wm8775 Wolfson Microelectronics plc, wm8775 Datasheet - Page 30

• Manufacturer Part Number: wm8775
• Description: 24-bit, 96khz Adc With 4 Channel I/p Multiplexer
• Manufacturer: Wolfson Microelectronics plc

WM8775
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When the noise gate is triggered, the PGA gain is held constant (preventing it from ramping up as it
would normally when the signal is quiet).
The table below summarises the noise gate control register. The NGTH control bits set the noise
gate threshold with respect to the ADC full-scale range. The threshold is adjusted in 6dB steps.
Levels at the extremes of the range may cause inappropriate operation, so care should be taken with
set–up of the function. Note that the noise gate only works in conjunction with the ALC function, and
always
Register bits AMX[3:0] control the left and right channel inputs into the stereo ADC. The default is
AIN1. One bit of AMX is allocated to each stereo input pair to allow the signals to be mixed before
being digitised by the ADC. For example, if AMX[3:0] is 0101, the input signal to the ADC will be
(AIN1L+AIN3L) on the left channel and (AIN1R+AIN3R) on the right channel.
However if the analogue input buffer is powered down, by setting AINPD, then all 4-channel mux
inputs are switched to buffered VMIDADC.
Table 12 ADC Input Mixer Control
Figure 24 ADC Input Mixer
ADC INPUT MUX AND POWERDOWN CONTROL
AMX[3:0]
REGISTER
Noise Gate
0001
0010
0100
1000
ADDRESS
AIN2L/R
AIN3L/R
AIN1L/R
AIN4L/R
R19(13h)
0010011
ADC Mux and
Control
Powerdown
REGISTER
ADDRESS
R21(15h)
0010101
Control
operates
LEFT ADC INPUT
on
AIN1L
AIN2L
AIN3L
AIN4L
AMX[0]
AMX[1]
AMX[2]
AMX[3]
4:2
BIT
0
the
BIT
3:0
NGTH[2:0]
same
LABEL
NGAT
AMX[3:0]
LABEL
RIGHT ADC INPUT
channel(s) as
AIN1R
AIN2R
AIN3R
AIN4R
DEFAULT
DEFAULT
000
0
0001
the
1 = enable
0 = disable
Noise gate threshold (with respect to
analogue input level)
000: -78dBFS
001: -72dBfs
… 6 dB steps
110: -42dBFS
111: -36dBFS
Noise gate function enable
ALC
ADC input mixer control bits (see
Table 12)
(left,
DESCRIPTION
DESCRIPTION
PD, Rev 4.4, October 2008
right,
both,
Production Data
or none).
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