WM8731 Wolfson Microelectronics Ltd., WM8731 Datasheet - Page 22

• Manufacturer Part Number: WM8731
• Description: WM8731 : Low Power Audio Codec With Integrated Headphone Driver
• Manufacturer: Wolfson Microelectronics Ltd.

WM8731
The software control for the MICIN is shown in Table 3. Note that the Microphone Mute only mutes
the input to the ADC, this will still allow the Microphone Input signal to pass to the line output in
Sidetone Mode.
Table 3 Microphone Input Software Control
The microphone input is biased internally through the operational amplifier to VMID. Whenever the
line inputs are muted the MICIN input is kept biased to VMID using special anti-thump circuitry. This
reduces any audible clicks that may otherwise be heard when re-activating the input.
The application drawing for the microphone is shown in Figure 12.
Figure 12 Microphone Input and Bias Application Drawing
Recommended component values are C1 = 220pF (npo ceramic), C2 = 1 F, R1 = 680 ohms, R2 =
47k. Rmic values depends on gain setting (see above).
R1 and R2 form part of the biasing network (refer to Microphone Bias section below). R1 connected
to MICBIAS is necessary only for electret type microphones that require a voltage bias. R2 should
always be present to prevent the microphone input from charging to a high voltage which may
damage the microphone on connection. R1 and R2 should be large so as not to attenuate the signal
from the microphone, which can have source impedance greater than 2k. C1 together with the
source impedance of the microphone and the input impedance of MICIN forms an RF filter. C2 is a
DC blocking capacitor to allow the microphone to be biased at a different DC voltage to the MICIN
signal.
MICROPHONE BIAS
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 Microphone Input
section for an application drawing and further description.
The scheme for MICBIAS is shown in Figure 13. Note that there is a maximum source current
capability of 3mA available for the MICBIAS. This limits the smallest value of external biasing
resistors that can safely be used.
Note that the MICBIAS output is not active in standby mode.
0000100
Analogue Audio
Path Control
REGISTER
ADDRESS
MICROPHONE
FROM
0
1
BIT
AGND
MICBOOST
MUTEMIC
LABEL
R1
R2
AGND
0
1
DEFAULT
AGND
C1
C2
Microphone Input Level Boost
1 = Enable Boost
0 = Disable Boost
Line Input Mute to ADC
1 = Enable Mute
0 = Disable Mute
MICBIAS
DESCRIPTION
Rmic
AI Rev 2.3 August 2001
Advanced Information
MICIN
22