Output Signal Path; Digital Hi-Fi Dac Volume Control - Cirrus Logic WM8940 Manual

OUTPUT SIGNAL PATH

DIGITAL
DIGITAL
AUDIO
GAIN
INTERFACE
Figure 19 DAC Digital Filter Path
Rev 4.4
The WM8940 output signal paths consist of digital application filters, up-sampling filters, a hi-fi DAC,
analogue mixers, speaker and mono output drivers. The digital filters and DAC are enabled by bit
DACEN. The mixers and output drivers can be separately enabled by individual control bits (see
Analogue Outputs). Thus it is possible to utilise the analogue mixing and amplification provided by the
WM8940, irrespective of whether the DACs are running or not.
The WM8940 DAC receives digital input data on the DACDAT pin. The digital filter block processes
the data to provide the following functions:
▪
Digital volume control
▪
A digital peak limiter.
▪
Sigma-Delta Modulation
The high performance sigma-delta audio DAC converts the digital data into an analogue signal.
DAC DIGITAL FILTERS
DIGITAL
DIGITAL
PEAK
FILTERS
LIMITER
The analogue output from the DAC can then be mixed with the AUX analogue input and the ADC
analogue input. The mix is fed to the output drivers, SPKOUTP/N, and MONOOUT.
MONOOUT: can drive a 16 or 32 headphone or line output or can be a buffered version of VMID
(When MONOMUTE=1).
SPKOUTP/N: can drive a 16 or 32 stereo headphone or stereo line output, or an 8 BTL mono
speaker.

DIGITAL HI-FI DAC VOLUME CONTROL

The signal volume from each hi-fi DAC can be controlled digitally. The gain and attenuation range is –
127dB to 0dB in 0.5dB steps. The level of attenuation for an eight-bit code X is given by:
0.5  (X-255) dB for 1  X  255;
REGISTER BIT
ADDRESS
R11 7:0 DACVOL
DAC Digital [7:0]
Volume
Table 29 DAC Volume
INTERP SDM
MUTE for X = 0
LABEL DEFAULT
11111111 DAC Digital Volume Control
( 0dB ) 0000 0000 = Digital Mute
0000 0001 = -127dB
0000 0010 = -126.5dB
... 0.5dB steps up to
1111 1111 = 0dB
WM8940
DAC
DESCRIPTION
41