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FEATURES Complete Microphone Conditioner in a 10-Lead Package Single 3 V Operation Low Shutdown Current < 2 A Adjustable Noise Gate Threshold Adjustable Compression Ratio Automatic Limiting Feature Prevents ADC Overload Low Noise and Distortion: 0.2% THD + N 20 kHz Bandwidth APPLICATIONS Desktop, Portable, or Palmtop Computers Telephone Conferencing Communication Headsets Two-Way Communications Surveillance Systems Karaoke and DJ Mixers
Low Voltage Microphone Preamplifier with Variable Compression and Noise Gating SSM2167
PIN CONFIGURATION 10-Lead MSOP (RM Suffix)
GND VCAIN SHUTDOWN BUF OUT INPUT
1 2 3 4 5 10
VDD OUTPUT COMPRESSION RATIO GATE THRS AVG CAP
SSM2167
9 8 7 6
GENERAL DESCRIPTION
The SSM2167 is a complete and flexible solution for conditioning microphone inputs in personal electronics and 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 10:1 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 amplification of background 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. The flexibility of setting the compression ratio and the time constant of the level detector, coupled with two values of rotation point, make the SSM2167 easy to integrate in a wide variety of microphone conditioning applications. The SSM2167 is available in two versions, with different amounts of fixed gain. The SSM2167-1 has 18 dB of fixed gain, while the SSM2167-2 features only 8 dB of fixed gain. The device is available in 10-lead MSOP package, and guaranteed for operation over the extended industrial temperature range of -40C to +85C.
LIMITING THRESHOLD (ROTATION POINT) DOWNWARD COMPRESSION REGION EXPANSION THRESHOLD 1 (NOISE GATE) r
LIMITING REGION
VCA GAIN
OUTPUT - dB
DOWNWARD EXPANSION REGION 1 1
VDE INPUT - dB
VRP
Figure 1. General Input/Output Characteristics
REV. A
Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781/329-4700 www.analog.com Fax: 781/326-8703 (c) Analog Devices, Inc., 2002
SSM2167-SPECIFICATIONS R
Parameter AUDIO SIGNAL PATH Voltage Noise Density Noise Total Harmonic Distortion + Noise Input Impedance Output Impedance Load Drive Input Voltage Range Output Voltage Range SSM2167-1 SSM2167-2* Gain Bandwidth Product SSM2167-1 SSM2167-2* CONTROL SECTION VCA Dynamic Gain Range VCA Fixed Gain SSM2167-1 SSM2167-2* Compression Ratio, Min Compression Ratio, Max Rotation Point SSM2167-1 SSM2167-2* Noise Gate Range POWER SUPPLY Supply Voltage Supply Current DC Output Voltage Power Supply Rejection Ratio SHUTDOWN Supply Current
*Preliminary Specifications subject to change without notice.
(@ VS = 3.0 V, f = 1 kHz, RL = 100 k , RCOMP = 0 GATE = 2 k , unless otherwise noted.)
Conditions 10:1 Compression 20 kHz Bandwidth, VIN = GND VIN = 100 mV rms Minimum Resistive Load Maximum Capacitive Load 0.4% THD 0.4% THD Min
, TA = 25 C, VIN = 100 mV rms,
Typ 20 -70 0.2 100 145 5 2 600 700 700 Max Unit nV/Hz dBV % k k nF mV rms mV rms mV rms MHz MHz dB dB dB
Symbol en THD + N ZIN ZOUT
1:1 Compression VCA G = 18 dB VCA G = 8 dB
1 1 40 18 8 1:1 10:1 63 100 -40 2.5 2.3 1.4 4.5 2 5.5 5
See Table I for RCOMP
Maximum Threshold VSY ISY PSRR ISY VSY = 2.5 V to 6 V Pin 3 = GND
mV rms mV rms dBV V mA V mV A
8
-2-
REV. A
SSM2167
ABSOLUTE MAXIMUM RATINGS*
Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 V Input Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 V Operating Temperature Range . . . . . . . . . . . -40C to +85C Junction Temperature Range . . . . . . . . . . . . . . . . . . . . 150C Lead Temperature Range (Soldering, 10 sec) . . . . . . . 300C
ESD RATINGS
Package Type 10-Lead MSOP (RM)
JA*
JC
Unit C/W
180
35
*JA is specified for worst-case conditions, i.e., JA is specified for device soldered in 4-layer circuit board for surface-mount packages.
