Microphone Preamplifier with Variable Compression and Noise Gating

a Microphone Preamplifier with Variable Compression and Noise Gating SSM2165* FUNCTIONAL BLOCK DIAGRAM C2 10 F + VCAIN RA 2 VCA BUFFER LEVEL DETECTOR CONTROL 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 ( 1 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 DESCRIPTION V+ V+ BUFOUT +1 AUDIO IN+ C1 0.1 F RA 2 VOUT SSM2165 AVG CAP C3 22 F + R1 25k COMPRESSION RATIO SET GND The SSM2165 is 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 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 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 18 dB, 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 (100 mV), *Patents pending. 1 All signals are in rms volts or dBu (0 dBu = 0.775 V rms). 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 µV), 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 18 dB, and adjustable rotation point, please refer to the SSM2166. 0 –10 –20 –30 –40 –50 –60 –80 OUTPUT – dBu –70 –60 –50 –40 INPUT – dBu –30 –20 –10 Figure 1a. SSM2165-1 Compression and Gating Characteristics 0 –10 –20 OUTPUT – dBu –30 –40 –50 –60 –70 –80 –70 –60 –50 –40 INPUT – dBu –30 –20 –10 Figure 1b. SSM2165-2 Compression and Gating 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 which 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 World Wide Web Site: http://www.analog.com Fax: 781/326-8703 © Analog Devices, Inc., 1999 SSM2165–SPECIFICATIONS (V+ = +5 V, f = 1 kHz, R = 100 k L , RCOMP = 0 , TA = +25 C, unless otherwise noted) Typ 17 –109 0.2 0.2 180 75 0.5 0.5 Max Units nV/√Hz2 dBu1 % % kΩ Ω kΩ nF V rms V rms kHz kHz dB dB dB mV rms mV rms Parameter AUDIO SIGNAL PATH Voltage Noise Density Noise Total Harmonic Distortion SSM2165-1 SSM2165-2 Input Impedance Output Impedance Load Drive Input Voltage Range Output Voltage Range Gain Bandwidth Product SSM2165-1 SSM2165-2 CONTROL SECTION VCA Dynamic Gain Range VCA Fixed Gain SSM2165-1 SSM2165-2 Rotation Point SSM2165-1 SSM2165-2 Compression Ratio, Min Compression Ratio, Max Control Feedthrough POWER SUPPLY Supply Voltage Range Supply Current Quiescent Output Voltage Level Power Supply Rejection Ratio2 NOTES 1 0 dBu = 0.775 V rms. 2 Referred to input. Specifications subject to change without notice. Symbol Conditions 15:1 Compression, VIN = GND 20 kHz Bandwidth, VIN = GND 2nd and 3rd Harmonics, VIN = –30 dBu 2nd and 3rd Harmonics, VIN = –20 dBu 22 kHz Low-Pass Filter Min en THD+N ZIN ZOUT Resistive Capacitive 1% THD 1% THD 1:1 Compression VCA G = 18 dB VCA G = 8 dB 5 2 1 1.4 300 100 40 18 8 40 100 1:1 15:1 ±5 4.5 7.5 2.2 50 5.5 10 15:1 Compression VS ISY PSRR mV V mA V dB –2– REV. A SSM2165 ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION Supply 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 RATINGS GND 1 VCAIN 2 8 V+ 7 OUTPUT TOP VIEW 6 COMP RATIO SET (Not to Scale) 5 AVG CAP AUDIO +IN 4 BUFOUT 3 SSM2165 883 (Human Body) Model . . . . . . . . . . . . . . . . . . . . . . . 