Datasheet

ManualsBrandsAnalog Devices ManualsAmplifiers & ComparatorsMicrophone Amplifier IC SO-14

Microphone Preamplifier with

Variable Compression and Noise Gating

SSM2166

Rev. D

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FEATURES

Complete microphone conditioner in a 14-lead SOIC package

Single 5 V operation

Adjustable noise gate threshold

Compression ratio set by external resistor

Automatic limiting feature—prevents ADC overload

Adjustable release time

Low noise and distortion

Power-down feature

20 kHz bandwidth (±1 dB)

APPLICATIONS

Microphone preamplifiers/processors

Computer sound cards

Public address/paging systems

Communication headsets

Telephone conferencing

Guitar sustain effects generators

Computerized voice recognition

Surveillance systems

Karaoke and DJ mixers

GENERAL DESCRIPTION

The SSM2166 integrates a complete and flexible solution for

conditioning 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

compression ratio is set by a single resistor and can be varied

from 1:1 to over 15:1 relative to a user-defined rotation point;

signals above the rotation point are limited to prevent overload

and to eliminate popping. In the 1:1 compression setting, the

SSM2166 can be programmed with a fixed gain of up to 20 dB;

this gain is in addition to the variable gain in other compression

settings. The input buffer can also be configured for front-end

gains of 0 dB to 20 dB. A downward expander (noise gate) prevents

amplification 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 may

add noise or impair accuracy of speech recognition algorithms.

The compression ratio and time constants are set externally. A

high degree of flexibility is provided by the VCA gain, rotation

point, and noise gate adjustment pins.

The SSM2166 is an ideal companion product for audio codecs

used in computer systems, such as the AD1845. The SSM2166 is

available in a 14-lead SOIC package and is guaranteed for

operation over the extended industrial temperature range of

−40°C to +85°C.

–10

–20

–30

–40

–50

–60

–70 –60 –50 –40 –30 –20 –10 0

INPUT (dBu)

OUTPUT (dBu)

00357-002

COMP RATIO = 10:1

COMP RATIO = 2:1

COMP RATIO = 1:1

Figure 1. Compression and Gating Characteristics with 10 dB of Fixed Gain (The

Gain Adjust Pin Can Be Used to Vary This Fixed Gain Amount)

FUNCTIONAL BLOCK DIAGRAM AND TYPICAL SPEECH APPLICATION

BUFFER

1kΩ

BUF OUT

VCA

V+

435 14

LEVEL

DETECTOR

VCA

CONTROL

8112

SSM2166

POWER DOWN GND AVG CAP COMP RATIO SET

GAIN

ADJUST

OUTPUT

NOISE GATE

SET

ROTATION

SET

–IN

AUDIO

+IN

0.1µF

10kΩ

1µF

10kΩ

10µF 10µF (OPTIONAL)

25kΩ22µF

500kΩ

2.3kΩ

17kΩ

00357-001

Figure 2.

Summary of content (20 pages)

