a Microphone Preamplifier with Variable Compression and Noise Gating SSM2165* GENERAL DESCRIPTION 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). C2 10mF + V+ AUDIO IN+ V+ BUFOUT C1 0.1mF +1 RA 2 VCAIN RA 2 VOUT VCA BUFFER LEVEL DETECTOR CONTROL SSM2165 AVG CAP C3 22mF GND + R1 25kV COMPRESSION RATIO SET 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 OUTPUT – dBu 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 FUNCTIONAL BLOCK DIAGRAM –20 –30 –40 –50 –60 –80 –70 –60 –50 –40 INPUT – dBu –30 –20 –10 Figure 1a. SSM2165-1 Compression and Gating Characteristics 0 –10 –20 OUTPUT – dBu 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 (ⴞ1 dB) Low Cost –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⍀, R L Parameter AUDIO SIGNAL PATH Voltage Noise Density Noise Total Harmonic Distortion SSM2165-1 SSM2165-2 Input Impedance Output Impedance Load Drive = 0 ⍀, TA = +25ⴗC, unless otherwise noted) Symbol Conditions en 15:1 Compression, VIN = GND 20 kHz Bandwidth, VIN = GND 17 –109 Min Typ 2nd and 3rd Harmonics, VIN = –30 dBu 2nd and 3rd Harmonics, VIN = –20 dBu 22 kHz Low-Pass Filter 0.2 0.2 Max Units nV/√Hz2 dBu1 THD+N Resistive Capacitive 1% THD 1% THD 1:1 Compression VCA G = 18 dB VCA G = 8 dB 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 0.5 0.5 180 75 ZIN ZOUT Input Voltage Range Output Voltage Range Gain Bandwidth Product SSM2165-1 SSM2165-2 POWER SUPPLY Supply Voltage Range Supply Current Quiescent Output Voltage Level Power Supply Rejection Ratio2 COMP VS ISY 1 1.4 kΩ Ω kΩ nF V rms V rms 300 100 kHz kHz 40 dB 18 8 dB dB 40 100 1:1 15:1 ±5 mV rms mV rms 5 2 15:1 Compression 4.5 7.5 2.2 50 PSRR % % mV 5.5 10 V mA V dB NOTES 1 0 dBu = 0.775 V rms. 2 Referred to input. Specifications subject to change without notice. –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 GND 1 VCAIN 2 8 V+ SSM2165 7 OUTPUT TOP VIEW 6 COMP RATIO SET (Not to Scale) 5 AVG CAP AUDIO +IN 4 BUFOUT 3 ESD RATINGS 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 # Mnemonic 1 2 GND VCAIN 3 ORDERING GUIDE Model Temperature Range Package Description Package Options SSM2165-1P SSM2165-2P SSM2165-1S SSM2165-2S –40°C to +85°C –40°C to +85°C –40°C to +85°C –40°C to +85°C Plastic DIP Plastic DIP Narrow SOIC Narrow SOIC 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. REV. A –3– Function WARNING! ESD SENSITIVE DEVICE SSM2165 –Typical Performance Characteristics 260 TA = +258C VS = +5V RL = 100kV 240 220 5mV 200 180 1s 100 RCOMP – kV 90 160 140 120 100 SSM2165–1 80 10 0% 60 SSM2165–2 40 TA = +258 C COMPRESSION RATIO = 15:1 NOISE BW = 20kHz 20 0 1:1 2:1 5:1 COMPRESSION RATIO 10:1 15:1 Figure 5. Wideband Output Noise Figure 2. Compression Ratio vs. RCOMP 70 5 COMP RATIO = 15:1 RCOMP = 0 VIN = 40mV rms 60 50 40 1 GAIN – dB THD+N – % TA = +258C COMP RATIO = 1:1 RL = 100kV/10kV VS = +5V SSM2165–1 30 20 10 G = 18dB G = 8dB 0 SSM2165–2 0.1 –10 0.050 0.01 0.1 INPUT – V rms –20 1k 1 1M –30 TA = +258C VS = +5V COMP RATIO = 1:1 V I N = –20dBu (–1) V I N = –30dBu (–2) RL = 100kV –40 PSRR – dB THD+N – % 1 100k FREQUENCY – Hz Figure 6. GBW Curves vs. VCA Gain Figure 3. THD + N (%) vs. Input (V rms) 5 10k V+ = 561V p-p –50 SSM2165–1 –60 0.1 SSM2165–2 0.050 20 100 1k FREQUENCY – Hz 10k –70 20 30k 100 1k FREQUENCY – Hz 10k 30k Figure 7. PSRR vs. Frequency, Referred to Input Figure 4. THD + N (%) vs. Frequency (Hz) –4– REV. A SSM2165 20mV 200mV 100 90 100 90 TA = +258C CAVG = 2.2mF RL = 10kV COMP RATIO = 1:1 VIN = 12.5mV (–1) VIN = 40mV (–2) 10 0% 10 0% 10ms Figure 8. Small Signal Transient Response TA = +258C CAVG = 2.2mF RL = 10kV COMP RATIO = 1:1 VIN = 125mV (–1) VIN = 400mV (–2) 10ms 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. Figure 10 illustrates the general transfer characteristic for the SSM2165 where the output level in dBu is plotted as a function of the input level in dBu (0 dBu = 0.775 V rms). 