WM8945
Production Data
APPLICATIONS INFORMATION
RECOMMENDED EXTERNAL COMPONENTS
AUDIO INPUT PATHS
The WM8945 provides up to 4 analogue audio inputs (including the auxiliary inputs AUX1 and AUX2).
Each of these inputs is referenced to the internal DC reference, VMID. A DC blocking capacitor is
required for each input pin used in the target application. The choice of capacitor is determined by the
filter that is formed between that capacitor and the input impedance of the input pin. The circuit is
illustrated in Figure 58. (Note that capacitors are not required on any unused audio input.)
Figure 58 Audio Input Path DC Blocking Capacitor
When the input impedance is known, and the cut-off frequency is known, then the minimum capacitor
value may be derived easily. For practical use, a 1F capacitance for all audio inputs can be
recommended for most cases. Tantalum electrolytic capacitors are particularly suitable as they offer
high stability in a small package size.
Ceramic equivalents are a cost effective alternative to the superior tantalum packages, but care must
be taken to ensure the desired capacitance is maintained at the LDOVOUT operating voltage. Also,
ceramic capacitors may show microphonic effects, where vibrations and mechanical conditions give
rise to electrical signals. This is particularly problematic for microphone input paths where a large
signal gain is required.
A single capacitor is required for a line input or single-ended microphone connection. In the case of a
differential microphone connection, a DC blocking capacitor is required on both input pins.
HEADPHONE / LINE OUTPUT PATHS
The WM8945 provides three outputs (LINEOUTL, SPKOUTL and SPKOUTR). Each of these outputs
is referenced to the internal DC reference, VMID. In any case where a line output is used in a single-
ended configuration (i.e. referenced to GND), a DC blocking capacitor is required in order to remove
the DC bias. In the case where a pair of line outputs is configured as a BTL differential pair, then the
DC blocking capacitor should be omitted.
The choice of capacitor is determined from the filter that is formed between the capacitor and the load
impedance. A 1F capacitance would be a suitable choice for a line or headphone load. Tantalum
electrolytic capacitors are again particularly suitable but ceramic equivalents are a cost effective
alternative. Care must be taken to ensure the desired capacitance is maintained at the appropriate
operating voltage.
Figure 59 DC-blocking Components for Line Output
PD, May 2011, Rev 4.1
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
