SSM2166SZ-REEL7 ,Complete Microphone Conditioner in a 14-Lead Package with Variable Compression & Noise GatingSPECIFICATIONS R1 = 0, R2 = , T = 25C, unless otherwise noted, V = 300 mV rms.) A INParameter S ..
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SSM2167-1RMZ-REEL ,Low Voltage Microphone Preamplifier with Variable Compression & Noise GatingGENERAL DESCRIPTIONLIMITINGLIMITINGREGIONThe SSM2167 is a complete and flexible solution for condit ..
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START405TR ,NPN SILICON RF TRANSISTORSTART405NPN Silicon RF Transistor• LOW NOISE FIGURE: NFmin = 1.1dB @ 1.8GHz, 2mA, 2V• COMPRESSION ..
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SSM2166SZ-SSM2166SZ-REEL7
Complete Microphone Conditioner in a 14-Lead Package with Variable Compression & Noise Gating
REV.B
Microphone Preamplifier with
Variable Compression and Noise Gating*Patents pending.
FEATURES
Complete Microphone Conditioner in a 14-Lead 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)
Low Cost
APPLICATIONS
Microphone Preamplifier/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 DESCRIPTIONThe 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 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 0dB to 20dB. A downward expander (noise gate)
prevents amplification of noise or hum. This results in opti-
mized signal levels prior to digitization, thereby eliminating the
need for additional gain or attenuation in the digital domain
that could 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 and AD1847.
The device is available in a 14-lead SOIC package, and is guar-
anteed for operation over the extended industrial temperature
range of –40°C to +85°C. For similar features and performance
in an 8-lead package, please refer to the SSM2165.
Figure 1.SSM2166 Compression and Gating Char-
acteristics with 10 dB of Fixed Gain (The Gain Adjust
Pin Can Be Used to Vary This Fixed Gain Amount)
Figure 2.Functional Block Diagram and Typical Speech Application
SSM2166–SPECIFICATIONS
(V+ = 5 V, f = 1 kHz, RL = 100 k�, RGATE = 600k�, RROTATION = 3 k�, RCOMP = 0 �,
R1 = 0�, R2 = ��, TA = 25�C, unless otherwise noted, VIN = 300 mV rms.)
NOTES0dBu = 0.775 V rms.Normal operation: Pin 12 = 0 V.
Specifications subject to change without notice.
ORDERING GUIDE
ABSOLUTE MAXIMUM RATINGSSupply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .+10 V
Audio Input Voltage . . . . . . . . . . . . . . . . . . . . .Supply Voltage
Operating Temperature Range . . . . . . . . . . . .–40°C to +85°C
Storage Temperature Range . . . . . . . . . . . . .–65°C to +150°C
Junction Temperature (TJ) . . . . . . . . . . . . . . . . . . . . . .150°C
Lead Temperature (Soldering, 60 sec) . . . . . . . . . . . . .300°C
ESD RATINGS883 (Human Body) Model . . . . . . . . . . . . . . . . . . . . . .2.0 kV
THERMAL CHARACTERISTICSThermal Resistance
14-Lead SOIC
θJA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .120°C/W
θJC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .36°C/W
CAUTIONESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000V readily
PIN FUNCTION DESCRIPTIONS
PIN CONFIGURATION
SSM2166–Typical Performance CharacteristicsTPC 1.Output vs. Input Characteristics
TPC 2.Noise Gate vs. RGATE (Pin 9 to V+)
TPC 3.Compression Ratio vs. RCOMP (Pin 10 to GND)
TPC 4.VCA Gain vs. RGAIN (Pin 2 to GND)
TPC 5.THD + N (%) vs. Input (V rms)
TPC 6.THD + N (%) vs. Frequency (Hz)
TPC 7.Rotation Point vs. RROT PT (Pin 11 to V+)
TPC 8a.Wideband Output Noise
TPC 8b.GBW Curves vs. VCA Gain
TPC 8c.PSRR vs. Frequency
TPC 9.Small Signal Transient Response
TPC 10.Large Signal Transient Response
SSM2166
APPLICATION INFORMATIONThe 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 30kHz 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 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- 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 OperationFigure 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 1dB 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 –1dB
change in the input signal level causes approximately a –3dB
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,
Figure 3.General Input/Output Characteristics of
the SSM2166
SSM2166 Signal PathFigure 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 kΩ 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 0.1µF 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 kΩ. An exter-
nal blocking capacitor (C6) must be used between the buffer’s
output and the VCA input. The 1 kΩ 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 kΩ, 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 2.2V.
Use a blocking capacitor for grounded loads.
bandwidth is unaffected). The GBW plots are shown in TPC 8b.
