package
size.
See Wolfson Applications Note WAN_0176 for further guidance on the choice of capacitor types.
POWER SUPPLY DECOUPLING
Electrical coupling exists particularly in digital logic systems where switching in one sub-system
causes fluctuations on the power supply. This effect occurs because the inductance of the power
supply acts in opposition to the changes in current flow that are caused by the logic switching. The
resultant variations (or ‘spikes’) in the power supply voltage can cause malfunctions and unintentional
behavior in other components. A decoupling (or ‘bypass’) capacitor can be used as an energy storage
component which will provide power to the decoupled circuit for the duration of these power supply
variations, protecting it from malfunctions that could otherwise arise.
