Datasheet SSM2166 (Analog Devices) - 8

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SSM2166 Data Sheet THEORY OF OPERATION
Figure 15 illustrates a typical transfer characteristic for the
APPLICATIONS INFORMATION
SSM2166 where the output level in decibels is plotted as a The SSM2166 is a complete microphone signal conditioning function of the input level in decibels. The dotted line indicates system on a single integrated circuit. Designed primarily for voice- the transfer characteristic for a unity-gain amplifier. For input band applications, this integrated circuit provides amplification, signals in the range of VDE (downward expansion) to VRP (rotation rms detection, limiting, variable compression, and downward point), an r dB change in the input level causes a 1 dB change in expansion. An integral voltage-controlled amplifier (VCA) the output level. Here, r is defined as the compression ratio. The provides up to 60 dB of gain in the signal path with approximately compression ratio can be varied from 1:1 (no compression) to 30 kHz bandwidth. An input buffer, op amp circuit can provide over 15:1 via a single resistor, RCOMP. Input signals above VRP additional gain because the circuit can be set anywhere from 0 dB are compressed with a fixed compression ratio of approximately to 20 dB for a total signal path gain of up to 80 dB. The device 15:1. This region of operation is the limiting region. Varying the operates on a single 5 V supply, accepts input signals up to 1 V compression ratio has no effect on the limiting region. The break- rms, and produces output signal levels >1 V rms (3 V p-p) into point between the compression region and the limiting region is loads >5 kΩ. The internal rms detector has a time constant set referred to as the limiting threshold or the rotation point and is by an external capacitor. user specified in the SSM2166. The rotation point term derives from the observation that the straight line in the compression The SSM2166 contains an input buffer and automatic gain region rotates about this point on the input/output characteristic as control (AGC) circuit for audio-band and voice-band signals. the compression ratio is changed. Circuit operation is optimized by providing a user-adjustable time constant and compression ratio. A downward expansion The gain of the system with an input signal level of VRP is fixed (noise gating) feature eliminates circuit noise in the absence of by RGAIN, regardless of the compression ratio, and is the nominal an input signal. The SSM2166 allows the user to set the downward gain of the system. The user can increase the nominal gain of expansion threshold, the limiting threshold (rotation point), the the system via the on-board VCA by up to 20 dB. Additionally, input buffer fixed gain, and the internal VCA nominal gain at the input buffer of the SSM2166 can provide fixed gains of 0 dB the rotation point. The SSM2166 also features a power-down to 20 dB with R1 and R2. mode and muting capability. Input signals below VDE are downward expanded; that is, a
SIGNAL PATH
−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 Figure 16 illustrates the block diagram of the SSM2166. The system is small for very small input signal levels, even though it audio input signal is processed by the input buffer and then may be quite large for small input signals above V by the VCA. The input buffer presents an input impedance of DE. The user sets the downward expansion threshold, V approximately 180 kΩ to the source. A dc voltage of approximately DE, externally via R 1.5 V is present at AUDIO +IN (Pin 7), requiring the use of a GATE at Pin 9 (NOISE GATE SET). The SSM2166 provides an active high, CMOS-compatible digital input whereby a power- blocking capacitor (C1) for ground referenced sources. A 0.1 μF down feature reduces the device supply current to less than 100 μA. capacitor is a good choice for most audio applications. The input buffer is a unity-gain stable amplifier that can drive the low
LIMITING THRESHOLD LIMITING
impedance input of the VCA.
(ROTATION POINT) REGION
The VCA is a low distortion, variable-gain amplifier (VGA) whose gain is set by the side-chain control circuitry. The input
COMPRESSION DOWNWARD REGION
to the VCA is a virtual ground in series with approximately 1 kΩ.
EXPANSION 1 VCA GAIN THRESHOLD B)
An external blocking capacitor (C6) must be used between the
d (NOISE GATE) r (
buffer output and the VCA input. The 1 kΩ impedance between
UT P
amplifiers determines the value of this capacitor, which is typically
UT O DOWNWARD
between 1 μF and 10 μF. An aluminum electrolytic capacitor is
EXPANSION REGION
an economical choice. The VCA amplifies the input signal current
1
flowing through C6 and converts this current to a voltage at the
1
OUTPUT pin (Pin 13). The net gain from input to output can be as high as 60 dB (without additional buffer gain), depending
V
5
VDE RP
01
INPUT (dB)
7- on the gain set by the control circuitry. 35 00 Figure 15. General Input/Output Characteristics Rev. F | Page 8 of 13 Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION FUNCTIONAL BLOCK DIAGRAM AND TYPICAL SPEECH APPLICATION TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS THERMAL RESISTANCE ESD CAUTION PIN CONFIGURATION AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION APPLICATIONS INFORMATION SIGNAL PATH LEVEL DETECTOR CONTROL CIRCUITRY Compression Ratio Rotation Point VCA Gain Setting and Muting Downward Expansion Threshold POWER-DOWN FEATURE PCB LAYOUT CONSIDERATIONS OUTLINE DIMENSIONS ORDERING GUIDE