WM8731 Wolfson Microelectronics Ltd., WM8731 Datasheet - Page 26

• Manufacturer Part Number: WM8731
• Description: WM8731 : Low Power Audio Codec With Integrated Headphone Driver
• Manufacturer: Wolfson Microelectronics Ltd.

WM8731
LINE OUTPUTS
The WM8731 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 Vrms at AVDD = 3.3 volts. Note that the DAC full scale level tracks
directly with AVDD. The scheme is shown in Figure 18. 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.
Figure 18 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.
The software control for the line outputs is shown in Table 7.
Table 7 Output Software Control
The recommended external components are shown in Figure 19.
0000100
Analogue
Audio Path
Control
REGISTER
ADDRESS
FROM MICROPHONE
INPUT
FROM LINE
FROM DAC
INPUTS
BIT
3
4
5
SIDETONE
BYPASS
DACSEL
LABEL
SIDETONE
DACSEL
BYPASS
VMID
DEFAULT
1
0
0
Bypass Switch
1 = Enable Bypass
0 = Disable Bypass
DAC Select
1 = Select DAC
0 = Don’t select DAC
Side Tone Switch
1 = Enable SideTone
0 = Disable Side Tone
DESCRIPTION
AI Rev 2.3 August 2001
Advanced Information
TO HEADPHONE AMP
LINEOUT
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