Rainbow Electronics MAX9814 - Manual (Page 8)

Attack Time
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
CT
seconds (where C CT is the external timing capacitor):
• Use a short attack time for the AGC to react quickly
to transient signals, such as snare drum beats
(music) or gun shots (DVD).
• Use a longer attack time to allow the AGC to ignore
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.
Hold Time
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.
Release Time
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
CT
and setting the attack-to-release time ratio by configur-
ing A/R as shown in Table 1:
• Use a small ratio to maximize the speed of the AGC.
• Use a large ratio to maximize the sound quality and
prevent repeated excursions above the threshold
from being independently adjusted by the AGC.
AGC Output Threshold
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.
Microphone Bias
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.
Applications Information
Programming Attack and Release Times
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.
MAX9814
Microphone Amplifier with AGC and
Low-Noise Microphone Bias
8 _______________________________________________________________________________________
10ms/div
V CT
500mV/div
V MICOUT
(AC-COUPLED)
1V/div
ATTACK
RELEASE
HOLD
0V
0V
A/R ATTACK/RELEASE RATIO
GND 1:500
V DD 1:2000
Unconnected 1:4000
Figure 2. Input Burst Exceeding AGC Limit
Table 1. Attack-and-Release Ratios
t RELEASE (ms)
C CT
t ATTACK
(ms)
A/R =
GND
A/R =
V
DD
A/R =
UNCONNECTED
22nF 0.05 25 100 200
47nF 0.11 55 220 440
68nF 0.16 80 320 640
100nF 0.24 120 480 960
220nF 0.53 265 1060 2120
470nF 1.1 550 2200 4400
680nF 1.63 815 3260 6520
1µF 2.4 1200 4800 9600
Table 2. Attack-and-Release Time