Low Voltage Microphone Preamplifier with Variable Compression and Noise Gating

a 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 –40°C to +85°C. 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 © 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 % kΩ Ω kΩ 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 . . . . . . . . . . . –40°C to +85°C Junction Temperature Range . . . . . . . . . . . . . . . . . . . . 150°C Lead Temperature Range (Soldering, 10 sec) . . . . . . . 300°C ESD RATINGS Package Type 10-Lead MSOP (RM) JA* JC Unit °C/W 180 35 *θJA is specified for worst-case conditions, i.e., θJA 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 –40°C to +85°C SSM2167-2RM-Reel* –40°C to +85°C *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 µ 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 µ 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 µ 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-