max98088 Maxim Integrated Products, Inc., max98088 Datasheet - Page 69

• Manufacturer Part Number: max98088
• Description: Stereo Audio Codec With Flexsound Technology
• Manufacturer: Maxim Integrated Products, Inc.

Table 5. ADC Input Mixer Register
The device’s record signal path includes both automatic
gain control (AGC) for the microphone inputs and a digi-
tal noise gate at the output of the ADC (Figure 10).
The IC’s AGC monitors the signal level at the output of
the ADC and then adjusts the MIC1 and MIC2 analog
PGA settings automatically. When the signal level is
below the predefined threshold, the gain is increased up
to its maximum (20dB). If the signal exceeds the thresh-
old, the gain is reduced to prevent the output signal level
exceeding the threshold. When AGC is enabled, the
microphone PGA is not user programmable. The AGC
provides a more constant signal level and improves the
available ADC dynamic range.
Figure 10. Record Path Signal Processing Block Diagram
REGISTER
0x23/0x24
PA1EN:
0/20/30dB
PA2EN:
0/20/30dB
PGAM1:
+20dB TO -6dB
PGAM2:
+20dB TO 0dB
BIT
7
6
5
4
3
2
1
0
Record Path Signal Processing
MIXADL/MIXADR
MIXADR
MIXADL
MIX
MIX
NAME
ADLEN
ADREN
ADCL
ADCR
AUTOMATIC
CONTROL
GAIN
AVLG: 0/6/
TO -15dB
AVL:0dB
Microphone AGC
12/18dB
SRMIX_
MODE1
MODE
AVFLT
Left/Right ADC Input Mixer
Selects which analog inputs are recorded by the left/right ADC.
1xxxxxxx = MIC1
x1xxxxxx = MIC2
xx1xxxxx = INA1 pin direct
xxx1xxxx = INA2 pin direct
xxxx1xxx = INA1
xxxxx1xx = INA2 (INADIFF = 0) or INA2 - INA1 (INADIFF = 1)
xxxxxx1x = INB1
xxxxxxx1 = INB2 (INBDIFF = 0) or INB2 - INB1 (INBDIFF = 1)
SAMPLE RATE
CONVERTER
NOISE GATE
with FLEXSOUND Technology
FILTERS
AUDIO/
VOICE
AVRG: 0/6/
12/18dB
AVR:0dB
TO -15dB
Since the AGC increases the levels of all signals below
a user-defined threshold, the noise floor is effectively
increased by 20dB. To counteract this, the noise gate
reduces the gain at low signal levels. Unlike typical noise
gates that completely silence the output below a defined
level, the noise gate in the IC applies downward expan-
sion. The noise gate attenuates the output at a rate of
1dB for each 2dB the signal is below the threshold.
The noise gate can be used in conjunction with the AGC
or on its own. When the AGC is enabled, the noise gate
reduces the output level only when the AGC has set the
gain to the maximum setting. Figure 11 shows the gain
response resulting from using the AGC and noise gate.
Figure 11. AGC and Noise Gate Input vs. Output Gain
Stereo Audio Codec
-100
-120
-20
-40
-60
-80
DESCRIPTION
0
-120
AGC AND NOISE GATE
-100
AMPLITUDE RESPONSE
AGC AND NOISE GATE
INPUT AMPLITUDE (dBFS)
-80
AGC ONLY
-60
NOISE GATE ONLY
AGC AND NOISE
GATE DISABLED
-40
-20
Noise Gate
0
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