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SSM2165-1P-SSM2165-1S-SSM2165-2S
Microphone Preamplifier with Variable Compression and Noise Gating
REV. A
FUNCTIONAL BLOCK DIAGRAM
FEATURES
Complete Microphone Conditioner in an 8-Lead Package
Single +5 V Operation
Preset Noise Gate Threshold
Compression Ratio Set by External Resistor
Automatic Limiting Feature Prevents ADC Overload
Adjustable Release Time
Low Noise and Distortion
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 Mixers
GENERAL DESCRIPTIONThe SSM2165 is a complete and flexible solution for condition-
ing 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 compres-
sion ratio is set by a single resistor and can be varied from 1:1 to
over 15:1 relative to the fixed rotation point. Signals above the
rotation point are limited to prevent overload and to eliminate
“popping.” A downward expander (noise gate) prevents amplifi-
cation of noise or hum. This results in optimized 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 flexibility
of setting the compression ratio and the time constant of the
level detector, coupled with two values of rotation point, make
the SSM2165 easy to integrate in a wide variety of microphone
conditioning applications.
The SSM2165 is an ideal companion product for audio codecs
used in computer systems, such as the AD1845 and AD1847.
The device is available in 8-lead SOIC and P-DIP packages, and
guaranteed for operation over the extended industrial temperature
range of –40°C to +85°C. As shown in Figure 1a, the SSM2165-1
has a rotation point of –25.7 dBu (40 mV)1, a VCA gain of 18dB,
and gives –7.7 dBu (320 mV) before limiting. As shown in Figure
1b, the SSM2165-2 has a rotation point of –17.8 dBu (100mV),
a VCA gain of 8 dB and gives –9.8 dBu (250 mV) before limiting.
Both have a noise gate threshold of –64 dBu (500 mV), below
which downward expansion reduces the gain with a ratio of
approximately 1:3. That is, a –3 dB reduction of output signal
occurs with a –1 dB reduction of input signal. For applications
requiring adjustable noise gate threshold, VCA gain up to 18dB,
and adjustable rotation point, please refer to the SSM2166.
INPUT – dBu
OUTPUT – dBu
–30Figure 1a. SSM2165-1 Compression and Gating Characteristics
INPUT – dBu
OUTPUT – dBu
–30Figure 1b. SSM2165-2 Compression and Gating Characteristics
Microphone Preamplifier with
Variable Compression and Noise Gating*Patents pending.All signals are in rms volts or dBu (0 dBu = 0.775 V rms).
SSM2165–SPECIFICATIONSCONTROL SECTION
NOTES0dBu = 0.775 V rms.Referred to input.
Specifications subject to change without notice.
(V+ = +5 V, f = 1 kHz, RL = 100 kV, RCOMP = 0 V, TA = +258C, unless otherwise noted)
CAUTIONESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000V readily
accumulate on the human body and test equipment and can discharge without detection.
Although the SSM2165 features proprietary ESD protection circuitry, permanent damage may
occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD
precautions are recommended to avoid performance degradation or loss of functionality.
PIN FUNCTION DESCRIPTIONS
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
8-Lead Plastic DIPJA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .103°C/WJC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .43°C/W
8-Lead SOICJA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .158°C/WJC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .43°C/W
ORDERING GUIDE
PIN CONFIGURATION
GND
AVG CAP
COMP RATIO SET
OUTPUT
VCAIN
BUFOUT
AUDIO +IN
SSM2165
COMPRESSION RATIO
1:115:12:15:110:1
COMP
– k
180Figure 2.Compression Ratio vs. RCOMP
INPUT – V rms
THD+N – %
0.050Figure 3.THD + N (%) vs. Input (V rms)
FREQUENCY – Hz
0.130k100
THD+N – %10k
0.050Figure 4.THD + N (%) vs. Frequency (Hz)
Figure 5.Wideband Output Noise
FREQUENCY – Hz
GAIN – dB
–201M10k100k
–10Figure 6.GBW Curves vs. VCA Gain
FREQUENCY – Hz30k1001k
10k
PSRR – dBFigure 7.PSRR vs. Frequency, Referred to Input
–Typical Performance Characteristics
THEORY OF OPERATIONFigure 10 illustrates the general transfer characteristic for the
SSM2165 where the output level in dBu is plotted as a function
of the input level in dBu (0 dBu = 0.775 V rms). 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 10:1. 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 “rotation point,” and is different for the
SSM2165-1 and SSM2165-2, see Table I.
Table I.Characteristics vs. Dash Number*At limiting.
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.
