REV. A

Information furnished by Analog Devices is believed to be accurate and

reliable. However, no responsibility is assumed by Analog Devices for its

use, nor for any infringements of patents or other rights of third parties

which may result from its use. No license is granted by implication or

otherwise under any patent or patent rights of Analog Devices.

a

SSM2165*

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.

Tel: 781/329-4700

World Wide Web Site: http://www.analog.com

Fax: 781/326-8703

© Analog Devices, Inc., 1999

FUNCTIONAL BLOCK DIAGRAM

VCA

+1

V+

V+

+

C2

10 F

BUFFER

LEVEL

DETECTOR

CONTROL

SSM2165

C1

0.1 F

AUDIO

IN+

R1

25k

C3

22 F

AVG CAP

COMPRESSION

RATIO SET

GND

2

2

+

FEATURES

Complete Microphone Conditioner in an 8-Lead Package

Single +5 V Operation

Preset Noise Gate Threshold

Compression Ratio Set by External Resistor

Automatic Limiting Feature Prevents ADC Overload

Adjustable Release Time

Low Noise and Distortion

20 kHz Bandwidth ( 1 dB)

Low Cost

APPLICATIONS

Microphone Preamplifier/Processor

Computer Sound Cards

Public Address/Paging Systems

Communication Headsets

Telephone Conferencing

Guitar Sustain Effects Generator

Computerized Voice Recognition

Surveillance Systems

Karaoke and DJ Mixers

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),

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

µV), 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.

INPUT – dBu

0

–10

–80

–70

–60

–50

–40

–30

–20

–10

–40

–50

–60

–20

–30

Figure 1a. SSM2165-1 Compression and Gating Characteristics

INPUT – dBu

0

–10

–80

–70

–60

–50

–40

–30

–20

–10

–40

–50

–60

–70

–20

–30

Figure 1b. SSM2165-2 Compression and Gating Characteristics

Microphone Preamplifier with

Variable Compression and Noise Gating

*Patents pending.