WM8988 datasheet(21/58 Pages) CIRRUS

21 / 58 page

WM8988

Rev 4.2

ADC DIGITAL FILTER

The ADC filters perform true 24-bit signal processing to convert the raw multi-bit oversampled data

from the ADC to the correct sampling frequency to be output on the digital audio interface. The digital

filter path is illustrated in Figure 8.

FROM ADC

DIGITAL

HPF

DIGITAL

FILTER

TO DIGITAL

AUDIO

INTERFACE

DIGITAL

DECIMATOR

ADCHPD

Figure 8 ADC Digital Filter

The ADC digital filters contain a digital high-pass filter, selectable via software control. The high-pass

filter response is detailed in the Digital Filter Characteristics section. When the high-pass filter is

enabled the DC offset is continuously calculated and subtracted from the input signal. By setting

HPOR, the last calculated DC offset value is stored when the high-pass filter is disabled and will

continue to be subtracted from the input signal. If the DC offset is changed, the stored and subtracted

value will not change unless the high-pass filter is enabled. This feature can be used for calibration

purposes. In addition the high-pass filter may be enabled separately on the left and right channels

(see Table 11).

The output data format can be programmed by the user to accommodate stereo or monophonic

recording on both inputs.

The polarity of the output signal can also be changed under software

control. The software control is shown in Table 10.

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

R5 (05h)

ADC and DAC

Control

6:5

ADCPOL

[1:0]

00 = Polarity not inverted

01 = L polarity invert

10 = R polarity invert

11 = L and R polarity invert

HPOR

Store dc offset when high-pass

filter disabled

1 = store offset

0 = clear offset

ADCHPD

ADCHPD and HPFLREN together

determine high-pass filter

behaviour (see Table 11)

R27 (1Bh)

HPFLREN

Table 10 ADC Signal Path Control

HPFLREN

ADCHPD

LEFT CHANNEL

RIGHT CHANNEL

HPF ON

HPF OFF

HPF ON

HPF OFF

HPF ON

Table 11 ADC High Pass Filter Modes

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