883 (Human Body) Model . . . . . . . . . . . . . . . . . . . . . . 500 V
*Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only; functional operation of the device at these or any other conditions above those listed in the operational sections of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
ORDERING GUIDE
Model
Temperature Range
Package Description 10-Lead Mini/micro SOIC (MSOP) 10-Lead Mini/micro SOIC (MSOP)
Package Option RM-10 RM-10
SSM2167-1RM-Reel -40C to +85C SSM2167-2RM-Reel* -40C to +85C
*Preliminary
CAUTION ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate on the human body and test equipment and can discharge without detection. Although the SSM2167 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.
WARNING!
ESD SENSITIVE DEVICE
REV. A
-3-
SSM2167 - Typical Performance Characteristics
100 TA = 25 C V+ = 3V RLOAD = 100k COMPRESSION RATIO 2:1 ROTATION POINT = 63mV rms
1
NOISE GATE - mV rms
10
THD + N - %
0.1 TA = 25 C V+ = 3V VIN FREQUENCY = 1kHz RLOAD = 100k COMPRESSION RATIO 1:1 ROTATION POINT = 63mV rms NOISE GATE SETTING = 1.4mV rms 0.1 INPUT VOLTAGE - V rms 1
1
0
500
1,000
1,500 2,000 RGATE -
2,500
3,000
3,500
0.01 10m
TPC 1. Noise Gate vs. RGATE
1 TA = 25 C V+ = 3V VIN = 24.5mV rms COMPRESSION RATIO 1:1 ROTATION POINT = 63mV rms NOISE GATE SETTING = 1.4mV rms 0
TPC 4. THD + N vs. Input
COMPRESSION RATIO 10:1 10 20 COMPRESSION RATIO 5:1
OUTPUT - dBV
THD + N - %
30 COMPRESSION RATIO 2:1 40 50 60 70 TA = 25 C V+ = 3V RL = 100k ROTATION POINT = 63mV rms NOISE GATE SETTING = 1.4mV rms 80 70 60 50 40 INPUT - dBV 30 20 10 COMPRESSION RATIO 1:1
0.1
0.01
20
100
1k FREQUENCY - Hz
10k 20k 30k
80
TPC 2. THD + N vs. Frequency
35
TPC 5. Output vs. Input Characteristics
10 V+ = 3V + 0.1 RGATE = 5k RCOMP = 0V
20
25
30
PSRR - dB
GAIN - dB
15
40
5
50
60
5 VIN = 2mV rms RCOMP = 175k ROTATION POINT = 63mV rms NOISE GATE SETTING = 1.4mV rms 10k 100k FREQUENCY - Hz 1M 10M
70 80 10
15
1k
100
1k FREQUENCY - Hz
10k
100k
TPC 3. GBW Curves vs. VCA Gain
TPC 6. PSRR vs. Frequency
-4-
REV. A
SSM2167
0 0 0
VOLTAGE - 50mV/DIV
0 0 TA = 25 C CSYS = 10 F SYSTEM GAIN = 19dB RLOAD = 100k COMPRESSION RATIO 1:1 0
VOLTAGE - 200mV/DIV
0 0 0 0 0 0 0
0 0 0 0 0 0 0
TA = 25 C CSYS = 10 F SYSTEM GAIN = 2.6dB RLOAD = 100k COMPRESSION RATIO 1:1
0
0
0 0 0 TIME - 10 s/DIV
0
0
0
0
0
0 0 0 TIME - 10 s/DIV
0
0
0
TPC 7. Small Signal Transient Response
0 0 0
VOLTAGE - 500mV/DIV
TPC 10. Large Signal Transient Response
0 0
VOLTAGE - 100mV/DIV
0 0 0 0 0 0 0
TA = 25 C CSYS = 10 F SYSTEM GAIN = 8.6dB RLOAD = 100k COMPRESSION RATIO 1:1
0 6dBV 0 0 0 0 0 0 0
66dBV 85dBV
0
0
0 0 0 TIME - 10 s/DIV
0
0
0
0
0
0
0 0 TIME - 1s/DIV
0
0
0
TPC 8. Large Signal Transient Response
TPC 11. RMS Level Detector Performance with CAVG = 22 F
0 0
0 0 0 TA = 25 C CSYS = 10 F SYSTEM GAIN = 8dB RLOAD = 100k COMPRESSION RATIO 1:1
0
VOLTAGE - 100mV/DIV
VOLTAGE - 50mV/DIV
6dBV 0 0 0 66dBV 0 85dBV 0 0 0
0 0 0 0 0 0 0
0
0
0 0 0 TIME - 10 s/DIV
0
0
0
0
0
0 0 0 TIME - 500ms/DIV
0
0
0
TPC 9. Small Signal Transient Response
TPC 12. RMS Level Detector Performance with CAVG = 2.2 F
REV. A
-5-
SSM2167
0 0 0
Theory of Operation
6dBV 0 0 0 66dBV 0 85dBV 0 0 0
0
0
0 0 0 TIME - 500ms/DIV
0
0
0
TPC 13. SSM2167-1 RMS Level Detector Performance with CAVG = 2.2 F
APPLICATIONS INFORMATION
The typical transfer characteristic for the SSM2167 is shown in Figure 1 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 10:1 via a single resistor, RCOMP. Input signals above VRP are compressed with a fixed compression ratio of approximately 10:1. 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." 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 the "fixed gain," 18 dBV for the SSM2167-1 and 8 dBV for the SSM2167-2, regardless of the compression ratio. 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 just above of VDE. The external resistor at Pin 7, RGATE, is used to set the downward expansion threshold VDE. Finally, the SSM2167 provides an active low, CMOS-compatible digital power-down feature that will reduce device supply current to typically less than 2 A.
