Microphone Preamplifier with Variable Compression and Noise Gating

a Microphone Preamplifier with Variable Compression and Noise Gating SSM2166* 20 dB; this gain is in addition to the variable gain in other compression settings. The input buffer can also be configured for frontend gains of 0 dB to 20 dB. 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 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 temperature range of –40°C to +85°C. For similar features/performance in an 8-lead package, please refer to the SSM2165. 10 0 –10 OUTPUT – dBu –20 –30 –40 –50 –60 –70 RATIO = 1:1 RATIO = 10:1 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 ( 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 RATIO = 2:1 The 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 setting the SSM2166 can be programmed with a fixed gain of up to R1 = 10k 5 BUFOUT 6 AUDIO +IN 7 0.1 F LEVEL DETECTOR R2 = 10k + 1 F BUFFER 10 F + 3 VCAIN 4 –60 –50 –40 –30 INPUT – dBu –20 –10 0 Figure 1. SSM2166 Compression and Gating Characteristics with 10 dB of Fixed Gain (The Gain Adjust Pin Can Be Used to Vary This Fixed Gain Amount) 10 F* + 14 VCAR 1k 1k VCA V+ 2 2.3k V+ VCA GAIN ADJ 13 VOUT 9 500k NOISE GATE SET 11 17k ROTATION POINT SET CONTROL SSM2166 12 POWER DOWN 1 GND 8 AVG CAP 22 F + 10 25k COMPRESSION RATIO SET *OPTIONAL *Patents pending. Figure 2. Functional Block Diagram and Typical Speech Application 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 =1 100 = 600 kΩ, R = 3 kΩ, SSM2166–SPECIFICATIONS (V+==0+5 V,R2f = ∞kHz,TR==+25 C,kΩ, R otherwise noted, V = 300 mVRrms.) = 0 Ω, R1 Ω, , unless L GATE ROTATION IN COMP A Parameter Symbol Conditions Min Typ Max Units AUDIO SIGNAL PATH Voltage Noise Density Noise Total Harmonic Distortion Input Impedance Output Impedance Load Drive Buffer Input Voltage Range Output Voltage Range VCA Input Voltage Range Output Voltage Range Gain Bandwidth Product CONTROL SECTION VCA Dynamic Gain Range VCA Fixed Gain Range Compression Ratio, Min Compression Ratio, Max Control Feedthrough POWER SUPPLY Supply Voltage Range Supply Current Quiescent Output Voltage Level Power Supply Rejection Ratio POWER DOWN Supply Current en THD+N ZIN ZOUT 15:1 Compression 20 kHz Bandwidth, VIN = GND 2nd and 3rd Harmonics, VIN = –20 dBu 22 kHz Low-Pass Filter 17 –109 0.25 180 75 0.5 nV/√Hz dBu1 % kΩ Ω kΩ nF V rms V rms V rms V rms MHz dB dB Resistive Capacitive 1% THD 1% THD 1% THD 1% THD 1:1 Compression, VCA G = 60 dB 5 2 1 1 1 1.4 30 60 –60 to +19 1:1 15:1 ±5 4.5 7.5 2.2 50 5.5 10 See Figure 5 for RCOMP/RROT 15:1 Compression, Rotation Point = –10 dBu VS ISY PSRR Pin 12 = V+2 mV V mA V dB µA 10 100 NOTES 1 0 dBu = 0.775 V rms. 2 Normal operation: Pin 12 = 0 V. Specifications subject to change without notice. ABSOLUTE MAXIMUM RATINGS ORDERING GUIDE Temperature Range Package Description Package Option 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 Model SSM2166P SSM2166S –40°C to +85°C –40°C to +85°C Plastic DIP Narrow SOIC N-14 SO-14 883 (Human Body) Model . . . . . . . . . . . . . . . . . . . . . . 