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.

• 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

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.

Microphone Amplifier with AGC and

Low-Noise Microphone Bias

8

_______________________________________________________________________________________

10ms/div

500mV/div

(AC-COUPLED)

1V/div

ATTACK

RELEASE

HOLD

0V

0V

A/R

ATTACK/RELEASE RATIO

GND

1:500

1:2000

Unconnected

1:4000

Figure 2. Input Burst Exceeding AGC Limit

Table 1. Attack-and-Release Ratios

(ms)

A/R =

GND

A/R =

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