WM8961GEFLSLASHV datasheet(53/116 Pages) WOLFSON

53 / 116 page

WM8961

Pre-Production

PP, August 2009, Rev 3.1

Write Left

Volume

Update

bit = 0

Write

Right

Volume

Update

bit = 1

Left PGA Input

Right PGA Input

Control Interface

Time

Left PGA Output

Right PGA Output

Write Right PGA Volume

with update

Write Left PGA Volume

without update

Timeout Counter

Timeout causes

volume update

Zero crossing causes

volume update

No zero crossing

TIMEOUT

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

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