Production Data
WM8501
DEVICE DESCRIPTION
GENERAL INTRODUCTION
The WM8501 is a high performance DAC with an integrated 1.7Vrms line driver from a 5V
analogue supply, designed for digital consumer audio applications.
The WM8501 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. Control of the internal functionality of the
device is provided by hardware control pins (pin programmed).
Operation using master clocks of 256fs, 384fs, 512fs or 768fs is supported, selection between
clock rates being automatically controlled. Sample rates (fs) from less than 8kHz to 96kHz are
allowed, provided the appropriate system clock is input. Support is also provided for up to 192kHz
using a master clock of 128fs or 192fs.
The audio data interface supports 16-bit right justified or 16-24-bit I2S (Philips left justified, one bit
delayed) interface formats. A DSP interface is also supported, enhancing the interface options for
the user.
Split analog and digital 2.7-5.5V supply may be used, the output amplitude scaling with absolute
analogue supply level. Low supply voltage operation and low current consumption combined with
the low pin count small package make the WM8501 attractive for many consumer applications. A
power down mode is provided, allowing power consumption to be minimised.
The device is packaged in a small 14-pin SOIC.
DAC CIRCUIT DESCRIPTION
The WM8501 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 being acceptable 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 WM8501 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 192kHz 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 prevent the generation of unwanted tones
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.
The voltage on the VMID 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 VMID pin. An
external reference could be used to drive into the VMID 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 capable of driving
into low impedance line loads as low as 820Ω or headphone loads down to 16Ω. The buffer
amplifier output voltage level is set to 1.7V rms for line level loads when using a 5V analogue
supply, avoiding the requirement for additional gain stages or higher voltage supplies in many
applications. When driving headphone loads the output voltage level is limited to 1Vrms. The
advanced multi-bit DAC used in WM8501 produces far less out of band noise than single bit
traditional sigma delta DACs, and so in most applications where a line level output is required, no
post DAC filter is required. Typically an AC coupling capacitor and a DC setting resistor to ground
are the only components required on the output of the chip.
PD, Rev 4.2, July 2009
9
w
WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
