WM8731SEDSSLASHR datasheet(25/63 Pages) WOLFSON

25 / 63 page

WM8731 / WM8731L

Production Data

PD Rev 4.3 August 2006

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

FROM LINE INPUT

ANALOG

INTEGRATOR

INSEL

MULTI

BITS

TO ADC DIGITAL FILTERS

Figure 15 Multi-Bit Oversampling Sigma Delta ADC Schematic

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