WM8781
Production Data
DEVICE DESCRIPTION
INTRODUCTION
The WM8781 is a stereo 24-bit ADC designed for demanding recording applications such as DVD
recorders, studio mixers, PVRs, and AV amplifiers. The WM8781 consists of stereo line level inputs,
followed by a sigma-delta modulator and digital filtering.
The device offers stereo line level inputs along with two control input pins (FORMAT, IWL) to allow
operation of the audio interface in three industry standard modes (left justified, right justified or I2S) .
An internal op-amp is integrated on the front end of the chip to accommodate analogue input signals
greater than 1Vrms. The device also has a high pass filter to remove residual DC offsets.
The WM8781 offers Master or Slave mode clocking schemes. A control input pin M/S is used to allow
Slave mode or Master mode operation. The WM8781 supports master clock rates from 128fs to
768fs and digital audio output word lengths from 16-24 bits. Sampling rates from 8kHz to 192kHz are
supported, delivering high SNR operating with 128x, 64x or 32x over-sampling, according to the
sample rate.
The line inputs are biased internally through the operational amplifier to VMID
.
ADC
The WM8781 uses a multi-bit over sampled sigma-delta ADC. A single channel of the ADC is
illustrated in Figure 4 Multi-Bit Oversampling Sigma Delta ADC Schematic.
ANALOG
INTEGRATOR
LIN/RIN
TO ADC DIGITAL FILTERS
MULTI
BITS
Figure 4 Multi-Bit Oversampling Sigma Delta ADC Schematic
The use of multi-bit feedback and high oversampling rates reduces the effects of jitter and high
frequency noise.
The ADC Full Scale input is 1.0V rms at AVDD = 5.0 volts. Any input voltage greater than full scale
will possibly overload the ADC and cause distortion. Note that the full scale input has a linear
relationship with AVDD. The internal op-amp and appropriate resistors can be used to reduce signals
greater than 1Vrms before they reach the ADC.
The ADC filters perform true 24 bit signal processing to convert the raw multi-bit oversampled data
from the ADC to the correct sampling frequency to be output on the digital audio interface.
ADC DIGITAL FILTER
The ADC digital filters contain a digital high pass filter. The high-pass filter response detailed in
Digital Filter Characteristics. The operation of the high pass filter removes residual DC offsets that
are present on the audio signal.
PD, April 2006, Rev 4.1
10
w
WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
