Advanced Information

WM8740

WOLFSON MICROELECTRONICS LTD

AI Rev 1.7 July 2000

7

DEVICE DESCRIPTION

The WM8740 is a high performance 128fs oversampling rate stereo DAC employing a novel 64 level

sigma delta DAC design which provides optimised signal-to-noise performance and clock jitter

tolerance. It is ideally suited to high quality audio applications such as CD, DVD-audio, home theatre

receivers and professional mixing consoles. The WM8740 supports sample rates from 8ks/s to

192ks/s.

The control functions of the WM8740 are either pin selected (hardware mode) or programmed via the

serial interface (software mode). Control functions that are available include: data input word length

and format selection (16-24 bits: I

selection (48kHz, 44.1kHz and 32kHz); differential output modes; a software or hardware mute and

independently digitally controllable attenuation on both channels.

The digital filtering may be bypassed entirely by selecting MODE8X. Data is then input directly to the

DAC, bypassing the digital filters. Left and right channels are input separately, using the MODE pin

as the right channel input. This mode allows the use of alternative digital filters, such as the Pacific

Microsonics PMD100 HDCD filter.

In addition to the normal stereo operating mode the WM8740 may also be used in dual differential

mode with either the left or right channel (selectable) being output dual differentially. Two WM8740s

can then be used in parallel to implement a stereo channel, each supporting a single channel

differentially. Note that this mode uses 2 pairs of differential outputs for each channel – the benefit is

SNR improved by 3dB. This mode is available in both software and hardware modes and may also

be used in conjunction with MODE8X.

SYSTEM CLOCK

Sample rates from 8ks/s up to 96ks/s are available, and automatically selected, with a system clock

of 256fs or 384fs. In addition a system clock of 128fs or 192fs may be used, with sample rates up to

192ks/s. With a 128fs or 192fs system clock 64x oversampling mode operation is automatically

selected and the first stage of the digital filter is bypassed.

WM8740 has an asynchronous monitor circuit, which in the event of removal of the master system

clock, resets the digital filters and analogue circuits, muting the output. Re-application of the system

clock re-starts the filters from an intitialised state. Control registers are not reset under this condition.

The WM8740 is tolerant of asynchronous bit clock jitter. The internal signal processing

resynchronises to the external LRCIN once the phase difference between bit clock and the system

clock exceeds half an LRCIN period. During this re-synch period the interpolating filters will either

miss or repeat an audio sample, minimising the audible effects of the operation. Table 1 shows the

typical system clock frequency inputs for the WM8740.

SYSTEM CLOCK FREQUENCY (MHZ)

SAMPLING

RATE

(LRCIN)

128fs

192fs

256fs

384fs

32kHz

4.096

6.144

8.192

12.288

44.1kHz

5.6448

8.467

11.2896

16.9340

48kHz

6.114

9.216

12.288

18.432

96kHz

12.288

18.432

24.576

36.864

192kHz

24.576

36.864

Unavailable

Unavailable

Table 1 System Clock Frequencies Versus Sampling Rate

AUDIO DATA INTERFACE

Data may be input at a rate corresponding to the system clock having a rate of 256fs or 384fs, in

which case an oversampling ratio of 128x is selected. Alternatively a rate of 128fs or 192fs may be

used, in which case the first filter stage is bypassed and an oversampling ratio of 64x results. Finally,

in MODE8X, data may be input at 8x the normal rate, in which case separate input pins are used to

input the two stereo channels of data (unless DIFFHW mode and MODE8X are both selected, in

which case only a mono channel is converted differentially). In MODE8X all filter stages are by-

passed, prior to the sigma delta modulator. Data is input MSB first in all modes.