MAX9814 データシートの表示（PDF） - Maxim Integrated

部品番号

コンポーネント説明

メーカー

MAX9814

Microphone Amplifier with AGC and Low-Noise Microphone Bias

Maxim Integrated 

Microphone Amplifier with AGC and

Low-Noise Microphone Bias

HOLD

ATTACK

RELEASE

0V

0V

VCT

500mV/div

VMICOUT

(AC-COUPLED)

1V/div

10ms/div

Figure 2. Input Burst Exceeding AGC Limit

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 CCT

seconds (where CCT 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 25ms. The

release time is set by picking an attack time using CCT

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.

Table 1. Attack-and-Release Ratios

A/R

GND

VDD

Unconnected

ATTACK/RELEASE RATIO

1:500

1:2000

1:4000

The attack and release times can be selected by utiliz-

ing the corresponding capacitances listed in Table 2.

Table 2. Attack-and-Release Time

CCT

tATTACK

(ms)

22nF

0.05

47nF

0.11

68nF

0.16

100nF

0.24

220nF

0.53

470nF

1.1

680nF

1.63

1µF

2.4

A/R =

GND

25

55

80

120

265

550

815

1200

tRELEASE (ms)

A/R =

A/R =

VDD UNCONNECTED

100

200

220

440

320

640

480

960

1060

2120

2200

4400

3260

6520

4800

9600

8 _______________________________________________________________________________________

Share Link: