WM8971L Wolfson Microelectronics Ltd., WM8971L Datasheet - Page 18

• Manufacturer Part Number: WM8971L
• Description: Stereo Codec for Portable Audio Applications
• Manufacturer: Wolfson Microelectronics Ltd.

WM8971L
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DEVICE DESCRIPTION
INTRODUCTION
INPUT SIGNAL PATH
The WM8971L is a low power audio codec offering a combination of high quality audio, advanced
features, low power and small size. These characteristics make it ideal for portable digital audio
applications such as Digital Still Cameras, MP3 and minidisk player / recorders. Stereo 24-bit multi-
bit delta sigma ADCs and DACs are used with oversampling digital interpolation and decimation
filters.
The device includes a stereo analogue input which can be switched internally. This input can be used
as either a line level input, as a microphone input or be selected as a mono differential input. A mono
input is also included which can be configured as a microphone or line input.
When recording a programmable gain amplifier with automatic level control (ALC) keeps the
recording volume constant. The on-chip stereo ADC and DAC are of a high quality using a multi-bit,
low-order oversampling architecture to deliver optimum performance with low power consumption.
The DAC output signal first enters an analogue mixer where an analogue input and/or the post-ALC
signal can be added to it. This mix is available on line and headphone outputs.
The WM8971L has a configurable digital audio interface where ADC data can be read and digital
audio playback data fed to the DAC. It supports a number of audio data formats including I
Mode (a burst mode in which frame sync plus 2 data packed words are transmitted), MSB-First, left
justified and MSB-First, right justified, and can operate in master or slave modes.
The WM8971L uses a unique clocking scheme that can generate many commonly used audio
sample rates from either a 12.00MHz USB clock or an industry standard 256/384 f
feature eliminates the common requirement for an external phase-locked loop (PLL) in applications
where the master clock is not an integer multiple of the sample rate. Sample rates of 8kHz,
11.025kHz, 12kHz, 16kHz, 22.05kHz, 24kHz, 32kHz, 44.1kHz, 48kHz, 88.2kHz and 96kHz can be
generated. The digital filters used for recording and playback are optimised for each sampling rate
used.
To allow full software control over all its features, the WM8971L offers a choice of 2 or 3 wire MPU
control interface. It is fully compatible and an ideal partner for a wide range of industry standard
microprocessors, controllers and DSPs.
The design of the WM8971L has given much attention to power consumption without compromising
performance. It operates at very low voltages, and includes the ability to power off parts of the
circuitry under software control, including standby and power off modes.
The input signal path for each channel consists of a switch to select between three analogue inputs,
followed by a PGA (programmable gain amplifier) and an optional microphone gain boost. A
differential input of (LINPUT1 – RINPUT1) may also be selected. The gain of the PGA can be
controlled either by the user or by the on-chip ALC function (see Automatic Level Control).
The signal then enters an ADC where it is digitised. Alternatively, the two channels can also be
mixed in the analogue domain and digitised in one ADC while the other ADC is switched off. The
mono-mix signal appears on both digital output channels.
SIGNAL INPUTS
The WM8971L has three high impedance, low capacitance AC coupled analogue inputs, LINPUT1,
RINPUT1, and MIC. The LINSEL and RINSEL control bits select between them. These inputs can be
configured as microphone or line inputs by enabling or disabling the microphone gain boost. A
differential input, LINPUT1-RINPUT1 may also be selected using L/RINSEL.
The signal inputs are biased internally to the reference voltage VREF. Whenever the line inputs are
muted or the device placed into standby mode, the inputs are kept biased to VREF using special
anti-thump circuitry. This reduces any audible clicks that may otherwise be heard when changing
inputs.
PD Rev 4.1 August 2005
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