2.0 kV THERMAL CHARACTERISTICS Thermal Resistance 14-Lead Plastic DIP θ JA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83°C/W θ JC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39°C/W 14-Lead SOIC θ JA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120°C/W θ JC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36°C/W 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 SSM2166 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. –2– WARNING! ESD SENSITIVE DEVICE REV. A SSM2166 PIN DESCRIPTION Pin # Mnemonic Function 1 2 GND GAIN ADJUST Ground VCA Gain Adjust Pin. A resistor from this pin to ground sets the fixed gain of the VCA. To check the setting of this pin the compression pin (Pin 10) should be grounded for no compression. The gain can be varied from 0 dB to 20 dB. For 20 dB leave the pin open. For 0 dB of fixed gain, a typical resistor value is approximately 1 kΩ. For 10 dB of fixed gain, the resistor value is approximately 2 kΩ–3 kΩ. For resistor values < 1 kΩ, the VCA can attenuate or mute. Refer to Figure 6. VCA Input Pin. A typical connection is a 10 µF capacitor from the buffer output pin (Pin 5) to this pin. Inverting Input to the VCA. This input can be used as a nonground reference for the audio input signal (see application notes). Input Buffer Amplifier Output Pin. Must not be loaded by capacitance to ground. Inverting Input to the Buffer. A 10 kΩ feedback resistor R1 from the buffer output Pin 5 to this input pin, and a resistor R2, from this pin through a 1 µF to ground gives gains of 6 dB to 20 dB for R2 = 10 kΩ to 1.1 kΩ. Input Audio Signal. The input signal should be ac-coupled (0.1 µF typical) into this pin. Detector Averaging Capacitor. A capacitor, 2.2 µF–22 µF, to ground from this pin is the averaging capacitor for the detector circuit. Noise Gate Threshold Set Point. A resistor to V+ sets the level below which input signals are downward-expanded. For a 0.7 mV threshold, the resistor value is approximately 380 kΩ. Increasing the resistor value reduces the threshold. See Figure 4. Compression Ratio Set Pin. A resistor to ground from this pin sets the compression ratio as shown in Figure 1. Figure 5 gives resistor values for various rotation points. Rotation Point Set Pin. This is set by a resistor to the positive supply. This resistor together with the gain adjust pin determines the onset of limiting. A typical value for this resistor is 17K for a 100 mV “rotation point.” Increasing the resistor value reduces the level at which limiting occurs. Refer to Figure 9. Power-Down Pin. Connect to ground for normal operation. Connect to positive supply for power-down mode. Output Signal. Positive Supply, +5 V Nominal. 3 4 5 6 VCAIN VCAR BUF OUT –IN 7 8 9 AUDIO +IN AVG CAP NOISE GATE SET 10 11 COMP RATIO SET ROTATION SET 12 13 14 POWER DOWN OUTPUT V+ PIN CONFIGURATION GND 1 GAIN ADJUST 2 VCAIN 3 VCAR 4 14 V+ 13 OUTPUT TOP VIEW 11 ROTATION SET BUF OUT 5 (Not to Scale) 10 COMP RATIO SET –IN 6 AUDIO +IN 7 9 NOISE GATE SET 8 AVG CAP SSM2166 12 POWER DOWN REV. A –3– SSM2166 0 –10 –20 OUTPUT – dBu –30 COMP RATIO = 2:1 20 COMP RATIO = 15:1 COMP RATIO = 10:1 COMP RATIO = 5:1 18 16 14 GAIN – dB 12 10 8 6 4 2 0 0 2 4 6 8 10 12 14 16 18 20 22 24 GAIN ADJUST RESISTOR – k 26 28 30 TA = +25°C V+ = 5V RL = 100k VIN = 100mV rms @ 1kHz NOISE GATE SETTING 550 V rms ROTATION POINT (PIN 11) 1V rms COMPRESSION RATIO = 1:1 –40 –50 –60 –70 –80 –80 COMP RATIO = 1:1 TA = +25°C V+ = 5V VIN = 300mV rms @ 1kHz RL = 100k NOISE GATE SETTING 550µV rms ROTATION POINT 300mV rms GAIN ADJUST (PIN 2) = 1.25k –70 –60 –50 –40 –30 INPUT – dBu –20 –10 0 Figure 3. Output vs. Input Characteristics Figure 6. VCA Gain vs. RGAIN (Pin 2 to GND) 100 TA = +25°C V+ = 5V RL = 100k COMPRESSION RATIO = 2:1 ROTATION POINT 1V rms GAIN ADJUST (PIN 2) = 1.25k THD