Microphone Biasing Circuit - Cirrus Logic WM8940 Manual

24
The Auxiliary amplifier path to the BOOST stage is controlled by the AUX2BOOSTVOL[2:0] register
bits. When AUX2BOOSTVOL=000 this path is completely disconnected from the BOOST stage.
Settings 001 through to 111 control the gain in 3dB steps from -12dB to +6dB.
The MICP path to the BOOST stage is controlled by the MICP2BOOSTVOL[2:0] register bits. When
MICP2BOOSTVOL=000 this input pin is completely disconnected from the BOOST stage. Settings
001 through to 111 control the gain in 3dB steps from -12dB to +6dB.
REGISTER BIT
ADDRESS
R47 6:4 MICP2BOOSTVOL
Input BOOST
control
2:0 AUX2BOOSTVOL
Table 7 Input BOOST Stage Control
The BOOST stage is enabled under control of the BOOSTEN register bit.
REGISTER BIT LABEL
ADDRESS
R2 4 BOOSTEN
Power
management 2
Table 8 Input BOOST Enable Control

MICROPHONE BIASING CIRCUIT

The MICBIAS output provides a low noise reference voltage suitable for biasing electret type
microphones and the associated external resistor biasing network. Refer to the Applications
Information section for recommended external components. The MICBIAS voltage can be altered via
the MBVSEL register bit.
MICBIAS=0.65*AVDD. The output can be enabled or disabled using the MICBEN control bit.
REGISTER BIT
ADDRESS
R1 4
Power
management 1
Table 9 Microphone Bias Enable
REGISTER BIT
ADDRESS
R44 8
Input Control
Table 10 Microphone Bias Voltage Control
LABEL DEFAUL
T
000 Controls the MICP pin to the input boost
stage (NB, when using this path set
MICP2INPPGA=0):
000=Path disabled (disconnected)
001=-12dB gain through boost stage
010=-9dB gain through boost stage
...
111=+6dB gain through boost stage
000 Controls the auxiliary amplifier to the input
boost stage:
000=Path disabled (disconnected)
001=-12dB gain through boost stage
010=-9dB gain through boost stage
...
111=+6dB gain through boost stage
DEFAULT
0 Input BOOST enable
0 = Boost stage OFF
1 = Boost stage ON
When MBVSEL=0, MICBIAS=0.9*AVDD and when MBVSEL=1,
LABEL DEFAULT
MICBEN 0 Microphone Bias Enable
0 = OFF (high impedance output)
1 = ON
LABEL DEFAULT
MBVSEL 0 Microphone Bias Voltage Control
0 = 0.9 * AVDD
1 = 0.65 * AVDD
WM8940
DESCRIPTION
DESCRIPTION
DESCRIPTION
DESCRIPTION
Rev 4.4