WM8961
Pre-Production
Left PGA Input
Left PGA Output
Zero crossing causes
volume update
Right PGA Input
No zero crossing
Right PGA Output
Timeout Counter
Timeout causes
volume update
TIMEOUT
Write
Right
Volume
Write Left
Volume
Control Interface
Update
bit = 0
Update
bit = 1
Time
Write Left PGA Volume
without update
Write Right PGA Volume
with update
Figure 26 Volume Update after Timeout
DC SERVO
The DC servo reduces any DC offset from the ground referenced headphone output and the output of
the microphone boost stage. The operation of the DC servo ensures that these outputs remain within
1.5mV of ground when enabled.
On the input stage the DC servo is used to null any DC offset at the input of the ADC (boost stage
output). This optimises the zero cross detection, particularly when using ALC. The DC servo is
referenced to the charge pump references VPOS and VNEG hence in order to permit the DC servo to
be used when recording, the headphone charge pump must be enabled (CP_ENA = 1) and MCLK
must be applied, see “Charge Pump” section. If the charge pump is not enabled or MCLK is not
supplied, large offsets will be present within the Input Path and the zero cross function may not
operate.
On the output stage the DC servo is used to null DC offsets at the headphone outputs. This results in
reduced power consumption since any deviation from ground at the output pin results in a DC current
flow through the load. Additionally, removal of DC offsets at these outputs helps to significantly reduce
pops and clicks at power up and down.
A block diagram of the DC Servo is shown in Figure 27
PP, August 2009, Rev 3.1
w
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
