WM8946ECSSLASHR datasheet(168/175 Pages) WOLFSON

168 / 175 page

WM8946

Production Data

PD, July 2012, Rev 4.3

168

APPLICATIONS INFORMATION

RECOMMENDED EXTERNAL COMPONENTS

AUDIO INPUT PATHS

The WM8946 provides up to 6 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 55. (Note that capacitors are not required on any unused audio input.)

Figure 55 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 1

F 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 WM8946 provides four outputs (LINEOUTL, LINEOUTR, 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 1

F capacitance would be a suitable choice for a line load. For a headphone load a

larger value (100uF for a 32 ohm load or 220uF for a 16 ohm load) would be required. 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.

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