WM8777
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LIMITER / AUTOMATIC LEVEL CONTROL (ALC)
The WM8777 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.
input
signal
PGA
gain
signal
Limiter
after
threshold
PGA
attack
time
decay
time
Figure 30 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.
input
signal
PGA
gain
signal
after
ALC
ALC
target
level
hold decay
time time
attack
time
Figure 31 ALC Operation
PP Rev 1.94 November 2004
68
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
