Production Data WM8521
w
PD, Rev 4.2, February 2013
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
2
S interface formats. A
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.
