SSM2166S ,Microphone Preamplifier with Variable Compression and Noise GatingSPECIFICATIONS R1 = 0 W , R2 = ¥ V, T = +258C, unless otherwise noted, V = 300 mV rms.) A INParamet ..
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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 ..
START405TR ,NPN SILICON RF TRANSISTORSTART405NPN Silicon RF Transistor• LOW NOISE FIGURE: NFmin = 1.1dB @ 1.8GHz, 2mA, 2V• COMPRESSION ..
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SSM2166P-SSM2166S
Microphone Preamplifier with Variable Compression and Noise Gating
Microphone Preamplifier with
Variable Compression and Noise Gating
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 (61 dB)
Low Cost
APPLICATIONS
Microphone Preamplifier/Processor
Computer Sound Cards
Public Address/Paging Systems
Communication Headsets
Telephone Conferencing
Guitar Sustain Effects Generator
Computerized Voice Recognition
Surveillance Systems
Karaoke and DJ Mixers20 dB; this gain is in addition to the variable gain in other com-
pression 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 14-lead SOIC and P-DIP packages,
and guaranteed for operation over the extended industrial tempera-
ture range of –40°C to +85°C. For similar features/performance
in an 8-lead package, please refer to the SSM2165.
Figure 1.SSM2166 Compression and Gating Characteris-
tics with 10 dB of Fixed Gain (The Gain Adjust Pin Can Be
Used to Vary This Fixed Gain Amount)
REV. A
*Patents pending.Figure 2.Functional Block Diagram and Typical Speech Application
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 set-
ting the SSM2166 can be programmed with a fixed gain of up to
10mF*
POINT SET
GND
10mFR1 = 10kV
POWER
DOWN
NOTES0dBu = 0.775 V rms.Normal operation: Pin 12 = 0 V.
Specifications subject to change without notice.
CAUTIONESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000V readily
SSM2166–SPECIFICATIONS
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 Plastic DIPJA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .83°C/WJC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .39°C/W
14-Lead SOICJA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .120°C/WJC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .36°C/W
(V+ = +5 V, f = 1 kHz, RL = 100 kW, RGATE = 600kW, RROTATION = 3 kW, RCOMP = 0 W,
R1 = 0W
, R2 = ¥V, TA = +258C, unless otherwise noted, VIN = 300 mV rms.)
PIN DESCRIPTION
PIN CONFIGURATION
SSM2166
INPUT – dBu
OUTPUT – dBu
INPUT – dBuFigure 3.Output vs. Input Characteristics
RGATE – kV
NOISE GATE – mV rms
100Figure 4.Noise Gate vs. RGATE (Pin 9 to V+)
Figure 5.Compression Ratio vs. RCOMP (Pin 10 to GND)
Figure 6.VCA Gain vs. RGAIN (Pin 2 to GND)
INPUT VOLTAGE – V rms
THD + N – %
0.05Figure 7.THD+N (%) vs. Input (V rms)
Figure 8.THD+N (%) vs. Frequency (Hz)
ROTATION POINT – V rms
RROT PT RESISTOR – kV
0.010455103540152025305055657075808590Figure 9.Rotation Point vs. RROT PT (Pin 11 to V+)
Figure 10a.Wideband Output Noise
FREQUENCY – Hz
GAIN – dB
–201k1M10k100k
–10Figure 10b.GBW Curves vs. VCA Gain
Figure 10c.PSRR vs. Frequency
Figure 11.Small Signal Transient Response
Figure 12.Large Signal Transient Response
SSM2166
APPLICATIONS INFORMATIONThe SSM2166 is a complete microphone signal conditioning
system on a single integrated circuit. Designed primarily for
voiceband applications, this integrated circuit provides amplifi-
cation, rms detection, limiting, variable compression, and down-
ward expansion. An integral voltage-controlled amplifier (VCA)
provides up to 60 dB of gain in the signal path with approxi-
mately 30kHz 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 kW. The internal rms detector has a time con-
stant set by an external capacitor.
The SSM2166 contains an input buffer and automatic gain con-
trol (AGC) circuit for audio- and voiceband signals. Circuit
operation is optimized by providing a user-adjustable time con-
stant and compression ratio. A downward expansion (noise gat-
ing) feature eliminates circuit noise in the absence of an input
signal. The SSM2166 allows the user to set the downward ex-
pansion threshold, the limiting threshold (rotation point), input
buffer fixed gain, and the internal VCA’s nominal gain at the ro-
tation point. The SSM2166 also features a power-down mode
and muting capability.
