WM8961 datasheet(55/116 Pages) WOLFSON

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WM8961

Pre-Production

PP, August 2009, Rev 3.1

DC SERVO CONFIGURATION FOLLOWING A HEADPHONE PGA VOLUME

UPDATE

When a PGA volume update is applied to the headphone output, its DC offset can change. To

maintain sub 1.5mV DC offsets, it may be necessary to re-calibrate the DC correction factors when

large changes are made to PGA settings in the signal path (6dB or greater). The DC Servo can be

configured to do this automatically as a background task using the DCS_TRIG_SERIES_HPL/R. This

function is not handled by the write control sequencer since it can happen at any time and is not part

of a defined sequence.

This function is only available on the headphone outputs since it is not required for inputs.

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

R61 (3Dh)

DC Servo 1

DCS_TRIG_SERIES_HPL

0= do not perform any series of updates on HPL

1= Perform a DCS_SERIES_NO_HP series of LSB

updates on channel HPL *see note 1

One channel completes before the next starts

Reading this register returns status of this task.

Read

0 = task completed.

1 = task being performed

DCS_TRIG_SERIES_HPR

0= do not perform any series of updates on HPR

1= Perform a DCS_SERIES_NO_HP series of LSB

updates on channel HPR *see note 1

One channel completes before the next starts

Reading this register returns status of this task.

Read

0 = task completed.

1 = task being performed

Table 38 DC Servo Series Updates following Volume Update

Note:

These bits are automatically reset to ‘0’ after 1 clock cycle.

The DCS_TRIG_SERIES_X will update the channel a “number of times” before moving onto the next

enabled channel. The “number of times” it updates a particular channel, is configured by

DCS_SERIES_NO_HP.

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

R65 (41h)

DC Servo 5

6:0

DCS_SERIES_NO_HP[6:0]

001_0000

Number of LSB updates in a series for channels HPL

and HPR

000_0000-000_1111 : Reserved

001_0000 : 16

…

111_1111 : 127

Table 39 DC Servo Number of Updates Control

After any large gain update and volume updates have completed, the DCS_TRIG_SERIES_X bit

should be set. The DC Servo will then examine each enabled output in order and apply any required

offset correction to each.

To prevent any audible artifacts on the analogue outputs, the DC Servo only applies any necessary

corrections in discreet 0.25mV steps. To allow this correction during audio playback, the DC Servo

includes a high order, low pass filter which removes any AC signal content from each output before it

measures the DC offset. This filter has a relatively long time constant which means that the servo will

take approx 0.275 seconds for each 0.25mV correction on each enabled analogue channel. This

means that the DC Servo may remain active, in the background, for several seconds after the

DCS_TRIG_SERIES_X bit is set. Refer to “DC Servo Filter Bandwidth” section for more information.

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