WM8731 datasheet(21/56 Pages) WOLFSON

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WM8731

Advanced Information

WOLFSON MICROELECTRONICS LTD

AI Rev 2.0 February 2001

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

MICBOOST

Microphone Input Level Boost

1 = Enable Boost

0 = Disable Boost

0000100

Analogue Audio

Path Control

MUTEMIC

Line Input Mute to ADC

1 = Enable Mute

0 = Disable Mute

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.

AGND

MICIN

MICBIAS

AGND

FROM

MICROPHONE

Rmic

Figure 12 Microphone Input and Bias Application Drawing

Recommended component values are C1 = 220pF (npo ceramic), C2 = 1

µF, 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.

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