WM8753 Wolfson Microelectronics plc, WM8753 Datasheet - Page 29

• Manufacturer Part Number: WM8753
• Description: HI FI AND TELEPHONY DUAL CODEC
• Manufacturer: Wolfson Microelectronics plc

WM8753L
w
Figure 9 ADC Digital Filter
ANALOGUE TO DIGITAL CONVERTER (ADC)
The WM8753L 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 9.
The characteristic of the digital decimation filter is selectable using control bit VXFILT for HiFi and
voice modes of operation. In Voice mode the filter has a steeper roll-off and reduced stopband
attenuation. The ADC digital filters also contain a digital high pass filter, selectable via software
control. The cut-off frequency of the highpass filter is selectable to suit the mode of operation. For
HiFi record mode the default cut-off of 3.4Hz (at fs=48kHz) is recommended and for voice record a
cut-off of 100Hz or 200Hz (at fs=8kHz) may be selected The frequency response of the highpass filter
will scale with sample rate so the 100Hz cut-off at 8KHz sample rate will scale to a 200Hz cut-off at
16kHz sample rate. The decimation and high-pass filter responses are 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
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 HPOR is reset. This feature can be used for calibration
purposes.
Table 17 Highpass Filter Cut-off Frequencies
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 18.
FROM ADC
HPMODE
00
01
10
11
fs=8kHz
DECIMATOR
DIGITAL
170
0.6
41
82
Cut-off (Hz)
fs=16kHz
164
340
1.1
DIGITAL
FILTER
82
fs=48kHz
ADCHPD
DIGITAL
HPF
1020
246
492
3.7
INTERFACE
TO DIGITAL
AUDIO
AI Rev 3.1 June 2004
Advanced Information
29