WM8727

Production Data

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PD Rev 4.1 April 2004

8

DEVICE DESCRIPTION

GENERAL INTRODUCTION

The WM8727 is a high performance DAC designed for digital consumer audio applications. The

range of features make it ideally suited for use in DVD players, AV receivers and other

consumer audio equipment.

The WM8727 is a complete 2-channel stereo audio digital-to-analogue converter, including

digital interpolation filter, multi-bit sigma delta with dither, and switched capacitor multi-bit

stereo DAC and output smoothing filters. It is fully compatible and an ideal partner for a range

of industry standard microprocessors, controllers and DSPs. A novel multi bit sigma-delta DAC

design is used, utilising a 128x oversampling rate, to optimise signal to noise performance and

offer increased clock jitter tolerance. (In ‘high-rate’ operation, the oversampling ratio is 64x for

system clocks of 128fs or 192fs)

Operation using master clocks of 256fs, 384fs, 512fs or 768fs is provided, selection between

clock rates being automatically controlled. Sample rates (fs) from less than 8ks/s to 96ks/s are

allowed, provided the appropriate system clock is input. Support is also provided for up to

192ks/s using a master clock of 128fs or 192fs.

The audio data interface supports 16-24-bit I

format. A DSP interface is also supported, enhancing the interface options for the user.

A single 2.7-5.5V supply may be used, the output amplitude scaling with absolute supply level.

Low supply voltage operation and low current consumption combined with the low pin count

small package make the WM8727 attractive for many consumer applications.

The device is packaged in a small 8-pin SOIC.

DAC CIRCUIT DESCRIPTION

The WM8727 DAC is designed to allow playback of 24-bit PCM audio or similar data with high

resolution and low noise and distortion. Sample rates up to 192ks/s may be used, with much

lower sample rates acceptable provided that the ratio of sample rate (LRCIN) to system clock

(MCLK) is maintained at one of the required rates.

The two DACs on the WM8727 are implemented using sigma-delta oversampled conversion

techniques. These require that the PCM samples are digitally filtered and interpolated to generate

a set of samples at a much higher rate than the up to 192ks/s input rate. This sample stream is

then digitally modulated to generate a digital pulse stream that is then converted to analogue

signals in a switched capacitor DAC. The advantage of this technique is that the DAC is

linearised using noise shaping techniques, allowing the 24-bit resolution to be met using non-

critical analogue components. A further advantage is that the high sample rate at the DAC output

means that smoothing filters on the output of the DAC need only have fairly crude characteristics

in order to remove the characteristic steps, or images, on the output of the DAC. To ensure that

generation of tones characteristic to sigma-delta convertors is not a problem, dithering is used in

the digital modulator and a higher order modulator is used. The multi-bit switched capacitor

technique used in the DAC reduces sensitivity to clock jitter, and dramatically reduces out of

band noise compared to switched current or single bit techniques used in other implementations.

The outputs of the 2 DACs are buffered out of the device by buffer amplifiers. These amplifiers

will source load currents of several mA and sink current up to 1.5mA allowing significant loads

to be driven. The output source is active and the sink is Class A, i.e. fixed value, so greater

loads might be driven if an external ‘pull-down’ resistor is connected at the output.

Typically an external low pass filter circuit will be used to remove residual out of band noise

characteristic of delta sigma converters. However, the advanced multi-bit DAC used in

WM8727 produces far less out of band noise than single bit traditional sigma delta DACs, and

so in many applications this filter may be removed, or replaced with a simple RC pole.