Theory of OperationFigure 13 illustrates a typical transfer characteristic for the
SSM2166 where the output level in dB is plotted as a func-
tion 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 adB 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 opera-
tion 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 “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 “nomi-
nal gain” of the system. The nominal gain of the system may be
increased by the user via the onboard VCA by up to 20 dB. Ad-
ditionally, 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 of VDE. The downward
expansion threshold, VDE, is set externally by the user via RGATE
Figure 13.General Input/Output Characteristics of the
SSM2166
The SSM2166 Signal PathFigure 14 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 kW 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 0.1mF 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 kW. An exter-
nal blocking capacitor (C6) must be used between the buffer’s
output and the VCA input. The 1 kW impedance between am-
plifiers determines the value of this capacitor which is typically
between 4.7 mF and 10 mF. An aluminum electrolytic capacitor
is an economical choice. The VCA amplifies the input signal
current flowing through C6 and converts this current to a volt-
age 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 Figure 6.
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 kW, 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 22, SSM2166
Evaluation Board for more details.
The output impedance of the SSM2166 is typically less that
75 W, and the external load on Pin 13 should be >5 kW. The
nominal output dc voltage of the device is approximately 2.2V.
Use a blocking capacitor for grounded loads.
GND
10mF
C7*
10mFFigure 14.Functional Block Diagram and Typical Application
bandwidth is unaffected). The GBW plots are shown in Figure
10b. The lower 3dB cutoff frequency of the SSM2166 is set by
the input impedance of the VCA (1 kW) 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 minimum when the gain is at a maximum, thereby optimiz-
ing the usable dynamic range of the part. A photograph of the
SSM2166’s wideband peak-to-peak output noise is illustrated in
Figure 10b.
The 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 mF. 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 aver-
aging 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 dy-
namics. Electrolytic capacitors are recommended here for low-
est cost and should be in the range of 2 mF to 47 mF. Capacitor
values from 18 mF to 22 mF have been found to be more appro-
priate in voiceband 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 mF. This time constant
controls both the steady-state averaging in the rms detector as
small signal. The attack time, the time it takes for the gain to be
reduced when a small signal is followed by a large signal, is con-
trolled partly by the AVG CAP value, but is mainly controlled
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 Fig-
ure 15 for a CAVG of 2.2 mF (Figure 15a) and 22 mF (Figure
15b). In each of these photographs, the input signal to the
SSM2166 (not shown) is a series of tone bursts in 6 successive
10 dB steps. The tone bursts range from –66 dBV (0.5 mV rms)
to –6 dBV (0.5 V rms). As illustrated in the photographs, 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 mF,
and 12 dB/s for a CAVG of 22 mF.
Figure 15a.RMS Level Detector Performance with
CAVG = 2.2 mF
SSM2166
6dBV
66dBV
85dBVFigure 15b.RMS Level Detector Performance with
CAVG = 22 mF
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 13.
Compression Ratio.Changing the scaling of the control sig-
nal fed to the VCA causes a change in the circuit’s compression
ratio, “r.” This effect is shown in Figure 16. The compression
ratio can be set by connecting a resistor between the COMP
RATIO pin (Pin 10) and GND. Lowering RCOMP gives smaller
compression ratios as indicated in Figure 5, with values of about
17 kW or less resulting in a compression ratio of 1:1. AGC per-
formance is achieved with compression ratios between 2:1 and
15:1, and is dependent on the application. A 100 kW potentiom-
eter may be used to allow this parameter to be adjusted. On the
evaluation board (Figure 22), an optional resistor can be used to
set the compression equal to 1:1 when the wiper of the potenti-
ometer is at its full CCW position.
Figure 16. Effect of Varying the Compression Ratio
Rotation Point.An internal dc reference voltage in the control
the rotation point may be varied from approximately 20 mV rms
to 1 V rms. From the figure, the rotation point is inversely propor-
tional to RROT PT. For example, a 1kW resistor would typically
set the rotation point at 1 V rms, whereas a 55kW 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 in-
put signal range. Occasional larger signal transients will then be
attenuated by the action of the limiter.
Figure 17.Effect of Varying the Rotation Point
VCA Gain Setting and Muting.The maximum gain of the
SSM2166 is set by the GAIN ADJUST pin (Pin 2) via RGAIN.
This resistor, with a range between 1 kW and 20 kW, will cause
the nominal VCA gain to vary from 0 dB to approximatelydB, respectively. To set the VCA gain to its maximum can
also be achieved by leaving the GAIN ADJUST pin in an
OPEN condition (no connect). Figure 18 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 kW resistor.
