WM8753 datasheet(29/87 Pages) WOLFSON

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WM8753L

Advanced Information

AI Rev 3.1 June 2004

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.

FROM ADC

DIGITAL

HPF

DIGITAL

FILTER

TO DIGITAL

AUDIO

INTERFACE

DIGITAL

DECIMATOR

ADCHPD

Figure 9 ADC Digital Filter

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.

Cut-off (Hz)

HPMODE

fs=8kHz

fs=16kHz

fs=48kHz

0.6

1.1

3.7

246

164

492

170

340

1020

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