When the compression is set to 2:1, a –2 dB change of the
input signal level in the compression region causes –1 dB
change of the output level. Likewise, at 10:1 compression, a
–10 dB change of the input signal level in the compression
region causes a –1 dB change in the output level. The gain of
the system with an input signal level of VRP is fixed regardless of
the compression ratio, and is different for the SSM2165-1 and
SSM2165-2 (see Figures 1a and 1b). The “nominal gain” of
the system is 18 dB for the SSM2165-1, and 8 dB for the
SSM2165-2. System gain is measured at VRP and is (VOUT – VIN)
in dB.
Input signals below VDE are downward expanded at a ratio of
approximately 1:3. As a result, the gain of the system is small
for very small input signal levels below VDE, even though it may
APPLICATIONS INFORMATIONThe SSM2165 is a complete microphone signal conditioning
system in a single integrated circuit. Designed primarily for
voiceband applications, this integrated circuit provides amplifi-
cation, rms detection, limiting, variable compression, and down-
ward expansion. The internal rms detector has a time constant
set by an external capacitor. An integral voltage-controlled
amplifier (VCA) provides up to 40 dB of gain in the signal path
with approximately 30 kHz bandwidth. The device operates on
a single +5 V supply, accepts input signals up to 1 V1, and pro-
duces output signal levels at limiting of 320 mV and 250 mV for
the SSM2165-1 and SSM2165-2 respectively, into loads > 5 kW.
The SSM2165 contains an input buffer and automatic gain
control (AGC) circuit for audio and voice band signals. Circuit
operation is optimized by providing user-adjustable compression
ratio and time constant. A downward expansion (noise gating)
feature reduces background and circuit noise below 500 mV.
The rotation point determines the output signal levels before
limiting (referred to the input), and is 40 mV for the SSM2165-1
and 100 mV for the SSM2165-2.
OUTPUT – dBFigure 10. General Input/Output Characteristics of the
SSM2165
Figure 8.Small Signal Transient Response
Figure 9.Large Signal Transient Response
SSM2165operation of the level detector down to 10 Hz, the value of the
capacitor should be around 22 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 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 re-
sponse times to signal dynamics. Electrolytic capacitors are
recommended here for lowest cost.
The rms detector filter time constant is approximately given by
10 · CAVG milliseconds where CAVG is in mF. This time con-
stant controls both the steady-state averaging in the rms detec-
tor 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 fol-
lowed by a 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 mainly controlled by internal circuitry that speeds up
the attack for large level changes, and controlled partly by the
AVG CAP value. This limits overload time to under 1 ms in
most cases.
The performance of the rms level detector is illustrated in Fig-
ure 12 for CAVG = 2.2 mF and Figure 13 for CAVG = 22 mF. In
each of these photographs, the input signal to the SSM2165
(not shown) is a series of tone bursts in 6 successive 10 dB
steps. The tone bursts range from –66 dBu (0.5 mV rms) to
–6 dBu (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 for CAVG and the input signal step size. The
rate of release is approximately 240 dB/s for a CAVG = 2.2 mF,
and 12 dB/s for a CAVG of 22 mF.
6dBV
66dBV
85dBVFigure 12. RMS Level Detector Performance with
CAVG = 2.2 mF
6dBV
66dBV
85dBV
The SSM2165 Signal PathFigure 11 illustrates the block diagram of the SSM2165. The
audio input signal is processed by the unity gain input buffer
and then by the VCA. The 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 4), requiring the
use of a blocking capacitor (C1) for ground-referenced sources.
A 0.1 mF capacitor is a good choice for most audio applications.
The buffer is designed to drive only the low impedance input of
the VCA, and must not be loaded by capacitance to ground.
The VCA is a low distortion, variable-gain amplifier whose gain
is set by the internal control circuitry. The input to the VCA is
a virtual ground in series with 500 W. An external blocking
capacitor (C2) must be used between the buffer’s output and
the VCA input. The desired low frequency response and the
total of 1 kW impedance between amplifiers determines the
value of this capacitor. For music applications, 10 mF will give
high pass fC = 16 Hz. For voice/communications applications,mF will give fC = 160 Hz. 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 SSM2165’s output (Pin7). The net gain from
input to output can be as high as 40dB for high compression
ratios and depending on the gain set by the control circuitry.
The output impedance of the SSM2165 is typically less thanW, and the external load on Pin 7 should be >5 kW. The
nominal output dc voltage of the device is approximately 2.2 V.
Use a dc blocking capacitor for grounded loads.
10mFAUDIO
IN+VOUTFigure 11.Functional Block Diagram and Typical Voice
Application
The bandwidth of the SSM2165 is quite wide at all gain set-
tings. The upper –3 dB point is approximately 300 kHz. The
GBW plots are shown in Figure 6. 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 optimizing the usable
dynamic range of the part. A photograph of the SSM2165’s
wideband peak-to-peak output noise is illustrated in Figure 5.
The Level DetectorThe SSM2165 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 5). Capacitor values from 18 mF to 22 mF have
