WM8904ECSSLASHR datasheet(42/188 Pages) WOLFSON

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WM8904

Pre-Production

PP, Rev 3.3, September 2012

ELECTRET CONDENSER MICROPHONE INTERFACE

Electret Condenser microphones may be connected as single-ended or differential inputs to the Input

PGAs described in the “Analogue Input Signal Path” section. The WM8904 provides a low-noise

reference voltage (MICBIAS) suitable for biasing electret condenser microphones.

MICBIAS CONTROL

The MICBIAS reference is provided on the MICBIAS pin. This reference voltage is enabled by setting

the MICBIAS_ENA register bit.

The MICBIAS output voltage is selected using the MICBIAS_SEL register. This register selects the

output voltage as a ratio of AVDD; the actual output voltage scales with AVDD.

The MICBIAS output is powered from the MICVDD supply pin, and uses VMID (ie. AVDD/2) as a

reference, as illustrated in Figure 24. In all cases, MICVDD must be at least 200mV greater than the

required MICBIAS output voltage.

Under the default setting of MICBIAS_SEL, the MICVDD supply may be connected directly to AVDD.

For other settings of MICBIAS_SEL, (ie. for higher MICBIAS voltages), the MICVDD supply must be

greater than AVDD.

The MICBIAS generator is illustrated in in Figure 24. The associated control registers are defined in

Table 6.

Figure 24 MICBIAS Generator

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

R6 (06h)

Mic Bias

Control 0

MICBIAS_ENA

MICBIAS Enable

0 = disabled

1 = enabled

R7 (07h)

Mic Bias

Control 1

2:0

MICBIAS_SEL [2:0]

000

Selects MICBIAS voltage

000 = 9/10 x AVDD (1.6V)

001 = 10/9 x AVDD (2.0V)

010 = 7/6 x AVDD (2.1V)

011 = 4/3 x AVDD (2.4V)

100 to 111 = 3/2 x AVDD (2.7V)

Note that the voltage scales with

AVDD. The value quoted in

brackets is correct for AVDD=1.8V.

Table 6 MICBIAS Control

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