Input Boost - Cirrus Logic WM8940 Manual

Rev 4.4
In mixer mode (AUXMODE=1) the on-chip input resistor is bypassed, this allows the user to sum in
multiple inputs with the use of external resistors. When used in this mode there will be gain variations
through this path from part to part due to the variation of the internal 20kΩ resistors relative to the
higher tolerance external resistors.
REGISTER BIT LABEL
ADDRESS
R1 6 AUXEN
Power
management 1
R44 3 AUXMODE
Input control
Table 5 Auxiliary Input Buffer Control

INPUT BOOST

The input BOOST circuit has 3 selectable inputs: the input microphone PGA output, the AUX amplifier
output and the MICP input pin (when not using a differential microphone configuration). These three
inputs can be mixed together and have individual gain boost/adjust as shown in Figure 8.
Output from
AUX amp
Output from
input PGA
MICP
Figure 8 Input Boost Stage
The input PGA path can have a +20dB boost (PGABOOST=1) a 0dB pass through (PGABOOST=0)
or be completely isolated from the input boost circuit (INPPGAMUTE=1).
REGISTER BIT
ADDRESS
R45 6 INPPGAMUTE
Input PGA gain
control
R47 8 PGABOOST
Input BOOST
control
Table 6 Input BOOST Stage Control
DEFAULT
0 Auxiliary input buffer enable
0 = OFF
1 = ON
0 0 = inverting buffer
1 = mixer (on-chip input resistor bypassed)
-12dB to + 6dB
AUX2BOOSTVOL
AUX2BOOSTVOL=000
R47[2:0]
0dB or +20dB
PGABOOST INPPGAMUTE
R47[8] R45[6]
-12dB to + 6dB
MICP2BOOSTVOL MICP2BOOSTVOL=000
R47[6:4]
LABEL DEFAUL
T
1 Mute control for input PGA:
0=Input PGA not muted, normal operation
1=Input PGA muted (and disconnected from
the following input BOOST stage).
0 0 = PGA output has +0dB gain through input
BOOST stage.
1 = PGA output has +20dB gain through
input BOOST stage.
WM8940
DESCRIPTION
To ADC input
and output
mixers
DESCRIPTION
23