ssm2165-1 Analog Devices, Inc., ssm2165-1 Datasheet

ssm2165-1

Manufacturer Part Number

ssm2165-1

Description

Microphone Preamplifier With Variable Compression And Noise Gating

Manufacturer

Analog Devices, Inc.

Datasheet

1.SSM2165-1.pdf (12 pages)

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GENERAL DESCRIPTION

The SSM2165 is a complete and flexible solution for condition-

ing microphone inputs in computer audio systems. It is also

excellent for improving vocal clarity in communications and

public address systems. A low noise voltage controlled amplifier

(VCA) provides a gain that is dynamically adjusted by a control

loop to maintain a set compression characteristic. The compres-

sion ratio is set by a single resistor and can be varied from 1:1 to

over 15:1 relative to the fixed rotation point. Signals above the

rotation point are limited to prevent overload and to eliminate

“popping.” A downward expander (noise gate) prevents amplifi-

cation of noise or hum. This results in optimized signal levels

prior to digitization, thereby eliminating the need for additional

gain or attenuation in the digital domain that could add noise or

impair accuracy of speech recognition algorithms. The flexibility

of setting the compression ratio and the time constant of the

level detector, coupled with two values of rotation point, make

the SSM2165 easy to integrate in a wide variety of microphone

conditioning applications.

The SSM2165 is an ideal companion product for audio codecs

used in computer systems, such as the AD1845 and AD1847.

The device is available in 8-lead SOIC and P-DIP packages, and

guaranteed for operation over the extended industrial temperature

range of –40 C to +85 C. As shown in Figure 1a, the SSM2165-1

has a rotation point of –25.7 dBu (40 mV)

and gives –7.7 dBu (320 mV) before limiting. As shown in Figure

1b, the SSM2165-2 has a rotation point of –17.8 dBu (100 mV),

All signals are in rms volts or dBu (0 dBu = 0.775 V rms).

Patents pending.

, a VCA gain of 18 dB,

Variable Compression and Noise Gating

a VCA gain of 8 dB and gives –9.8 dBu (250 mV) before limiting.

Both have a noise gate threshold of –64 dBu (500 V), below

which downward expansion reduces the gain with a ratio of

approximately 1:3. That is, a –3 dB reduction of output signal

occurs with a –1 dB reduction of input signal. For applications

requiring adjustable noise gate threshold, VCA gain up to 18 dB,

and adjustable rotation point, please refer to the SSM2166.

AUDIO

Microphone Preamplifier with

IN+

0.1 F

–10

–20

–30

–40

–50

–60

–70

–10

–20

–30

–40

–50

–60

FUNCTIONAL BLOCK DIAGRAM

–80

SSM2165

–70

GND

V+

BUFFER

BUF

–60

LEVEL

DETECTOR

OUT

INPUT – dBu

AVG CAP

–50

10 F

22 F

–40

VCA

SSM2165

25k

–30

CONTROL

VCA

–20

COMPRESSION

RATIO SET

–10

OUT

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