WM8971LEFL datasheet(19/56 Pages) WOLFSON

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Advanced Information

WM8971L

AI Rev 3.0 March 2004

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 output can be enabled or disables

using the MICB control bit (see also the “Power Management” section).

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

R25 (19h)

Power

Management (1)

MICB

Microphone Bias Enable

0 = OFF (high impedance output)

1 = ON

Table 5 Microphone Bias Control

The internal MICBIAS circuitry is shown below. Note that the is a maximum source current capability

for MICBIAS is 3mA. The external biasing resistors therefore must be large enough to limit the

MICBIAS current to 3mA.

AGND

MICBIAS

= 1.8 x VMID

= 0.9 X AVDD

VMID

internal

resistor

internal

resistor

MICB

Figure 6 Microphone Bias Schematic

PGA CONTROL

The PGA matches the input signal level to the ADC input range. The PGA gain is logarithmically

adjustable from +30dB to –17.25dB in 0.75dB steps. Each PGA can be controlled either by the user

or by the ALC function (see Automatic Level Control). When ALC is enabled for one or both

channels, then writing to the corresponding PGA control register has no effect.

The gain is independently adjustable on both Right and Left Line Inputs. However, by setting the

LIVU or RIVU bits whilst programming the PGA gain, both channels are simultaneously updated.

This reduces the required number of software writes required. Setting the LZCEN and RZCEN bits

enables a zero-cross detector which ensures that PGA gain changes only occur when the signal is at

zero, eliminating any zipper noise. If zero cross is enabled a timeout is also available to update the

gain if a zero cross does not occur. This function may be enabled by setting TOEN in register R23

(17h).

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