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Microphone Preamplifier with
a
Variable Compression and Noise Gating
SSM2166*
FEATURES 20 dB; this gain is in addition to the variable gain in other com-
Complete Microphone Conditioner in a 14-Lead Package pression settings. The input buffer can also be configured for front-
Single +5 V Operation end gains of 0 dB to 20 dB. A downward expander (noise gate)
Adjustable Noise Gate Threshold prevents amplification of noise or hum. This results in opti-
Compression Ratio Set by External Resistor miz
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(V+ = +5 V, f = 1 kHz, R = 100 kW , R = 600 kW , R = 3 kW , R = 0 W , L GATE ROTATION COMP SSM2166–SPECIFICATIONS R1 = 0 W , R2 = ¥ V, T = +258C, unless otherwise noted, V = 300 mV rms.) A IN Parameter Symbol Conditions Min Typ Max Units AUDIO SIGNAL PATH Voltage Noise Density e 15:1 Compression 17 nV/ Hz n 1 Noise 20 kHz Bandwidth, V = GND –109 dBu IN Total Harmonic Distortion THD+N 2nd and 3rd Harmonics, V = –20 dBu 0.25 0.5 % IN 22 kHz Low-Pass Filter Input Impedance Z 180 kW IN Output Im
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SSM2166 PIN DESCRIPTION Pin # Mnemonic Function 1 GND Ground 2 GAIN ADJUST 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 com- pression. 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 kW . For 10 dB of fixed gain, the resistor value is approximately 2 kW –3␣ kW . For resistor
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SSM2166 20 0 COMP RATIO = 15:1 COMP RATIO = 10:1 18 COMP RATIO = 5:1 –10 16 –20 14 –30 12 COMP RATIO = 2:1 T = +25°C A 10 –40 V+ = 5V R = 100kV L 8 T = +25°C A –50 V = 100mV rms @ 1kHz IN V+ = 5V 6 NOISE GATE SETTING 550mV rms V = 300mV rms @ 1kHz IN ROTATION POINT (PIN 11) 1V rms –60 R = 100kV L 4 COMPRESSION RATIO = 1:1 NOISE GATE SETTING 550μV rms ROTATION POINT 300mV rms –70 COMP RATIO = 1:1 GAIN ADJUST (PIN 2) = 1.25kV 2 0 –80 02 2 4 6 8 10 12 14 16 18 20 22 246 28 30 –80 –70 –60 –50 –4
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SSM2166 1.0 –10 R = 0 T = +25°C COMP A R = 1.24kV GAIN V+ = 5V –20 R = 500kV GATE R = 100kV L R = 1.74kV ROT COMPRESSION RATIO = 1:1 –30 NOISE GATE SETTING 550mV rms GAIN ADJUST (PIN 2) = 1.25kV V+ = 5±1V p-p –40 0.1 –50 V+ = 5±0.5V p-p –60 –70 0.01 –80 04 510 15 20 25 3035405 50 55 60 65 70 75 80 85 90 20 100 1k 10k 30k R RESISTOR – kV FREQUENCY – Hz ROT PT Figure 10c. PSRR vs. Frequency Figure 9. Rotation Point vs. R (Pin 11 to V+) ROT PT 20mV 5mV 1s 100 100 90 90 T = +25 C A C = 2.2mF AVG 10
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SSM2166 APPLICATIONS INFORMATION LIMITING LIMITING REGION The SSM2166 is a complete microphone signal conditioning THRESHOLD (ROTATION POINT) system on a single integrated circuit. Designed primarily for voiceband applications, this integrated circuit provides amplifi- COMPRESSION DOWNWARD REGION EXPANSION cation, rms detection, limiting, variable compression, and down- VCA GAIN THRESHOLD 1 ward expansion. An integral voltage-controlled amplifier (VCA) (NOISE GATE) r provides up to 60 dB of ga
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SSM2166 C6 C7* 10mF 10mF V+ 53 4 14 BUF VCA OUT VCA R1 = 10kV IN R 1kV 1kV INPUT BUFFER OUTPUT 13 V VCA OUT 7 AUDIO GAIN +IN ADJUST 2 V+ 0.1mF R SSM2166 GAIN R2 = 10kV + R GATE NOISE 1mF RMS GATE 9 CONTROL LEVEL CIRCUITRY DETECTOR ROTATION R ROT PT POINT ADJUST 11 POWER DOWN 12 POWER DOWN AVG COMPRESSION GND CAP RATIO SET 1 8 10 C AVG R *OPTIONAL COMP 2.2mF GND Figure 14. Functional Block Diagram and Typical Application bandwidth is unaffected). The GBW plots are shown in Figure small signal. T
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SSM2166 the rotation point may be varied from approximately 20 mV rms 1S 100mV to 1 V rms. From the figure, the rotation point is inversely propor- 100 6dBV tional to R . For example, a 1 kW resistor would typically ROT PT 90 set the rotation point at 1 V rms, whereas a 55 kW resistor would typically set the rotation point at approximately 30 mV rms. Since limiting occurs for signals larger than the rotation point 66dBV (V > V ), the rotation point effectively sets the maximum IN RP 10 output
