SSM2166SZ-REEL71 datasheet(8/20 Pages) AD

Part #	SSM2166SZ-REEL71

Description	Microphone Preamplifier with Variable Compression and Noise Gating

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SSM2166

Rev. D | Page 8 of 20

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. The

compression ratio can be varied from 1:1 (no compression) to

over 15:1 via a single resistor, RCOMP. Input signals above VRP

are compressed with a fixed compression ratio of approximately

15:1. This region of operation is the limiting region. Varying the

compression ratio has no effect on the limiting region. The break-

point between the compression region and the limiting region is

referred to as the limiting threshold or the rotation point and is

user specified in the SSM2166. The term “rotation point” derives

from the observation that the straight line in the compression

region rotates about this point on the input/output characteristic as

the compression ratio is changed.

The gain of the system with an input signal level of VRP is fixed

by RGAIN, regardless of the compression ratio, and is the nominal

gain of the system. The nominal gain of the system can be

increased by the user via the on-board VCA by up to 20 dB.

Additionally, the input buffer of the SSM2166 can be configured

to provide fixed gains of 0 dB to 20 dB with R1 and R2.

Input signals below VDE are downward expanded; that is, a

−1 dB change in the input signal level causes approximately a

−3 dB change in the output level. As a result, the gain of the

system is small for very small input signal levels, even though it

may be quite large for small input signals above VDE. The down-

ward expansion threshold, VDE, is set externally by the user via

RGATE at Pin 9 (NOISE GATE SET). The SSM2166 provides an

active high, CMOS-compatible digital input whereby a power-

down feature reduces the device supply current to less than 100 μA.

LIMITING

REGION

LIMITING

THRESHOLD

(ROTATION POINT)

COMPRESSION

REGION

1

r

1

DOWNWARD

EXPANSION

THRESHOLD

(NOISE GATE)

DOWNWARD

EXPANSION

REGION

VCA GAIN

VRP

INPUT (dB)

VDE

Figure 15. General Input/Output Characteristics

APPLICATIONS INFORMATION

The SSM2166 is a complete microphone signal conditioning

system on a single integrated circuit. Designed primarily for voice-

band applications, this integrated circuit provides amplification,

rms detection, limiting, variable compression, and downward

expansion. An integral voltage-controlled amplifier (VCA)

provides up to 60 dB of gain in the signal path with approximately

30 kHz bandwidth. Additional gain is provided by an input

buffer, op amp circuit that can be set anywhere from 0 dB to 20 dB

for a total signal path gain of up to 80 dB. The device operates

on a single 5 V supply, accepts input signals up to 1 V rms, and

produces output signal levels >1 V rms (3 V p-p) into loads >5 kΩ.

The internal rms detector has a time constant set by an external

capacitor.

The SSM2166 contains an input buffer and automatic gain

control (AGC) circuit for audio-band and voice-band signals.

Circuit operation is optimized by providing a user-adjustable

time constant and compression ratio. A downward expansion

(noise gating) feature eliminates circuit noise in the absence of

an input signal. The SSM2166 allows the user to set the downward

expansion threshold, the limiting threshold (rotation point), the

input buffer fixed gain, and the internal VCA nominal gain at

the rotation point. The SSM2166 also features a power-down

mode and muting capability.

SIGNAL PATH

Figure 16 illustrates the block diagram of the SSM2166. The

audio input signal is processed by the input buffer and then

by the VCA. The input buffer presents an input impedance of

approximately 180 kΩ to the source. A dc voltage of approximately

1.5 V is present at AUDIO +IN (Pin 7), requiring the use of a

blocking capacitor (C1) for ground referenced sources. A 0.1 μF

capacitor is a good choice for most audio applications. The input

buffer is a unity-gain stable amplifier that can drive the low

impedance input of the VCA.

The VCA is a low distortion, variable-gain amplifier (VGA)

whose gain is set by the side-chain control circuitry. The input

to the VCA is a virtual ground in series with approximately 1 kΩ.

An external blocking capacitor (C6) must be used between the

buffer output and the VCA input. The 1 kΩ impedance between

amplifiers determines the value of this capacitor, which is typically

between 1 μF and 10 μF. An aluminum electrolytic capacitor is

an economical choice. The VCA amplifies the input signal current

flowing through C6 and converts this current to a voltage at the

OUTPUT pin (Pin 13). The net gain from input to output can

be as high as 60 dB (without additional buffer gain), depending

on the gain set by the control circuitry.

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