WM8281ECSSLASHR datasheet(47/392 Pages) CIRRUS

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WM8281

Rev 4.0

ANALOGUE MICROPHONE INPUT

Up to six analogue microphones can be connected to the WM8281, either in single-ended or

differential mode. The applicable mode is selected using the INn_MODE registers, as described later.

Note that the mode is configurable for each stereo pair of inputs; the Left and Right channels of any

pair of inputs are always in the same mode.

The WM8281 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 one of the Right channel 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 or INnRP). 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 or INnRP)

, whilst the inverted (or ‘noisy ground’) signal is connected to the inverting

input pins (INnLN or INnRN).

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 14

and Figure 15. The integrated MICBIAS generators provide a low noise reference for biasing the

ECMs.

To ADC

PGA

VMID

IN1xN,

IN2xN,

IN3xN

IN1xP,

IN2xP,

IN3xP

MIC

GND

MICBIAS

To ADC

PGA

VMID

IN1xN,

IN2xN,

IN3xN

IN1xP,

IN2xP,

IN3xP

MIC

GND

MICBIAS

Figure 14 Single-Ended ECM Input

Figure 15 Differential ECM Input

Analogue MEMS microphones can be connected to the WM8281 in a similar manner to the ECM

configurations described above; typical configurations are illustrated in Figure 16 and Figure 17. In

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

microphones.

To ADC

PGA

VREF

IN1xN,

IN2xN,

IN3xN

IN1xP,

IN2xP,

IN3xP

MEMS

Mic

MICBIAS

VDD

OUT

GND

To ADC

PGA

VREF

IN1xN,

IN2xN,

IN3xN

IN1xP,

IN2xP,

IN3xP

MEMS

Mic

MICBIAS

VDD

OUT-P

GND

OUT-N

Figure 16 Single-Ended MEMS Input

Figure 17 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.

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