The SSM2167 is a complete microphone signal conditioning system on a single integrated circuit. Designed primarily for voice band applications, this integrated circuit provides amplification, limiting, variable compression, and noise gate. User adjustable compression ratio, noise gate threshold, and two different fixed gains optimize circuit operation for a variety of applications. The SSM2167 also features a low power shutdown mode for battery-powered applications.
VDD
VOLTAGE - 100mV/DIV
+
10 F
+ 10 F
OUTPUT 100k
SSM2167 Signal Path
GND 10 F VDD 500k RCOMP +
SSM2167
VDD RGATE
SHUTDOWN INPUT 0.1 F
10 F
Figure 3 illustrates the block diagram of the SSM2167. The audio input signal is processed by the input buffer and then by the VCA. The input buffer presents an input impedance of approximately 100 k to the source. A dc voltage of approximately 1.5 V is present at INPUT (Pin 5 of the SSM2167), 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 and an internal rms detector. The VCA is a low distortion, variable-gain amplifier whose gain is set by the side-chain control circuitry. An external blocking capacitor (C2) must be used between the buffer's output and the VCA input. The 1 k impedance between amplifiers determines the value of this capacitor, which is typically between 4.7 F and 10 F. An aluminum electrolytic capacitor is an economical choice. The VCA amplifies the input signal current flowing through C2 and converts this current to a voltage at the SSM2167's output pin (Pin 9). The net gain from input to output can be as high as 40 dB for the SSM2167-1 and 30 dB for the SSM2167-2, depending on the gain set by the control circuitry. The output impedance of the SSM2167 is typically less than 145 , and the external load on Pin 9 should be > 5 k. The nominal output dc voltage of the device is approximately 1.4 V, so a blocking capacitor for grounded loads must be used.
GND
Figure 2. Typical Application Circuit
VDD C2 10 F + BUFOUT 1k INPUT C1 0.1 F +1 BUFFER LEVEL DETECTOR NOISE GATE AND COMPRESSION SETTINGS VCAIN 1k VCA OUTPUT
CONTROL
CAVG R C3 + G 10 F SHUTDOWN GND
RC
VDD
Figure 3. Functional Block Diagram
-6-
REV. A
SSM2167
The bandwidth of the SSM2167 is quite wide at all gain settings. The upper 3 dB point is over 1 MHz at gains as high as 30 dB. The GBW plots are shown in TPC 3. The lower 3 dB cutoff frequency of the SSM2167 is set by the input impedance of the VCA (1 k) and C2. 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 maximizing the usable dynamic range of the part.
Level Detector
15:1 5:1 2:1 VCA GAIN
The SSM2167 incorporates a full-wave rectifier and a patentpending, true rms level detector circuit whose averaging time constant is set by an external capacitor (CAVG) 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 F. Some experimentation with larger values for CAVG 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 F to 22 F. The rms detector filter time constant is approximately given by 10 CAVG milliseconds where CAVG is in F. 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 increase due to a decrease in input signal. The attack time, the time it takes for the gain to be reduced because of a sudden increase in input level, is controlled mainly by internal circuitry that speeds up the attack for large level changes. This limits overload time to less than 1 ms in most cases. The performance of the rms level detector is illustrated in TPC 12 for a CAVG of 2.2 F and TPC 11 for a CAVG of 22 F. In each of these photographs, the input signal to the SSM2167 (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 -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 F, and 12 dB/s for a CAVG of 22 F.
