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SSM2166 Datasheet(PDF) 6 Page - Analog Devices

Part # SSM2166
Description  Microphone Preamplifier with Variable Compression and Noise Gating
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Manufacturer  AD [Analog Devices]
Direct Link  http://www.analog.com
Logo AD - Analog Devices

SSM2166 Datasheet(HTML) 6 Page - Analog Devices

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REV. B
–6–
SSM2166
APPLICATION 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 amplifi-
cation, 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
0dB 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 k
Ω. 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- 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),
input buffer fixed gain, and the internal VCA’s nominal gain at
the rotation point. The SSM2166 also features a power-down
mode and muting capability.
Theory of Operation
Figure 3 illustrates a typical transfer characteristic for the
SSM2166 where the output level in dB is plotted as a function
of the input level in dB. 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 may 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
breakpoint 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 “rota-
tion 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 may 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 downward expansion
threshold, VDE, is set externally by the user via RGATE at Pin 9
(NOISE GATE). Finally, the SSM2166 provides an active high,
CMOS compatible digital input whereby a power-down feature
will reduce device supply current to less than 100
µA.
INPUT – dB
LIMITING
REGION
LIMITING
THRESHOLD
(ROTATION POINT)
COMPRESSION
REGION
1
r
1
1
DOWNWARD
EXPANSION
THRESHOLD
(NOISE GATE)
DOWNWARD
EXPANSION
REGION
VDE
VRP
VCA GAIN
Figure 3. General Input/Output Characteristics of
the SSM2166
SSM2166 Signal Path
Figure 4 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 k
Ω to the source. A dc voltage of approxi-
mately 1.5 V is present at AUDIO +IN (Pin 7 of the SSM2166),
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 ampli-
fier that can drive the low impedance input of the VCA.
The VCA is a low distortion, variable-gain amplifier 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
Ω. An exter-
nal blocking capacitor (C6) must be used between the buffer’s
output and the VCA input. The 1 k
Ω impedance between
amplifiers determines the value of this capacitor, which is typically
between 4.7
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 SSM2166’s 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.
The gain of the VCA at the rotation point is set by the value of a
resistor connected between Pin 2 and GND, RGAIN. The rela-
tionship between the VCA gain and RGAIN is shown in TPC 4.
The AGC range of the SSM2166 can be as high as 60 dB. The
VCAIN pin (Pin 3) on the SSM2166 is the noninverting input
terminal to the VCA. The inverting input of the VCA is also
available on the SSM2166’s Pin 4 (VCAR) and exhibits an input
impedance of 1 k
Ω, as well. As a result, this pin can be used for
differential inputs or for the elimination of grounding problems
by connecting a capacitor whose value equals that used in series
with the VCAIN pin, to ground. See Figure 12, Evaluation
Board, for more details.
The output impedance of the SSM2166 is typically less than
75
Ω, and the external load on Pin 13 should be >5 kΩ. The
nominal output dc voltage of the device is approximately 2.2 V.
Use a blocking capacitor for grounded loads.
The bandwidth of the SSM2166 is quite wide at all gain set-
tings. The upper 3 dB point is approximately 30 kHz at gains as
high as 60 dB (using the input buffer for additional gain, circuit


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