2.0 kV THERMAL CHARACTERISTICS Thermal Resistance 8-Lead Plastic DIP θJA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103°C/W θJC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43°C/W 8-Lead SOIC θJA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158°C/W θJC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43°C/W PIN FUNCTION DESCRIPTIONS Pin # 1 2 Mnemonic GND VCAIN Function 3 ORDERING GUIDE Model SSM2165-1P SSM2165-2P SSM2165-1S SSM2165-2S Temperature Range –40°C to +85°C –40°C to +85°C –40°C to +85°C –40°C to +85°C Package Description Plastic DIP Plastic DIP Narrow SOIC Narrow SOIC Package Options N-8 N-8 SO-8 SO-8 4 5 6 7 8 Ground VCA Input Pin. A typical connection is a 1 µF–10 µF capacitor from the buffer output pin (Pin 3) to this pin. Input Buffer Amplifier Output BUFOUT Pin. Must not be loaded by capacitance to ground. AUDIO +IN Input Audio Signal. The input signal should be ac-coupled (0.1 µF typical) into this pin. AVG CAP Detector Averaging Capacitor. A capacitor, 2.2 µF–22 µF, to ground from this pin is the averaging capacitor for the detector circuit. COMP RATIO SET Compression Ratio Set Pin. A resistor to ground from this pin sets the compression ratio as shown in Figure 1. OUTPUT Output Signal. V+ Positive Supply, +5 V Nominal. 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 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. WARNING! ESD SENSITIVE DEVICE REV. A –3– SSM2165 –Typical Performance Characteristics 260 240 220 200 180 160 RCOMP – k 100 90 TA = +25 C VS = +5V RL = 100k 5 V 1s 140 120 100 80 60 40 20 0 1:1 2:1 5:1 COMPRESSION RATIO 10:1 15:1 SSM2165–2 SSM2165–1 10 0% TA = +25 C COMPRESSION RATIO = 15:1 NOISE BW = 20kHz Figure 2. Compression Ratio vs. RCOMP Figure 5. Wideband Output Noise 5 TA = +25 C COMP RATIO = 1:1 RL = 100k /10k VS = +5V 1 70 60 50 40 COMP RATIO = 15:1 RCOMP = 0 VIN = 40 V rms THD+N – % GAIN – dB 30 20 10 0 G = 18dB G = 8dB SSM2165–1 0.1 SSM2165–2 –10 –20 1k 0.050 0.01 0.1 INPUT – V rms 1 10k 100k FREQUENCY – Hz 1M Figure 3. THD + N (%) vs. Input (V rms) Figure 6. GBW Curves vs. VCA Gain 5 THD+N – % 1 TA = +25 C VS = +5V COMP RATIO = 1:1 V I N = –20dBu (–1) V I N = –30dBu (–2) RL = 100k PSRR – dB –30 –40 –50 V+ = 5 1V p-p SSM2165–1 –60 0.1 SSM2165–2 0.050 20 100 1k FREQUENCY – Hz 10k 30k –70 20 100 1k FREQUENCY – Hz 10k 30k Figure 4. THD + N (%) vs. Frequency (Hz) Figure 7. PSRR vs. Frequency, Referred to Input –4– REV. A SSM2165 20mV 100 90 100 90 200mV 10 0% TA = +25 C CAVG = 2.2 F RL = 10k COMP RATIO = 1:1 VIN = 12.5mV (–1) VIN = 40mV (–2) 10 0% 10 s TA = +25 C CAVG = 2.2 F RL = 10k COMP RATIO = 1:1 VIN = 125mV (–1) VIN = 400mV (–2) 10 s Figure 8. Small Signal Transient Response Figure 9. Large Signal Transient Response APPLICATIONS INFORMATION THEORY OF OPERATION The SSM2165 is a complete microphone signal conditioning system in a single integrated circuit. Designed primarily for voiceband applications, this integrated circuit provides amplification, rms detection, limiting, variable compression, and downward 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 produces output signal levels at limiting of 320 mV and 250 mV for the SSM2165-1 and SSM2165-2 respectively, into loads > 5 kΩ. The SSM2165 contains an input buffer and automatic gain control (AGC) circuit fo