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WM8521 Datasheet(PDF) 9 Page - Wolfson Microelectronics plc |
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WM8521 Datasheet(HTML) 9 Page - Wolfson Microelectronics plc |
9 / 20 page Product Preview WM8521 w PP Rev 1.3 December 2004 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 96kHz 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 2S (Philips left justified, one bit delayed) 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 96kHz 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. |
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