WM8974GEFLSLASHV datasheet(23/87 Pages) WOLFSON

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Production Data

WM8974

PD Rev 4.2 March 2007

INPUT LIMITER / AUTOMATIC LEVEL CONTROL (ALC)

The WM8974 has an automatic PGA gain control circuit, which can function as an input peak limiter

or as an automatic level control (ALC).

The Automatic Level Control (ALC) provides continuous adjustment of the input PGA in response to

the amplitude of the input signal. A digital peak detector monitors the input signal amplitude and

compares it to a register defined threshold level (ALCLVL).

If the signal is below the threshold, the ALC will increase the gain of the PGA at a rate set by

ALCDCY. If the signal is above the threshold, the ALC will reduce the gain of the PGA at a rate set

by ALCATK.

The ALC has two modes selected by the ALCMODE register: normal mode and peak limiter mode.

The ALC/limiter function is enabled by setting the register bit R32[8] ALCSEL.

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

2:0

ALCMIN

[2:0]

000 (-12dB)

Set minimum gain of PGA

000 = -12dB

001 = -6dB

010 = 0dB

011 = +6dB

100 = +12dB

101 = +18dB

110 = +24dB

111 = +30dB

5:3

ALCMAX

[2:0]

111 (+35.25dB) Set Maximum Gain of PGA

111 = +35.25dB

110 = +29.25dB

101 = +23.25dB

100 = +17.25dB

011 = +11.25dB

010 = +5.25dB

001 = -0.75dB

000 = -6.75dB

R32 (20h)

ALC Control 1

ALCSEL

ALC function select

0 = ALC disabled

1 = ALC enabled

R33 (21h)

ALC Control 2

3:0

ALCLVL

[3:0]

1011

(-12dB)

ALC target – sets signal level at ADC input

1111 = -6dBFS

1110 = -7.5dBFS

1101 = -9dBFS

1100 = -10.5dBFS

1011 = -12dBFS

1010 = -13.5dBFS

1001 = -15dBFS

1000 = -16.5dBFS

0111 = -18dBFS

0110 = -19.5dBFS

0101 = -21dBFS

0100 = -22.5dBFS

0011 = -24dBFS

0010 = -25.5dBFS

0001 = -27dBFS

0000 = -28.5dBFS

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