WM8731 / WM8731L
Production Data
FROM DAC
DIGITAL
FILTERS
TO LINE OUTPUT
Figure 18 Multi-Bit Oversampling Sigma Delta Schematic
The DAC converts the multi-level digital audio data stream from the DAC digital filters into high
quality analogue audio.
LINE OUTPUTS
The WM8731/L provides two low impedance line outputs LLINEOUT and RLINEOUT, suitable for
driving typical line loads of impedance 10K and capacitance 50pF. The line output is used to
selectively sum the outputs from the DAC or/and the Line inputs in bypass mode.
The LLINEOUT and RLINEOUT outputs are only available at a line output level and are not level
adjustable in the analogue domain, having a fixed gain of 0dB. The level is fixed such that at the DAC
full scale level the output level is 1.0Vrms at AVDD = 3.3 volts. Note that the DAC full scale level
tracks directly with AVDD. The scheme is shown in Figure 19. The line output includes a low order
audio low pass filter for removing out-of band components from the sigma-delta DAC. Therefore no
further external filtering is required in most applications.
SIDETONE
FROM MICROPHONE
INPUT
BYPASS
FROM LINE
INPUTS
DACSEL
FROM DAC
LINEOUT
VMID
TO HEADPHONE AMP
Figure 19 Line Output Schematic
The DAC output, Line Input and microphone are summed into the Line Output. In DAC mode only the
output from the DAC is routed to the line outputs. In Bypass mode the Line Input is summed into the
Line Outputs. In Side Tone mode the Microphone Input is summed into the Line Output. These
features can be used for either over-dubbing or, if the DAC is muted, as a pure analogue bypass or
Side Tone feature, so avoiding any digital signal processing.
The line output is muted by either muting the DAC (analogue) or Soft Muting (digital) and disabling
the BYPASS and SIDETONE paths. Refer to the DAC section for more details. Whenever the DAC
is muted or the device placed into standby mode the DC voltage is maintained at the line outputs to
prevent any audible clicks from being present.
PD, Rev 4.8, April 2009
28
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
