WM8994ECSSLASHR datasheet(45/363 Pages) CIRRUS

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WM8994

Rev 4.6

MICROPHONE INPUTS

Up to four microphones can be connected to the WM8994, either in single-ended or differential mode.

A dedicated PGA is provided for each microphone input. Two low noise microphone bias circuits are

provided, reducing the need for external components.

For single-ended microphone inputs, the microphone signal is connected to the inverting input of the

PGAs (IN1LN, IN2LN, IN1RN or IN2RN). The non-inverting inputs of the PGAs are internally

connected to VMID in this configuration. The non-inverting input pins IN1LP, IN2LP, IN1RP and

IN2RP are free to be used as line connections to the input or output mixers in this configuration.

For differential microphone inputs, the non-inverted microphone signal is connected to the non-

inverting input of the PGAs (IN1LP, IN2LP, IN1RP or IN2RP)

, whilst the inverted (or ‘noisy ground’)

signal is connected to the inverting input pins (IN1LN, IN2LN, IN1RN and IN2RN).

The gain of the input PGAs is controlled via register settings, as defined in Table 4. Note that the input

impedance of both inverting and non-inverting inputs changes with the input PGA gain setting, as

described unde

r “Electrical Characteristics”. See also the “Applications Information” for details of input

resistance at all PGA Gain settings.

The microphone input configurations are illustrated in Figure 13 and Figure 14. Note that any PGA

input pin that is used in either microphone configuration is not available for use as a line input path at

the same time.

MIC

GND

input

mixers

PGA

VMID

MICBIAS

IN1LN,

IN2LN,

IN1RN,

IN2RN

IN1LP,

IN2LP,

IN1RP,

IN2RP

MIC

GND

input

mixers

PGA

VMID

MICBIAS

IN1LN,

IN2LN,

IN1RN,

IN2RN

IN1LP,

IN2LP,

IN1RP,

IN2RP

Figure 13 Single-Ended Microphone Input

Figure 14 Differential Microphone Input

MICROPHONE BIAS CONTROL

There are two MICBIAS generators which provide low noise reference voltages suitable for powering

silicon (MEMS) microphones or biasing electret condenser (ECM) type microphones via an external

resistor.

The MICBIAS voltages can be independently enabled using the MICB1_ENA and MICB2_ENA control

bits; the voltage of each can be selected using the MICB1_LVL and MICB2_LVL register bits as

detailed in Table 1.

An external decoupling capacitor is required on each of the MICBIAS outputs, and must be connected

whenever the associated MICBIAS output is enabled. Enabling either of the MICBIAS outputs with no

external capacitor may result in degraded device performance and is not recommended. Refer to the

“Applications Information” section for recommended external components.

When a MICBIAS output is disabled, the output pin can be configured to be floating or to be actively

discharged. This is selected using the MICB1_DISCH and MICB2_DISCH register btis.

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