wm8775 Wolfson Microelectronics plc, wm8775 Datasheet - Page 26

• Manufacturer Part Number: wm8775
• Description: 24-bit, 96khz Adc With 4 Channel I/p Multiplexer
• Manufacturer: Wolfson Microelectronics plc

WM8775
LIMITER / AUTOMATIC LEVEL CONTROL (ALC)
w
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 22 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 23 ALC Operation
signal
signal
signal
signal
input
input
PGA
PGA
PGA
after
ALC
after
gain
gain
attack
hold
time
time
decay
time
decay
time
attack
time
target
ALC
level
PD, Rev 4.4, October 2008
threshold
Limiter
Production Data
26