WM8776 datasheet(35/55 Pages) WOLFSON

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WM8776

PP Rev 1.91 June 2004

MAXIMUM GAIN (ALC ONLY) AND MAXIMUM ATTENUATION

To prevent low level signals being amplified too much by the ALC, the MAXGAIN register

sets the upper limit for the gain. This prevents low level noise being over-amplified. The

MAXGAIN register has no effect on the limiter operation.

The MAXATTEN register has different operation for the limiter and for the ALC. For the

limiter it defines the maximum attenuation below the static (user programmed) gain. For the

ALC, it defines the lower limit for the gain.

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

R16 (10h)

0010000

ALC Control 1

6:4

MAXGAIN

111

(+24dB)

Set maximum gain for the PGA (ALC

only)

111 : +24dB

110 : +20dB

…..(-4dB steps)

010 : +4dB

001 : 0dB

000 : 0dB

Maximum attenuation of PGA

R20 (14h)

0010100

Limiter Control

3:0

MAXATTEN

0110

Limiter

(attenuation

below static)

0011 or lower:

-3dB

0100: -4dB

…. (-1dB steps)

1100 or higher:

-12dB

ALC (lower PGA

gain limit)

1010 or lower:

-1dB

1011 : -5dB

….. (-4dB steps)

1110 : -17dB

1111 : -21dB

HOLD TIME (ALC ONLY)

The ALC also has a hold time, which is the time delay between the peak level detected being

below target and the PGA gain beginning to ramp up. It can be programmed in power-of-two

n) steps, e.g. 2.67ms, 5.33ms, 10.67ms etc. up to 43.7ms. Alternatively, the hold time can

also be set to zero. The hold time only applies to gain ramp-up, there is no delay before

ramping the gain down when the signal level is above target.

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

R17 (11h)

0010001

ALC Control 2

3:0

HLD[3:0]

0000

ALC hold time before gain is

increased.

0000: 0ms

0001: 2.67ms

0010: 5.33ms

… (time doubles with every step)

1111: 43.691s

OVERLOAD DETECTOR (ALC ONLY)

To prevent clipping when a large signal occurs just after a period of quiet, the ALC circuit

includes an overload detector. If the ADC input signal exceeds 87.5% of full scale (–1.16dB),

the PGA gain is ramped down at the maximum attack rate (as when ATK = 0000), until the

signal level falls below 87.5% of full scale. This function is automatically enabled whenever

the ALC is enabled.

(Note: If ATK = 0000, then the overload detector makes no difference to the operation of the

ALC. It is designed to prevent clipping when long attack times are used).

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