WM8904
Pre-Production
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
REGISTER
ADDRESS
BIT
LABEL
DEFAULT
DESCRIPTION
MICBIAS Enable
R6 (06h)
0
MICBIAS_ENA
0
Mic Bias
Control 0
0 = disabled
1 = enabled
R7 (07h)
Mic Bias
Control 1
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)
2:0
MICBIAS_SEL [2:0]
000
Note that the voltage scales with
AVDD. The value quoted in
brackets is correct for AVDD=1.8V.
Table 6 MICBIAS Control
PP, Rev 3.3, September 2012
42
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
