WM8775 datasheet(23/36 Pages) WOLFSON

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WM8775

PP Rev 1.8, June 2004

LIMITER / AUTOMATIC LEVEL CONTROL (ALC)

The WM8775 has an automatic pga gain control circuit, which can function as a peak limiter or as an

automatic level control (ALC). In peak limiter mode, a digital peak detector detects when the input

signal goes above a predefined level and will ramp the pga gain down to prevent the signal becoming

too large for the input range of the ADC. When the signal returns to a level below the threshold, the

pga gain is slowly returned to its starting level. The peak limiter cannot increase the pga gain above

its static level.

Figure 19 Limiter Operation

In ALC mode, the circuit aims to keep a constant recording volume irrespective of the input signal

level. This is achieved by continuously adjusting the PGA gain so that the signal level at the ADC

input remains constant. A digital peak detector monitors the ADC output and changes the PGA gain

if necessary.

Figure 20 ALC Operation

hold

time

decay

time

attack

time

input

signal

after

ALC

PGA

gain

ALC

target

level

input

signal

after

PGA

gain

Limiter

threshold

attack

time

decay

time

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