WM8940
Rev 4.4
19
DEVICE DESCRIPTION
INTRODUCTION
The WM8940 is a low power audio codec combining a high quality mono audio DAC and ADC, with
flexible line and microphone input and output processing. Applications for this device include digital
still cameras or camcorders with mono audio, record and playback capability.
FEATURES
The chip offers great flexibility in use, and so can support many different modes of operation as
follows:
MICROPHONE INPUTS
Two microphone inputs are provided, allowing for either a differential microphone input or a single
ended microphone to be connected. These inputs have a user programmable gain range of -12dB to
+35.25dB using internal resistors. After the input PGA stage comes a boost stage which can add a
further 20dB of gain. A microphone bias is output from the chip which can be used to bias the
microphones. The signal routing can be configured to allow manual adjustment of mic levels, or to
allow the ALC loop to control the level of mic signal that is transmitted.
Total gain through the microphone paths of up to +55.25dB can be selected.
PGA AND ALC OPERATION
A programmable gain amplifier is provided in the input path to the ADC. This may be used manually
or in conjunction with a mixed analogue/digital automatic level control (ALC) which keeps the
recording volume constant.
AUX INPUT
The device includes a mono input, AUX, that can be used as an input for warning tones (beep) etc.
The output from this circuit can be summed into the mono output and/or the speaker output paths, so
allowing
for mixing of audio with ‘backing music’ etc as required. This path can also be summed into
the input in a flexible fashion, either to the input PGA as a second microphone input or as a line input.
The configuration of this circuit, with integrated on-chip resistors allows several analogue signals to be
summed into the single AUX input if required.
ADC
The mono ADC uses a multi-bit high-order over sampling architecture to deliver optimum performance
with low power consumption. Various sample rates are supported, from the 8ks/s rate typically used in
voice dictation, up to the 48ks/s rate used in high quality audio applications.
HI-FI DAC
The hi-fi DAC provides high quality audio playback suitable for all portable mono audio type
applications.
DIGITAL FILTERING
Advanced Sigma Delta Converters are used along with digital decimation and interpolation filters to
give high quality audio at sample rates from 8ks/s to 48ks/s.
Application specific digital filters are also available which help to reduce the effect of specific noise
sources such as wind noise or narrowband noise from other parts of the system. The filters include a
programmable ADC high pass filter and four fully programmable ADC notch filters.
OUTPUT MIXING AND VOLUME ADJUST
Flexible mixing is provided on the outputs of the device; a mixer is provided for the speaker outputs,
and an additional mono summer for the mono output. These mixers allow the output of the DAC, the
output of the ADC volume control and the Auxiliary input to be combined. The output volume can be
adjusted using the integrated digital volume control and there is additional analogue gain adjustment
capability on the speaker output.
AUDIO INTERFACES
The WM8940 has a standard audio interface, to support the transmission of audio data to and from
the chip. This interface is a 4-wire standard audio interface which supports a number of audio data
