Pre-Production
WM8904
DIFFERENTIAL MICROPHONE INPUT
The Differential Mic PGA configuration is illustrated in Figure 23 for the Left channel. The available
gain in this mode is from +12dB to +30dB in 3dB linear steps. The input to the ADC is in phase with
the input pin selected by L_IP_SEL_N. The input to the ADC is phase inverted with respect to the
input pin selected by L_IP_SEL_P.
Note that the inverting input pin is selected using L_IP_SEL_P and the non-inverting input pin is
selected using L_IP_SEL_N. This is not the same as for the Differential Line mode.
The equivalent configuration is also available on the Right channel; this can be selected
independently of the Left channel mode.
Figure 23 Differential Microphone Mode
INPUT PGA GAIN CONTROL
The volume control gain for the Left and Right channels be independently controlled using the
LIN_VOL and RIN_VOL register fields as described in Table 3. The available gain range varies
according to the selected PGA Mode as detailed in Table 4. Note that the value ‘00000’ must not be
used in Differential Mic Mode, as the PGA will not function correctly under this setting. In single-
ended mode (L_MODE / R_MODE = 00b), the conversion from single-ended to differential within the
WM8904 adds a further 6dB of gain to the signal path.
Each input channel can be independently muted using LINMUTE and RINMUTE.
It is recommended to not adjust the gain dynamically whilst the signal path is enabled; the signal
should be muted at the input or output stage prior to adjusting the volume control.
REGISTER
ADDRESS
BIT
LABEL
DEFAULT
DESCRIPTION
R44 (2Ch)
7
LINMUTE
1
Left Input PGA Mute
Analogue Left
Input 0
0 = not muted
1 = muted
4:0
7
LIN_VOL [4:0]
RINMUTE
00101
1
Left Input PGA Volume
(See Table 4 for volume range)
Right Input PGA Mute
0 = not muted
R45 (2Dh)
Analogue
Right Input 0
1 = muted
4:0
RIN_VOL [4:0]
00101
Right Input PGA Volume
(See Table 4 for volume range)
Table 3 Input PGA Volume Control
PP, Rev 3.3, September 2012
39
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