PAGE 1
Microphone Preamplifier with Variable Compression and Noise Gating SSM2166 SSM2166 can be programmed with a fixed gain of up to 20 dB; this gain is in addition to the variable gain in other compression settings. The input buffer can also be configured for front-end gains of 0 dB to 20 dB. A downward expander (noise gate) prevents amplification of noise or hum.
PAGE 2
SSM2166 TABLE OF CONTENTS Features .............................................................................................. 1 Signal Path ......................................................................................8 Applications ....................................................................................... 1 Level Detector ................................................................................9 General Description ........................................................
PAGE 3
SSM2166 SPECIFICATIONS V+ = 5 V, f = 1 kHz, RL = 100 kΩ, RGATE = 600 kΩ, RROT PT = 3 kΩ, RCOMP = 0 Ω, R1 = 0 Ω, R2 = ∞ Ω, TA = 25°C, unless otherwise noted; VIN = 300 mV rms. Table 1. Parameter AUDIO SIGNAL PATH Voltage Noise Density Noise Total Harmonic Distortion and Noise Input Impedance Output Impedance Load Drive Symbol Conditions en 15:1 Compression 20 kHz bandwidth, VIN = GND Second and third harmonics, VIN = −20 dBu, 22 kHz low-pass filter THD + N 1 2 Max Unit 0.
PAGE 4
SSM2166 ABSOLUTE MAXIMUM RATINGS THERMAL RESISTANCE Table 2. Parameter Supply Voltage Audio Input Voltage Operating Temperature Range Storage Temperature Range Junction Temperature (TJ) Lead Temperature (Soldering, 60 sec) Rating 10 V Supply voltage −40°C to +85°C −65°C to +150°C 150°C 300°C Table 3. Package Type 14-Lead SOIC ESD CAUTION Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device.
PAGE 5
SSM2166 PIN CONFIGURATION AND FUNCTION DESCRIPTIONS GND 1 GAIN ADJUST 2 VCAIN 3 SSM2166 14 V+ 13 OUTPUT 12 POWER DOWN TOP VIEW 11 ROTATION SET (Not to Scale) BUF OUT 5 10 COMP RATIO SET –IN 6 9 NOISE GATE SET AUDIO +IN 7 8 AVG CAP 00357-003 VCAR 4 Figure 3. Pin Configuration Table 4. Pin Function Descriptions Pin No.
PAGE 6
SSM2166 TYPICAL PERFORMANCE CHARACTERISTICS 0 5 COMP RATIO = 15:1 –20 COMP RATIO = 2:1 1 COMP RATIO = 1:1 THD + N (%) –40 –50 TA = 25°C V+ = 5V VIN = 300mV rms @ 1kHz RL = 100kΩ NOISE GATE SETTING ≈ 550µV rms ROTATION SET ≈ 300mV rms GAIN ADJUST = 1.25kΩ –70 –80 –80 –70 –60 –50 –40 –30 INPUT (dBu) –20 –10 RL = 100kΩ 0.1 0 0.05 0.01 00357-004 –60 RL = 10kΩ Figure 4. Output vs.
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SSM2166 5µV 100 • • • • • • • • 1s • • • • • • • • • • • • • • • • • • • • • • • • 20mV • • • • • • • • 100 • • • • 90 • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • TA = 25°C CAVG = 2.
PAGE 8
SSM2166 THEORY OF OPERATION Figure 15 illustrates a typical transfer characteristic for the SSM2166 where the output level in decibels is plotted as a function of the input level in decibels. The dotted line indicates the transfer characteristic for a unity-gain amplifier. For input signals in the range of VDE (downward expansion) to VRP (rotation point), an r dB change in the input level causes a 1 dB change in the output level. Here, r is defined as the compression ratio.
PAGE 9
SSM2166 The gain of the VCA at the rotation point is set by the value of a resistor, RGAIN, connected between Pin 2 and GND. The relationship between the VCA gain and RGAIN is shown in Figure 6. The AGC range can be as high as 60 dB. The VCAIN pin (Pin 3) is the noninverting input terminal to the VCA. The inverting input of the VCA is available at the VCAR pin (Pin 4) and exhibits an input impedance of 1 kΩ, as well.
PAGE 10
SSM2166 The performance of the rms level detector is illustrated for a CAVG of 2.2 μF in Figure 17 and for a CAVG of 22 μF in Figure 18. In each of these images, the input signal to the SSM2166 (not shown) is a series of tone bursts in six successive 10 dB steps. The tone bursts range from −66 dBV (0.5 mV rms) to −6 dBV (0.5 V rms).
PAGE 11
SSM2166 Because limiting occurs for signals larger than the rotation point (VIN > VRP), the rotation point effectively sets the maximum output signal level. It is recommended that the rotation point be set at the upper extreme of the range of typical input signals so that the compression region covers the entire desired input signal range. Occasional larger signal transients are then attenuated by the action of the limiter. The gain of the VCA can be reduced below 0 dB by making RGAIN smaller than 1 kΩ.
PAGE 12
SSM2166 The supply current of the SSM2166 can be reduced to less than 100 μA by applying an active high, 5 V CMOS-compatible input to the POWER DOWN pin (Pin 12). In this state, the input and output circuitry of the SSM2166 assumes a high impedance state; as such, the potentials at the input pin and the output pin are determined by the external circuitry connected to the SSM2166. The SSM2166 takes approximately 200 ms to settle from a powerdown to power-on command.
PAGE 13
SSM2166 on the SSM2166 by connecting the GAIN ADJUST pin (Pin 2) through a 330 Ω resistance to ground. This is provided on the evaluation board via R11 and S1. Capacitor C5, connected between Pin 2 and ground and provided on the evaluation board, can be used to avoid audible clicks when using the mute function. EVALUATION BOARD A schematic diagram of the SSM2166 evaluation board is illustrated in Figure 26.
PAGE 14
SSM2166 IC2 00357-026 00357-028 IC1 Figure 27. Evaluation Board Topside Silkscreen (Not to Scale) Figure 29. Evaluation Board Backside Metallization (Not to Scale) EVALUATION BOARD EXAMPLES To illustrate how easy it is to program the SSM2166, a practical example is provided. The SSM2166 was used to interface an electret-type microphone to a postamplifier. The evaluation board or the circuit configuration shown in Figure 26 can be used.
PAGE 15
SSM2166 Note that the SSM2166 processes the output of the buffer, which in the previous example is 20 dB or 10 times the input level. Use the oscilloscope to verify that the buffer is not being driven into clipping with excessive input signals. In the application, take the minimum gain in the buffer consistent with the average source level as well as the crest factor (ratio of peak to rms). Step 3: Testing Setup With the power on, adjust the generator for an input level of 15 mV, 1 kHz.
PAGE 16
SSM2166 SETUP SUMMARY The transfer condition of Figure 2 has been implemented. For inputs below the 100 μV noise gate threshold, circuit and background noise is minimized. Above it, the output increases at a rate of 1 dB for each 2 dB input increase until the 500 mV rotation point is reached at an input of approximately 15 mV. For higher inputs that drive the output beyond 500 mV, limiting occurs and there is little further increase.
PAGE 17
SSM2166 OUTLINE DIMENSIONS 8.75 (0.3445) 8.55 (0.3366) 8 14 1 7 1.27 (0.0500) BSC 0.25 (0.0098) 0.10 (0.0039) COPLANARITY 0.10 0.51 (0.0201) 0.31 (0.0122) 6.20 (0.2441) 5.80 (0.2283) 0.50 (0.0197) 0.25 (0.0098) 1.75 (0.0689) 1.35 (0.0531) SEATING PLANE 45° 8° 0° 0.25 (0.0098) 0.17 (0.0067) 1.27 (0.0500) 0.40 (0.
PAGE 18
SSM2166 NOTES Rev.
PAGE 19
SSM2166 NOTES Rev.