WM8945ECSSLASHR datasheet(164/169 Pages) WOLFSON

164 / 169 page

WM8945

Production Data

PD, May 2011, Rev 4.1

164

Due to the wide tolerance of many types of ceramic capacitors, care must be taken to ensure that the

selected components provide the required capacitance across the required temperature and voltage

ranges in the intended application. For most application the use of ceramic capacitors with capacitor

dielectric X5R is recommended.

MICROPHONE BIAS CIRCUIT

The WM8945 is designed to interface easily with electret microphones. These may be connected in

single-ended or differential configurations. The single-ended method allows greater capability for the

connection of multiple audio sources simultaneously, whilst the differential method provides better

performance due to its rejection of common-mode noise.

In either configuration, the microphone requires a bias current (electret condenser microphones) or

voltage supply (silicon microphones), which can be provided by MICBIAS. This reference is

generated by an output-compensated amplifier, which requires an external capacitor in order to

guarantee accuracy and stability. The recommended capacitance is 4.7

F, although it may be

possible to reduce this to 1

F if the analogue supply (LDOVOUT) is not too noisy. A ceramic type is a

suitable choice here, providing that care is taken to choose a component that exhibits this

capacitance at the intended MICBIAS voltage.

Note that the MICBIAS voltage may be adjusted using register control to suit the requirements of the

microphone. Also note the WM8945 supports a maximum current of 3mA. If more than one

microphone is connected to the MICBIAS, then combined current must not exceed 3mA.

A current-limiting resistor is also required when using an electret condenser microphone (ECM). The

resistance should be chosen according to the minimum operating impedance of the microphone and

MICBIAS voltage so that the maximum bias current of the WM8945 is not exceeded. Wolfson

recommends a 2.2k

 current limiting resistor as it provides compatibility with a wide range of

microphone models.

The recommended connections for single-ended and differential microphone modes are illustrated in

Figure 61 and Figure 62.

Figure 61 Single-Ended Microphone Connection

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