WM8714
Production Data
DEVICE DESCRIPTION
GENERAL INTRODUCTION
The WM8714 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 WM8714 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 DSPs and audio controllers. 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)
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 less than 8ks/s to 96ks/s are
allowed, provided the appropriate system clock is input.
The audio data interface supports 16-bit right justified or 16-24-bit I2S (Philips left justified, one bit
delayed) interface formats.
Single 2.7-5.5V supplies 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 WM8714 attractive for many consumer applications.
The device is packaged in a small 14-pin SOIC.
DAC CIRCUIT DESCRIPTION
The WM8714 DAC is designed to allow playback of 24-bit PCM audio or similar data with high
resolution and low noise and distortion. The two DACs on the WM8714 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
96ks/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 converters 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.
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 WM8714
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.
PD Rev 4.0 November 2004
9
w
WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
