WM8944BECSSLASHR datasheet(160/165 Pages) CIRRUS

160 / 165 page

WM8944B

160

Rev 4.3

MICROPHONE BIAS CIRCUIT

The WM8944B 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 WM8944B 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 WM8944B is not exceeded. Cirrus Logic

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 55 and Figure 56.

IN1/

DMICDAT

PGA

MIC

2k2

4.7uF

MICBIAS

1uF

GND

AUX

Figure 55 Single-Ended Microphone Connection

IN1/

DMICDAT

PGA

MIC

2k2

4.7uF

MICBIAS

1uF

GND

1uF

AUX

Figure 56 Pseudo-Differential Microphone Connection

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