WM8737L Production Data
w
PD, Rev 4.4, January 2012
14
DEVICE DESCRIPTION
INTRODUCTION
The WM8737L is a low power analogue to digital converter (ADC) designed for audio recording. Its
features, performance and low power consumption make it ideal for recordable CD players, MP3
players, portable MD players and PDAs.
The device includes three stereo analogue inputs with a multiplexer to select between inputs. Each
input can be used as either a line level input or as a microphone input with on-chip microphone pre-
amplifiers. A programmable gain amplifier provides additional gain or attenuation, and can be used for
automatic level control (ALC), keeping the recording volume constant. It is also possible to use the
WM8737L as a mono device, or to mix the two channels to mono, either in the analogue or in the
digital domain.
The ADC is of a high quality using a multi-bit high-order oversampling architecture delivering high
SNR at low power consumption. It can operate at oversampling rates of 64fs (low power mode) or
128fs (normal power mode), allowing users to design for low power consumption or high
performance. The ADC also includes a digital high pass filter to remove unwanted DC components
from the audio signal. This filter may be turned off for DC measurements.
The output from the ADC is available on a configurable digital audio interface. It supports a number of
audio data formats including I
2
S, DSP Mode, Left justified and Right justified, and can operate in
master or slave modes.
The WM8737L master clock can be either an industry standard 256/384 f
s
clock or a 12MHz/24MHz
USB clock. Sample rates of 16kHz, 22.05kHz, 24kHz, 32kHz, 44.1kHz, 48kHz, 88.2kHz and 96kHz
can be generated directly from the master clock, without an external PLL. The digital filters are
optimised for each sample rate.
The WM8737L can be controlled through a 2 wire or 3 wire MPU control interface. It is fully
compatible and an ideal partner for a range of industry standard microprocessors, controllers and
DSPs.
The design of the WM8737L has minimised power consumption without compromising performance.
It can operate at very low voltages, can power off parts of the circuitry under software control and
includes standby and power off modes.
INPUT SIGNAL PATH
The signal path consists of a multiplexer switch to select between three sets of analogue inputs,
followed by a microphone boost preamplifier with selectable gain settings of 13dB, 18dB, 28dB and
33dB.
The microphone preamplifier feeds into a PGA (programmable gain amplifier) via an external
capacitor which removes dc offsets that could otherwise produce zipper noise when the PGA gain
changes. Alternatively, for line input signals, the microphone preamplifier can be bypassed to reduce
power consumption and noise. The PGA gain can be controlled either by the user or by the on-chip
ALC function (see Automatic Level Control).
The output signal from each PGA (left and right) enters an ADC where it is digitised. The two
channels can also be mixed in the analogue domain and digitised in one ADC while the other ADC is
switched off to reduce power consumption (see “Power Management” section). The mono-mix signal
appears on both digital output channels.
LEFT AND RIGHT CHANNEL SIGNAL INPUTS
The WM8737L has two sets of low capacitance ac coupled analogue inputs, LINPUT1, LINPUT2,
LINPUT3 and RINPUT1, RINPUT2, RINPUT3. 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 preamplifier. All inputs have high impedance when the preamplifier is used and their
impedance is between 1.8k and 50k (depending on PGA gain) if the preamplifier is bypassed.
The microphone preamplifier has very high gain settings. Care should be taken to prevent the input
signal to the microphone preamplifier from exceeding the supply rails which will saturate the
microphone preamplifier and may cause the preamplifier to oscillate. This is more likely to happen at
the higher gain settings of +28dB and +33dB. The input signal should be limited to prevent the input
signal from exceeding the supply rails and saturating the microphone preamplifier.
