WM8731 / WM8731L
Production Data
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 14. 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.
VMID
MICBIAS
AGND
Figure 14 Microphone Bias Schematic
ADC
The WM8731/L uses a multi-bit oversampled sigma-delta ADC. A single channel of the ADC is
illustrated in the Figure 15.
FROM MICROPHONE
INPUT
ANALOG
INTEGRATOR
TO ADC DIGITAL FILTERS
FROM LINE INPUT
MULTI
BITS
INSEL
Figure 15 Multi-Bit Oversampling Sigma Delta ADC Schematic
PD, Rev 4.9, October 2012
25
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
