WM8762
Preliminary Technical Data
DEVICE DESCRIPTION
GENERAL INTRODUCTION
The WM8762 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 WM8762 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 8kHz to 192KHz
are allowed, provided the appropriate system clock is input.
The audio data interface supports a 16-24-bit left justified interface format. 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 WM8762 attractive for many consumer applications.
The device is packaged in a small 8-pin SOIC.
DAC CIRCUIT DESCRIPTION
The WM8762 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 192kHz may be used, with much
lower sample rates acceptable provided that the ratio of sample rate (LRCIN) to system master
clock (MCLK) is maintained at one of the required rates.
The two DACs on the WM8762 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 converters 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
WM8762 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.
PTD Rev 2.0 August 2003
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
