WM9090ECSSLASHR datasheet(27/81 Pages) WOLFSON

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Production Data

WM9090

PD, November 2010, Rev 4.1

AUTOMATIC GAIN CONTROL (AGC)

The Speaker Output PGA incorporates an Automatic Gain Control (AGC) circuit. This feature

provides an automatic reduction in the speaker path gain in order to prevent clipping or power

overload at the loudspeaker. The AGC circuit provides two separate detection mechanisms to

identify clipping or power overload respectively. Each of these two mechanisms can be

independently configured to suit the loudspeaker characteristics and the desired audio response. The

two control mechanisms operate together to provide a flexible and effective automatic gain control

feature.

AGC CONTROL

AGC is enabled by setting the AGC_ENA register bit, as defined in Table 12.

The AGC can provide attenuation in the speaker output path - note that it can never apply additional

gain to boost the signal level. The maximum extent of the AGC attenuation can be controlled by

setting the AGC_MINGAIN register. This field sets the lowest gain level that can be selected by the

AGC under signal clipping or power limiting conditions.

When the signal conditions trigger the AGC to apply attenuation, the Speaker PGA gain is controlled

automatically by the AGC. In order to prevent ‘zipper noise’ from the gain adjustment, the PGA gain

is only changed when a signal zero-cross is detected. When AGC_RAMP = 1, then the gain

adjustment is restricted to a single gain step on each zero-cross. When AGC_RAMP = 0, then

multiple gain steps may be applied, if necessary, on each zero-cross.

Selecting single gain steps only will result in a more gradual gain adjustment, but the AGC may also

be slower to remove signal clipping under this selection. Note that the AGC attenuation has a step

size of 0.5dB, providing a high resolution of signal level control.

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

R3 (03h)

Power

Management

(3)

AGC_ENA

AGC Enable

0 = Disabled

1 = Enabled

R100 (64h)

AGC Control

AGC_RAMP

AGC Ramp Control

Selects how the AGC gain

adjustment is applied

0 = Multiple gains steps per zero-

cross

1 = Single gain step per zero-cross

5:0

AGC_MINGAIN [5:0]

00000

AGC Minimum Gain

-57dB to +6dB in 1dB steps

(See Table 11 for AGC Minimum

Gain range)

Table 12 AGC Control

AGC ANTI-CLIP

The AGC incorporates two mechanisms for monitoring the signal conditions. One of these is the anti-

clip threshold detection. The anti-clip function measures the speaker supply voltage, SPKVDD, and

compares this with the output signal level. The difference between these voltages is referred to as

the headroom; to avoid clipping, the signal level must always be less than the supply voltage. If the

headroom is small (ie. the signal level is very close to the supply voltage), then clipping and distortion

will occur.

The anti-clip function can be disabled using the AGC_CLIP_ENA bit. It is enabled by default.

The headroom threshold at which the AGC will apply attenuation is set using the AGC_CLIP_THR

300mV, the distortion (THD) is approximately 1%. Therefore, if the anti-clip threshold is set to

300mV, then the AGC would aim to limit the distortion to be no worse than 1% under maximum

signal conditions. Selecting a larger headroom threshold will avoid clipping across a wider range of

operating conditions.

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