WM1811A datasheet(121/328 Pages) WOLFSON

121 / 328 page

Production Data

WM1811A

PD, November 2013, Rev 4.1

121

DC SERVO

The WM1811A provides a DC servo circuit on the headphone outputs HPOUT1L and HPOUT1R in

order to remove DC offset from these ground-referenced outputs. When enabled, the DC servo

ensures that the DC level of these outputs remains within 1mV of ground. Removal of the DC offset is

important because any deviation from GND at the output pin will cause current to flow through the

load under quiescent conditions, resulting in increased power consumption. Additionally, the presence

of DC offsets can result in audible pops and clicks at power up and power down.

DC SERVO ENABLE AND START-UP

The DC Servo circuit is enabled on HPOUT1L and HPOUT1R by setting DCS_ENA_CHAN_0and

DCS_ENA_CHAN_1respectively. When the DC Servo is enabled, the DC offset correction can be

commanded in two different ways, as described below.

The DCS_TIMER_PERIOD_01 register must be set to 0000 for correct operation of the DC Servo.

This register must be set to 0000 before enabling the DC Servo.

Writing a logic 1 to DCS_TRIG_STARTUP_n initiates a series of DC offset measurements and applies

the necessary correction to the associated output; (‘n’ = 0for Left channel, 1for Right channel). On

completion, the headphone output will be within 1mV of AGND. This mode is recommended for typical

applications. Completion of the DC offset correction triggered in this way is indicated by the

DCS_STARTUP_COMPLETE field, as described in Table 70. Typically, this operation takes 86ms per

channel.

For correct operation of the DC Servo Start-Up mode, it is important that there is no active audio

signal present on the signal path while the mode is running. The DC Servo Start-Up mode should be

scheduled at the correct position within the Headphone Output Enable sequence, as described in the

Analogue Output Signal Path” section. All other stages of the analogue signal path should be fully

enabled prior to commanding the Start-Up mode; the DAC Digital Mute function should be used,

where appropriate, to ensure there is no active audio signal present during the DC Servo

measurements.

Writing a logic 1 to DCS_TRIG_DAC_WR_n causes the DC offset correction to be set to the value

contained in the DCS_DAC_WR_VAL_n fields in Register R89. This mode is useful if the required

offset

correction

has

already

been

determined

and

stored;

faster

than

the

DCS_TRIG_STARTUP_n mode, but relies on the accuracy of the stored settings. Completion of the

DC offset correction triggered in this way is indicated by the DCS_DAC_WR_COMPLETE field, as

described in Table 70. Typically, this operation takes 2ms per channel.

For pop-free operation of the DC Servo DAC Write mode, it is important that the mode is scheduled at

the correct position within the Headphone Output Enable sequence, as described in the “Analogue

Output Signal Path” section.

The current DC offset value for each Headphone output channel can be read from the

DCS_DAC_WR_VAL_nfields. These values may form the basis of settings that are subsequently

used by the DC Servo in DAC Write mode. Note that these fields have a different definition for Read

and Write, as described in Table 70.

When using either of the DC Servo options above, the status of the DC offset correction process is

indicated

the

DCS_CAL_COMPLETE

field;

this

the

logical

the

DCS_STARTUP_COMPLETE and DCS_DAC_WR_COMPLETE fields.

The DCS_DAC_WR_COMPLETE bits can be used as inputs to the Interrupt control circuit or used to

generate an external logic signal on a GPIO pin. See “Interrupts” and “General Purpose Input/Output”

for further details.

The DC Servo control fields associated with start-up operation are described in Table 70. It is

important to note that, to minimise audible pops/clicks, the Start-Up and DAC Write modes of DC

Servo operation should be commanded as part of a control sequence which includes muting and

shorting of the headphone outputs.

WM1811A Datasheet(PDF) 121 Page - Wolfson Microelectronics plc

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