WM8994ECSSLASHR datasheet(353/363 Pages) CIRRUS

353 / 363 page

WM8994

Rev 4.6

353

CHARGE PUMP COMPONENTS

A fly-back capacitor is required between the CPCA and CPCB pins. The required capacitance is

2.2µF at 2V.

A decoupling capacitor is required on CPVOUTP and CPVOUTN; the recommended value is 2.2µF at

2V.

The positioning of the Charge Pump capacitors is important, particularly the fly-back capacitor. These

capacitors should be placed as close as possible to the WM8994.

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 applications, the use of ceramic capacitors with capacitor

dielectric X5R is recommended.

MICROPHONE BIAS CIRCUIT

The WM8994 is designed to interface easily with up to four analogue microphones. These may be

connected in single-ended or differential configurations, as illustrated in Figure 89. 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 MICBIAS1 or MICBIAS2.

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 WM8994 is not exceeded. Cirrus Logic

recommends a 2.2k

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

microphone models.

MIC

input

mixers

PGA

Line Input

VMID

IN1LN,

IN2LN,

IN1RN,

IN2RN

IN1LP,

IN2LP,

IN1RP,

IN2RP

MICBIAS1/2

2k2

4.7uF

AGND

MIC

input

mixers

PGA

Line Input

VMID

IN1LN,

IN2LN,

IN1RN,

IN2RN

IN1LP,

IN2LP,

IN1RP,

IN2RP

MICBIAS1/2

2k2

4.7uF

AGND

2k2

Figure 89 Single-Ended and Differential Analogue Microphone Connections

The WM8994 also supports up to four digital microphone inputs. The MICBIAS1 generator is suitable

for use as a low noise supply for digital microphones, as shown in Figure 90.

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