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| 型號： | SSM2166SZ-REEL7 |
| 廠商： | Analog Devices Inc |
| 文件頁數(shù)： | 19/20頁 |
| 文件大?。?/td> | 0K |
| 描述： | IC PREAMP AUDIO MONO MIC 14SOIC |
| 產(chǎn)品變化通告： | Material Change 25/Mar/2009 |
| 標準包裝： | 1 |
| 類型： | AB 類 |
| 輸出類型： | 1-通道（單聲道） |
| 電源電壓： | 4.5 V ~ 5.5 V |
| 特點： | 麥克風 |
| 安裝類型： | 表面貼裝 |
| 供應商設備封裝： | 14-SOICN |
| 封裝/外殼： | 14-SOIC（0.154"，3.90mm 寬） |
| 包裝： | 標準包裝 |
| 產(chǎn)品目錄頁面： | 775 (CN2011-ZH PDF) |
| 其它名稱： | SSM2166SZ-REEL7DKR |
SSM2166
Data Sheet
Rev. E | 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
active high, CMOS-compatible digital input whereby a power-
down feature reduces the device supply current to less than 100 A.
LIMITING
REGION
LIMITING
THRESHOLD
(ROTATION POINT)
COMPRESSION
REGION
1
r
1
DOWNWARD
EXPANSION
THRESHOLD
(NOISE GATE)
DOWNWARD
EXPANSION
REGION
VCA GAIN
VRP
OU
TP
U
T
(
dB
)
INPUT (dB)
00357-
015
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
produces output signal levels >1 V rms (3 V p-p) into loads >5 k.
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
audio input signal is processed by the input buffer and then
by the VCA. The input buffer presents an input impedance of
approximately 180 k 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 F
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 k.
An external blocking capacitor (C6) must be used between the
buffer output and the VCA input. The 1 k impedance between
amplifiers determines the value of this capacitor, which is typically
between 1 F and 10 F. 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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