wm8988 Wolfson Microelectronics plc, wm8988 Datasheet - Page 20

• Manufacturer Part Number: wm8988
• Description: Stereo Codec For Portable Audio Applications
• Manufacturer: Wolfson Microelectronics plc

WM8988
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ANALOGUE TO DIGITAL CONVERTER (ADC)
The WM8988 uses a multi-bit, oversampled sigma-delta ADC for each channel. The use of multi-bit
feedback and high oversampling rates reduces the effects of jitter and high frequency noise. The
ADC Full Scale input level is proportional to AVDD. With a 3.3V supply voltage, the full scale level is
1.0 Volts r.m.s. Any voltage greater than full scale may overload the ADC and cause distortion.
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.
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.
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.
Table 10 ADC Signal Path Control
Table 11 ADC High Pass Filter Modes
R5 (05h)
ADC and DAC
Control
R27 (1Bh)
FROM ADC
REGISTER
ADDRESS
HPFLREN
0
0
1
1
DECIMATOR
DIGITAL
6:5
4
0
5
BIT
ADCHPD
ADCPOL
[1:0]
HPOR
ADCHPD
HPFLREN
0
1
0
1
LABEL
DIGITAL
FILTER
00
0
0
0
DEFAULT
ADCHPD
DIGITAL
HPF
LEFT CHANNEL
If the DC offset is changed, the stored and
HPF OFF
HPF OFF
HPF ON
HPF ON
INTERFACE
TO DIGITAL
AUDIO
00 = Polarity not inverted
01 = L polarity invert
10 = R polarity invert
11 = L and R polarity invert
Store dc offset when high-pass
filter disabled
1 = store offset
0 = clear offset
ADCHPD and HPFLREN together
determine high-pass filter
behaviour (see Table 11)
DESCRIPTION
PD, Rev 4.0, October 2008
RIGHT CHANNEL
HPF OFF
HPF OFF
HPF ON
HPF ON
Production Data
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