WM8788 Datasheet, PDF (18/22 Pages) Wolfson Microelectronics plc – High Performance 24-bit Stereo ADC

WM8788
Preliminary Technical Data
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 RIN and RFB should not exceed 10kΩ.
For 1Vrms (0dBV) maximum input signal level when AVDD = 3.3V, the recommended input
components are RIN = 2kΩ, RFB = 2kΩ.
For 2Vrms (6dBV) maximum input signal level when AVDD = 3.3V, the recommended input
components are RIN = 2kΩ, RFB = 1kΩ.
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, RIN. 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 4.7μF 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.
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PTD, December 2010, Rev 2.2
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