max9814etdt Maxim Integrated Products, Inc., max9814etdt Datasheet - Page 8

• Manufacturer Part Number: max9814etdt
• Description: Max9814 Microphone Amplifier With Agc And Low-noise Microphone Bias
• Manufacturer: Maxim Integrated Products, Inc.

Microphone Amplifier with AGC and
Low-Noise Microphone Bias
The attack time is the time it takes for the AGC to
reduce the gain after the input signal has exceeded the
threshold level. The gain attenuation during the attack
time is exponential, and defined as one-time constant.
The time constant of the attack is given by 2400 x C
seconds (where C
Hold time is the delay after the signal falls below the
threshold level before the release phase is initiated.
Hold time is internally set to 30ms and nonadjustable.
The hold time is cancelled by any signal exceeding the
set threshold level, and the attack time is reinitiated.
The release time is how long it takes for the gain to
return to its normal level after the output signal has fall-
en below the threshold level and 30ms hold time has
expired. Release time is defined as release from a
20dB gain compression to 10% of the nominal gain set-
ting after the input signal has fallen below the TH
threshold and the 30ms hold time has expired. Release
time is adjustable and has a minimum of 25s. The
release time is set by picking an attack time using C
Figure 2. Input Burst Exceeding AGC Limit
• Use a short attack time for the AGC to react quickly
• Use a longer attack time to allow the AGC to ignore
8
to transient signals, such as snare drum beats
(music) or gun shots (DVD).
short-duration peaks and only reduce the gain when
a noticeable increase in loudness occurs. Short-
duration peaks are not reduced, but louder pas-
sages are. This allows the louder passages to be
reduced in volume, thereby maximizing output
dynamic range.
_______________________________________________________________________________________
0V
0V
ATTACK
CT
HOLD
is the external timing capacitor):
RELEASE
10ms/div
Release Time
V
500mV/div
V
(AC-COUPLED)
1V/div
Attack Time
CT
MICOUT
Hold Time
CT
CT
and setting the attack-to-release time ratio by configur-
ing A/R as shown in Table 1:
The output threshold that activates AGC is adjustable
through the use of an external resistive divider. Once
the divider is set, AGC reduces the gain to match the
output voltage to the voltage set at the TH input.
The MAX9814 features an internal low-noise micro-
phone bias voltage capable of driving most electret
condenser microphones. The microphone bias is regu-
lated at 2V to provide that the input signal to the low-
noise preamplifier does not clip to ground.
Attack and release times are set by selecting the
capacitance value between CT and GND, and by set-
ting the logic state of A/R (Table 1). A/R is a tri-level
logic input that sets the attack-to-release time ratio.
The attack and release times can be selected by utiliz-
ing the corresponding capacitances listed in Table 2.
• Use a small ratio to maximize the speed of the AGC.
• Use a large ratio to maximize the sound quality and
Table 1. Attack-and-Release Ratios
Table 2. Attack-and-Release Time
100nF
220nF
470nF
680nF
22nF
47nF
68nF
C
1µF
prevent repeated excursions above the threshold
from being independently adjusted by the AGC.
Programming Attack and Release Times
CT
Unconnected
GND
A/R
V
DD
t
ATTACK
(ms)
0.05
0.11
0.16
0.24
0.53
1.63
1.1
2.4
Applications Information
A/R =
GND
1200
120
265
550
815
25
55
80
ATTACK/RELEASE RATIO
AGC Output Threshold
t
A/R =
1060
2200
3260
4800
RELEASE
V
100
220
320
480
DD
Microphone Bias
1:2000
1:4000
1:500
UNCONNECTED
(ms)
A/R =
2120
4400
6520
9600
200
440
640
960