WM8998 datasheet(41/307 Pages) WOLFSON

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WM8998

PD, October 2014, Rev 4.0

ANALOGUE MICROPHONE INPUT

Up to six analogue microphones can be connected to the WM8998, either in single-ended or

differential mode. The applicable mode, and input pin selection, is controlled using the INnx_SRC

registers, as described later.

The WM8998 includes external accessory detection circuits, which can detect the presence of a

microphone, and the status of a hookswitch or other push-buttons. When using this function, it is

recommended to use the IN2B analogue microphone input paths, to ensure best immunity to electrical

transients arising from the push-buttons.

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

(INnLP, INnRP, or INnP). The inverting inputs of the PGAs are connected to an internal reference in

this configuration.

For differential input, the non-inverted microphone signal is connected to the non-inverting input of the

PGAs (INnLP, INnRP, or INnP), whilst the inverted (or ‘noisy ground’) signal is connected to the

inverting input pins (INnLN, INnRN, or INnN).

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

impedance of the analogue input paths is fixed across all PGA gain settings.

The Electret Condenser Microphone (ECM) analogue input configurations are illustrated in Figure 11

and Figure 12. The integrated MICBIAS generators provide a low noise reference for biasing the

ECMs.

To ADC

PGA

VMID

IN1AxN,

IN1BxN,

IN2AN,

IN2BN

IN1AxP,

IN1BxP,

IN2AP,

IN2BP

MIC

GND

MICBIAS

To ADC

PGA

VMID

IN1AxN,

IN1BxN,

IN2AN,

IN2BN

IN1AxP,

IN1BxP,

IN2AP,

IN2BP

MIC

GND

MICBIAS

Figure 11 Single-Ended ECM Input

Figure 12 Differential ECM Input

Analogue MEMS microphones can be connected to the WM8998 in a similar manner to the ECM

configurations described above; typical configurations are illustrated in Figure 13 and Figure 14. In

this configuration, the integrated MICBIAS generators provide a low-noise power supply for the

microphones.

To ADC

PGA

VREF

IN1AxN,

IN1BxN,

IN2AN,

IN2BN

IN1AxP,

IN1BxP,

IN2AP,

IN2BP

MEMS

Mic

MICBIAS

VDD

OUT

GND

To ADC

PGA

VREF

IN1AxN,

IN1BxN,

IN2AN,

IN2BN

IN1AxP,

IN1BxP,

IN2AP,

IN2BP

MEMS

Mic

MICBIAS

VDD

OUT-P

GND

OUT-N

Figure 13 Single-Ended MEMS Input

Figure 14 Differential MEMS Input

Note that the MICVDD pin can also be used (instead of MICBIASn) as a reference or power supply for

external microphones. The MICBIAS outputs are recommended, as these offer better noise

performance and independent enable/disable control.

WM8998 Datasheet(PDF) 41 Page - Wolfson Microelectronics plc

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