WM8734 datasheet(15/42 Pages) ETC

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WM8734

Advanced Information

AI Rev 2.2 November 2001

The use of multi-bit feedback and high oversampling rates reduces the effects of jitter and high

frequency noise.

The ADC Full Scale input is 1.0V rms at AVDD = 3.3 volts. Any voltage greater than full scale will

possibly overload the ADC and cause distortion. Note that the full scale input tracks directly with

AVDD. The device employs a pair of ADCs. The two channels cannot be selected independently.

The digital data from the ADC is fed for signal processing to the ADC Filters.

ADC FILTERS

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. Figure 11

illustrates the digital filter path.

FROM ADC

DIGITAL

HPF

DIGITAL

DECIMATION

FILTER

TO DIGITAL

AUDIO

INTERFACE

DIGITAL

DECIMATOR

HPFEN

Figure 11 ADC Digital Filter

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

filter response detailed in Digital Filter Characteristics. 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 changed, the stored and subtracted value will not

change unless the high-pass filter is enabled. The software control is shown in Table 2.

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

ADCHPD

ADC High Pass Filter Enable

(Digital)

1 = Disable High Pass Filter

0 = Enable High Pass Filter

00000101

Digital Audio

Path Control

HPOR

Store dc offset when High Pass

Filter disabled

1 = store offset

0 = clear offset

Table 2 ADC Software Control

There are several types of ADC filters, frequency and phase responses of these are shown in Digital

Filter Characteristics. The filter types are automatically configured depending on the sample rate

chosen. Refer to the sample rate section for more details.

DAC FILTERS

The DAC filters perform true 24 bit signal processing to convert the incoming digital audio data from

the digital audio interface at the specified sample rate to multi-bit oversampled data for processing by

the analogue DAC. Figure 12 illustrates the DAC digital filter path.

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