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SSM2166 The gain of the VCA can be reduced below 0 dB by making Power-Down Feature smaller than 1 kW . Switching Pin 2 through 330 W or less The supply current of the SSM2166 can reduced to under R GAIN to ground will mute the output. Either a switch connected to 100 m A by applying an active HIGH, 5 V CMOS compatible ground or a transistor may be used, as shown in Figure 19. To input to the SSM2166’s POWER DOWN pin (Pin 12). In this avoid audible “clicks” when using this MUTE feature, a ca
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SSM2166 The value of the C7 should be the same as C6, the capacitor value used between BUF and VCA . This connection makes OUT IN C1 0.1mF the source ground noise appear as a common-mode signal to the 7 AUDIO IN +IN VCA, allowing the common-mode noise to be rejected by the (R > 5kV) S C X VCA’s differential input circuitry. C7 can also be useful in SSM2166 1000pF reducing ground loop problems and in reducing noise coupling from the power supply by balancing the impedances connected to the input
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SSM2166 Figure 23a. Evaluation Board Topside Silkscreen Figure 23c. Evaluation Board Backside Metallization (Not to Scale) (Not to Scale) Signal sources are connected to the SSM2166 through a 1/8" phone jack where a 0.1 m F capacitor couples the input signal to the SSM2166’s +IN pin (Pin 7). As shown in Figure 22 and in microphone applications, the phone jack shield can be optionally connected to the board’s ground plane (Jumper J1 inserted into board socket pins labeled “1” and “2”) or to the
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SSM2166 or an audio cable, then the onboard OP113 can be used. To Evaluation Board use the OP113 buffer, insert Jumper J4 into board socket pins If you build your own breadboard, keep the leads to Pins 3, 4, labeled “4” and “5” and insert Jumper J5 into board socket pins and 5 short. A convenient evaluation board is available from labeled “6” and “7.” If the output buffer is not required, re- your sales representative. The R and C designations refer to the move Jumper J5 and insert Jumper J4
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SSM2166 STEP 9. Record Values STEP 5. Adjusting the Rotation Point With the power removed from the test fixture, measure and Set the input level to 15 mV, and observe the output on the os- record the values of all potentiometers, including any fixed resis- cilloscope. Adjust R3—ROTATION PT ADJ CW until the tance in series with them. If you have changed the averaging output level just begins to drop, then reverse so that the output is capacitor, C4, note its value too. 500 mV. You have now set
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SSM2166 SSM2166 Demo Board Parts List R1 10k* Feedback R2 10k Input R3 50k Pot Rotation Point, Adj. R4 1k Rotation Point, Fixed R5 0 Comp Ratio, Fixed R6 100k Pot Comp Ratio, Adj. R7 1M Pot Noise Gate, Adj. R8 1k Noise Gate, Fixed R9 1k Gain Adj., Fixed R10 20k Pot Gain Adj. R11 330 Mute R12 100k Power Down Pull-Up C1 0.1* Input DC Block C2 1 Buffer Low f g = 1 C3 0.1 m F +V Bypass C4 2.2–22 Avg. Cap C5 0.01 Mute Click Suppress C6 10 Coupling C7 10 VCA Noise/DC Balance IC1 SSM2166P Mic Preamp IC
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SSM2166 OUTLINE DIMENSIONS Dimensions shown in inches and (mm). 14-Lead Plastic DIP 14-Lead Narrow-Body SOIC (N-14) (SO-14) 0.795 (20.19) 0.3444 (8.75) 0.725 (18.42) 0.3367 (8.55) 14 8 0.280 (7.11) 14 8 0.1574 (4.00) 0.2440 (6.20) 0.240 (6.10) 17 0.325 (8.25) 1 7 0.1497 (3.80) 0.2284 (5.80) 0.195 (4.95) 0.300 (7.62) 0.060 (1.52) 0.115 (2.93) PIN 1 0.015 (0.38) 0.210 (5.33) 0.0688 (1.75) PIN 1 0.0196 (0.50) x 45° MAX 0.0098 (0.25) 0.0532 (1.35) 0.130 0.0099 (0.25) 0.160 (4.06) (3.30) 0.0040 (0.10
