Production Data

WM8521

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PD Rev 4.1 August 2006

9

DEVICE DESCRIPTION

GENERAL INTRODUCTION

The WM8521 is a high performance DAC designed for digital consumer audio applications requiring

a 2Vrms output. The range of features make it ideally suited for use in DVD players, Digital TV,

Digital Set Top Boxes, AV receivers and other consumer audio equipment.

The WM8521 is a complete 2-channel stereo audio digital-to-analogue converter, including digital

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

output smoothing filters combined with 2Vrms outputs. 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.

Control of internal functionality of the device is provided by hardware control (pin programmed).

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

rates being automatically controlled. Sample rates (fs) from 8kHz to 192kHz are allowed provided the

appropriate system clock is input.

The audio data interface supports 16-bit right justified or 16-, 20-, 24-, 32-bit I

16bit DSP interface is also supported, enhancing the interface options for the user.

The device is packaged in a small 14-pin SOIC.

DAC CIRCUIT DESCRIPTION

The WM8521 DAC is designed to allow playback of 24-bit PCM audio or similar data with high

resolution and low noise and distortion. Sample rates from 8kHz to 192kHz may be used provided

that the ratio of sample rate (LRCLK) to master clock (MCLK) is maintained at one of the required

rates.

The two DACs on the WM8521 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 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 along with a higher

order modulator. 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 voltage on the CAP pin is used as the reference for the DACs. Therefore the amplitude of the

signals at the DAC outputs will scale with the amplitude of the voltage at the CAP pin. An external

reference could be used to drive into the CAP pin if desired, with a value typically of about midrail

ideal for optimum performance. However driven in normal operation, an internal divider will set a

valve of AVDD/2 on the cap pin.

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 WM8521

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.