WM9090
Production Data
w
PD, November 2010, Rev 4.1
48
DC SERVO
The WM9090 provides a DC servo circuit on the headphone outputs HPOUTL and HPOUTR 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.
The recommended usage of the DC Servo is initialised by running the default Start-Up sequence as
described in the “Control Write Sequencer” section. The default Start-Up sequence executes a series
of DC offset corrections, after which the measured offset correction is maintained on the headphone
output channels. If a different usage is required, eg. if a periodic DC offset correction is required,
then the default Start-Up sequence may be modified according to specific requirements. The relevant
control fields are described in the following paragraphs and are defined in Table 28.
DC SERVO ENABLE AND START-UP
The DC Servo circuit is enabled on HPOUTL and HPOUTR by setting DCS_ENA_CHAN_0 and
DCS_ENA_CHAN_1 respectively. When the DC Servo is enabled, the DC offset correction can be
commanded in a number of different ways, including single-shot and periodically recurring events.
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’ = 0 for Left channel, 1 for Right
channel). On completion, the headphone output will be within 1mV of GND. This is the DC Servo
mode selected by the default Start-Up sequence. Completion of the DC offset correction triggered in
this way is indicated by the DCS_STARTUP_COMPLETE field, as described in Table 28. Typically,
this operation takes 25ms per channel.
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 R87. This mode is useful if the required
offset correction has already been determined and stored; it is 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 28. Typically, this operation takes 2ms per channel.
When using either of the DC Servo options above, the status of the DC offset correction process is
indicated by the DCS_CAL_COMPLETE field; this is the logical OR of the
DCS_STARTUP_COMPLETE and DCS_DAC_WR_COMPLETE fields.
The DC Servo control fields associated with start-up operation are described in Table 28. 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; a suitable sequence is defined in the default Start-Up sequence.