The lower 3dB cutoff frequency of the SSM2166 is set by the
input impedance of the VCA (1 kΩ) and C6. While the noise of
the input buffer is fixed, the input referred noise of the VCA is a
function of gain. The VCA input noise is designed to be a mini-
mum when the gain is at a maximum, thereby optimizing the usable
dynamic range of the part. An image of the SSM2166’s wideband
peak-to-peak output noise is illustrated in TPC 8a.
Level DetectorThe SSM2166 incorporates a full-wave rectifier and a patent-
pending, true rms level detector circuit whose averaging time
constant is set by an external capacitor connected to the AVG
CAP pin (Pin 8). For optimal low frequency operation of the
level detector down to 10 Hz, the value of the capacitor should
be 2.2 µF. Some experimentation with larger values for the
AVG CAP may be necessary to reduce the effects of excessive
low frequency ambient background noise. The value of the
averaging capacitor affects sound quality: too small a value for
this capacitor may cause a “pumping effect” for some signals,
while too large a value can result in slow response times to
signal dynamics. Electrolytic capacitors are recommended here
for lowest cost and should be in the range of 2 µF to 47 µF.
Capacitor values from 18 µF to 22 µF have been found to be
more appropriate in voice-band applications, where capacitors
on the low end of the range seem more appropriate for music
program material.
The rms detector filter time constant is approximately given by
10 � CAVG milliseconds where CAVG is in µF. This time constant
controls both the steady-state averaging in the rms detector as
well as the release time for compression; that is, the time it takes
for the system gain to react when a large input is followed by a
small signal. The attack time, the time it takes for the gain to be
by internal circuitry that speeds up the attack for large level
changes. This limits overload time to under 1 ms in most cases.
The performance of the rms level detector is illustrated in Figure 5
for a CAVG of 2.2 µF (Figure 5a) and 22 µF (Figure 5b). 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 –6dBV (0.5 V rms).
As illustrated in the images, the attack time of the rms level
detector is dependent only on CAVG, but the release times are
linear ramps whose decay times are dependent on both CAVG
and the input signal step size. The rate of release is approximately
240 dB/s for a CAVG of 2.2 µF, and 12 dB/s for a CAVG of 22 µF.
Figure 5a.RMS Level Detector Performance with
CAVG = 2.2 µF
Figure 4.Functional Block Diagram and Typical Application
SSM2166Figure 5b.RMS Level Detector Performance
with CAVG = 22 µF
Control CircuitryThe output of the rms level detector is a signal proportional to
the log of the true rms value of the buffer output with an added
dc offset. The control circuitry subtracts a dc voltage from this
signal, scales it, and sends the result to the VCA to control the
gain. The VCA’s gain control is logarithmic—a linear change in
control signal causes a dB change in gain. It is this control law
that allows linear processing of the log rms signal to provide the
flat compression characteristic on the input/output characteristic
shown in Figure 3.
Compression RatioChanging the scaling of the control signal fed to the VCA causes
a change in the circuit’s compression ratio, “r.” This effect is
shown in Figure 6. The compression ratio can be set by con-
necting a resistor between the COMP RATIO pin (Pin 10) and
GND. Lowering RCOMP gives smaller compression ratios as
indicated in TPC 3, with values of about 17 kΩ or less resulting
in a compression ratio of 1:1. AGC performance is achieved
with compression ratios between 2:1 and 15:1, and is dependent
on the application. A 100 kΩ potentiometer may be used to
allow this parameter to be adjusted. On the evaluation board
(Figure 12), an optional resistor can be used to set the compres-
sion equal to 1:1 when the wiper of the potentiometer is at its
full CCW position.
Rotation PointAn internal dc reference voltage in the control circuitry, used to
set the rotation point, is user specified, as illustrated in TPC 7.
The effect on rotation point is shown in Figure 7. By varying a
resistor, RROT PT, connected between the positive supply and the
ROTATION POINT SET pin (Pin 11), the rotation point may
be varied from approximately 20 mV rms to 1 V rms. From the
figure, the rotation point is inversely proportional to RROT PT.
For example, a 1kΩ resistor would typically set the rotation
point at 1 V rms, whereas a 55kΩ resistor would typically set
the rotation point at approximately 30 mV rms.
Since 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 will cover the entire desired input
signal range. Occasional larger signal transients will then be
attenuated by the action of the limiter.
Figure 7.Effect of Varying the Rotation Point
VCA Gain Setting and MutingThe maximum gain of the SSM2166 is set by the GAIN ADJUST
pin (Pin 2) via RGAIN. This resistor, with a range between 1 kΩ
and 20 kΩ, will cause the nominal VCA gain to vary from 0 dB
to approximately 20dB, respectively. Setting the VCA gain to
its maximum can also be achieved by leaving the GAIN ADJUST
pin in an OPEN condition (no connect). Figure 8 illustrates the
effect on the transfer characteristic by varying this parameter. For
low level signal sources, the VCA should be set to maximum gain
using a 20 kΩ resistor.
