WM8739 / WM8739L

Advanced Information

AI Rev 2.2 September 2001

14

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

FILTER

TO DIGITAL

AUDIO

INTERFACE

DIGITAL

DECIMATOR

HPFEN

Figure 11 ADC Digital Filter

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 changed, the stored and subtracted

value will not change unless the high-pass filter is enabled. The software control is shown in Table 3.

REGISTER

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

0

ADCHPD

0

ADC High Pass Filter Enable (Digital)

1 = Disable High Pass Filter

0 = Enable High Pass Filter

0000101

Audio Path Control

4

HPOR

0

Store dc offset when High Pass Filter

disabled

1 = store offset

0 = clear offset

Table 3 ADC Software Control

There are several types of ADC filter, the frequency and phase responses of which 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.

CLOCKING SCHEMES

In a typical digital audio system there is only one central clock source producing a reference clock to

which all audio data processing is synchronised. This clock is often referred to as the audio system’s

Master Clock. To allow WM8739 to be used in a centrally clocked system, the WM8739 is capable of

either generating this ‘system’ clock itself or receiving it from an external source as will be discussed.

For applications where it is desirable that the WM8739 is the system clock source, then clock

generation is achieved through the use of a suitable crystal connected between the XTI/MCLK input

and XTO output pins (see CRYSTAL OSCILLATOR section).