max9856 Maxim Integrated Products, Inc., max9856 Datasheet - Page 28

• Manufacturer Part Number: max9856
• Description: Max9856 Low-power Audio Codec With Directdrive Headphone Amplifiers
• Manufacturer: Maxim Integrated Products, Inc.

Low-Power Audio CODEC with
DirectDrive Headphone Amplifiers
The MAX9856 AGC continuously adjusts the analog
microphone PGAs to maintain constant signal level. When
the AGC is enabled, manual control of the input PGA is
not possible. The PGA includes zero-cross detection,
which prevents gain changes, from being audible.
The AGC process consists of three main sections.
When the AGC threshold is exceeded, the gain is
reduced exponentially with a time constant referred to
as the attack time. Once the large signal has passed,
Table 9. Digital Highpass Filter Cutoff Frequencies
Table 10. Automatic Gain Control Registers
AGC Register Bit Description
28
LRCLK (kHz)
______________________________________________________________________________________
11.025
22.05
0x0C
0x0D
REG
44.1
12
16
24
32
48
64
88
96
8
AGCRLS
BITS
000
Off
Off
Off
Off
Off
Off
Off
Off
Off
Off
Off
Off
B7
0
0
AGC Release Time. The release time is the time it takes for the gain to return to its normal level after the
input signal has fallen below the threshold and the hold time has passed:
000—78ms
001—156ms
010—312ms (recommended)
011—625ms
100—1.25s
101—2.5s
110—5s
111—10s
Automatic Gain Control
001 (Hz)
10
14
15
20
28
30
40
55
60
5
7
8
B6
0
010 (Hz)
AGCRLS
111
120
10
14
15
20
28
30
40
55
60
80
B5
0
011 (Hz)
AGCSRC
111
121
161
222
241
20
28
30
40
55
60
80
ADCHP/DACHP
B4
the AGC waits the specified hold time before reducing
the gain. The time required to reduce the gain from
maximum attenuation to minimum attenuation is known
as the release time.
The AGC circuitry only operates on the PGA in the micro-
phone path, but the digital level detector is based on the
mixed signal. Only use the AGC when input signals from
the LINEIN and AUXIN are excluded or attenuated.
Table 10 lists the AGC registers and shows the AGC
register bit description.
FUNCTION
100 (Hz)
112
122
162
224
244
325
448
487
41
56
61
81
B3
AGCATK
101 (Hz)
114
124
165
227
247
330
455
495
660
909
990
82
B2
AGCSTH
110 (Hz)
1021
1362
1877
2043
170
235
255
340
469
511
681
938
B1
AGCHLD
111 (Hz)
1002
1091
1455
2005
2182
2909
4009
4364
364
501
545
727
B0