WM8948ECSSLASHR datasheet(182/189 Pages) WOLFSON

182 / 189 page

WM8948

Production Data

PD, May 2011, Rev 4.1

182

APPLICATIONS INFORMATION

RECOMMENDED EXTERNAL COMPONENTS

AUDIO INPUT PATHS

The WM8948 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 59. (Note that capacitors are not required on any unused audio

input.)

Figure 59 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

μF 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 WM8948 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

μF 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.

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