Control Circuitry
OUTPUT - dB
1:1
1 1
VDE
INPUT - dB
VRP
Figure 4. Effect of Varying the Compression Ratio
Setting the Compression Ratio
Changing 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 4. Connecting a resistor (RCOMP) between Pin 8 and VDD sets the compression ratio. Lowering RCOMP gives smaller compression ratios as indicated in Table I. AGC performance is achieved with compression ratios between 2:1 and 10:1, and is dependent on the application. Shorting RCOMP will disable the AGC function, setting the compression equal to 1:1. If using a compression resistor, using a value greater than 5 k is recommend. If lower than 5 k is used, the device may interpret this as a short, 0 .
Table I. Setting Compression Ratio
Compression Ratio 1:1 2:1 3:1 5:1 10:1
Value of RCOMP 0 (short to V+) 15 k 35 k 75 k 175 k
r:1 VCA GAIN
The 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 1.
OUTPUT - dB
1 1
VDE2 VDE1 VDE3
INPUT - dB
VRP
Figure 5. Effects of Varying the Downward Expansion (Noise Gate) Threshold
REV. A
-7-
SSM2167
Setting the Noise Gate Threshold (Downward Expansion) Shutdown Feature
Noise gate threshold is another programmable point using an external resistor (RGATE) that is connected between Pin 7 (NOISE GATE THRS) and VDD. The downward expansion threshold may be set between -40 dBV and -55 dBV, as shown in Table II. The downward expansion threshold is inversely proportional to the value of this resistance: setting this resistance to 0 sets the threshold at approximately 10 mV rms (-40 dBV), whereas a 5 k resistance sets the threshold at approximately 1 mV rms (-55 dBV). This relationship is illustrated in Figure 5. We do not recommend more than 5 k for the RGATE resistor as the noise floor of the SSM2167 prevents the noise gate from being lowered further without causing problems.
Table II. Setting Noise Gate Threshold
The supply current of the SSM2167 can be reduced to under 10 A by applying an active LOW, 0 V CMOS compatible input to the SSM2167's /SHUTDOWN Pin (Pin 3). In this state, the input and output circuitry of the SSM2167 will assume a high impedance state; as such, the potentials at the input pin and the output pin will be determined by the external circuitry connected to the SSM2167. The SSM2167 takes approximately 200 ms to settle from a SHUTDOWN to POWER-ON command. For POWER-ON to SHUTDOWN, the SSM2167 requires more time, typically less than 1 s. Cycling the power supply to the SSM2167 can result in quicker settling times: the off-to-on settling time of the SSM2167 is less than 200 ms, while the on-to-off settling time is less than 1 ms. The SSM2167 shutdown current is related to both temperature and voltage.
PC Board Layout Considerations
Noise Gate (dBV) -40 -48 -54 -55
Rotation Point (Limiting)
Value of RGATE 0 (short to V+) 1 k 2 k 5 k
Since the SSM2167 is capable of wide bandwidth operation and can be configured for as much as 60 dB of gain, special care must be exercised in the layout of the PC board that contains the IC and its associated components. The following applications hints should be considered for the PC board. The layout should minimize possible capacitive feedback from the output of the SSM2167 back to its input. Do not run input and output traces adjacent to each other. A single-point ("star") ground implementation is recommended in addition to maintaining short lead lengths and PC board runs. In applications where an analog ground and a digital ground are available, the SSM2167 and its surrounding circuitry should be connected to the system's analog ground. As a result of these recommendations, wire-wrap board connections and grounding implementations are to be explicitly avoided.
Input signals above a particular level, "the rotation point," are attenuated (limited) by internal circuitry. This feature allows the SSM2167 to limit the maximum output, preventing clipping of the following stage, such as a CODEC or ADC. The rotation point for SSM2167 is set internally to -24 dBV (63 mV rms) for SSM2167-1 and -20 dBV (100 mV rms) for SSM2167-2.
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
10-Lead MSOP (RM-10)
0.124 (3.15) 0.112 (2.84)
10
6
0.124 (3.15) 0.112 (2.84)
1 5
0.199 (5.05) 0.187 (4.75)
PIN 1 0.0197 (0.50) BSC 0.122 (3.10) 0.110 (2.79) 0.043 (1.09) 0.037 (0.94) 0.016 (0.41) 0.006 (0.15) 6 SEATING 0 0.011 (0.28) PLANE 0.003 (0.08) 0.022 (0.56) 0.021 (0.53) 0.120 (3.05) 0.112 (2.84)
0.038 (0.97) 0.030 (0.76)
0.006 (0.15) 0.002 (0.05)
Revision History
Location Data Sheet changed from REV. 0 to REV. A. Page
Edits to Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Edits to Figures 2 and 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 -8- REV. A
PRINTED IN U.S.A.
C02628-0-3/02(A)

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