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 over 15:1 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 “rotation point,” and is different for the SSM2165-1 and SSM2165-2, see Table I. The SSM2165 contains an input buffer and automatic gain control (AGC) circuit for audio and voice band signals. Circuit operation is optimized by providing user-adjustable compression ratio and time constant. A downward expansion (noise gating) feature reduces background and circuit noise below 500 µV. The rotation point determines the output signal levels before limiting (referred to the input), and is 40 mV for the SSM2165-1 and 100 mV for the SSM2165-2. LIMITING REGION OUTPUT – dB LIMITING THRESHOLD (ROTATION POINT) DOWNWARD COMPRESSION EXPANSION REGION THRESHOLD 1 (NOISE GATE) r VCA GAIN Vrp Figure 10. General Input/Output Characteristics of the SSM2165 Output* –1 –2 40 mV (–25.7 dBu) 100 mV (–17.7 dBu) 18 dB 8 dB 320 mV (–6 dBu) 250 mV (–8 dBu) Input signals below VDE are downward expanded at a ratio of approximately 1:3. As a result, the gain of the system is small for very small input signal levels below VDE, even though it may be quite large for input signals above VDE. The downward expansion threshold, VDE, is fixed at 500 µV (–64 dBu) for both dash versions. All signals are in rms volts or dBu (0 dBu = 0.775 V rms). REV. A Gain When the compression is set to 2:1, a –2 dB change of the input signal level in the compression region causes –1 dB change of the output level. Likewise, at 10:1 compression, a –10 dB change of the input signal level in the compression region causes a –1 dB change in the output level. The gain of the system with an input signal level of VRP is fixed regardless of the compression ratio, and is different for the SSM2165-1 and SSM2165-2 (see Figures 1a and 1b). The “nominal gain” of the system is 18 dB for the SSM2165-1, and 8 dB for the SSM2165-2. System gain is measured at VRP and is (VOUT – VIN) in dB. 1 1 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. 1 VDE SSM2165 *At limiting. DOWNWARD EXPANSION REGION INPUT – dB Table I. Characteristics vs. Dash Number –5– SSM2165 operation of the level detector down to 10 Hz, the value of the capacitor should be around 22 µF. Some experimentation with larger values for the AVG CAP 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. The SSM2165 Signal Path Figure 11 illustrates the block diagram of the SSM2165. The audio input signal is processed by the unity gain input buffer and then by the VCA. The buffer presents an input impedance of approximately 180 kΩ to the source. A dc voltage of approximately 1.5 V is present at AUDIO +IN (Pin 4), requiring the use of a blocking capacitor (C1) for ground-referenced sources. A 0.1 µF capacitor is a good choice for most audio applications. The buffer is designed to drive only the low impedance input of the VCA, and must not be loaded by capacitance to ground. The VCA is a low distortion, variable-gain amplifier whose gain is set by the internal control circuitry. The input to the VCA is a virtual ground in series with 500 Ω. An external blocking capacitor (C2) must be used between the buffer’s output and the VCA input. The desired low frequency response and the total of 1 kΩ impedance between amplifiers determines the value of this capacitor. For music applications, 10 µF will give high pass fC = 16 Hz. For voice/communications applications, 1 µF will give fC = 160 Hz. An aluminum electrolytic capacitor is an economical choice. The VCA amplifies the input signal current flowing through C6 and converts this current to a voltage at the SSM2165’s output (Pin 7). The net gain from input to output can be as high as 40 dB for high compression ratios and depending on the gain set by the control circuitry. The output impedance of the SSM2165 is typically less than 75 Ω, and the external load on Pin 7 should be >5 kΩ. The nominal output dc voltage of the device is approximately 2.2 V. Use a dc blocking capacitor for grounded loads. AUDIO IN+ C1 0.1mF V+ C2 10mF + V+ BUFOUT 500V 500V The rms detector filter time constant is approximately given by 10 × CAVG milliseconds where CAVG is in µF. 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 react when a large input is followed by a small signal. The attack time, the time it takes for the gain to be reduced when a small signal is followed by a large signal, is mainly controlled by internal circuitry that speeds up the attack for large level changes, and controlled partly by the AVG CAP value. This limits overload time to under 1 ms in most cases. The performance of the rms level detector is illustrated in Figure 12 for CAVG = 2.2 µF and Figure 13 for CAVG = 22 µF. In each of these photographs, the input signal to the SSM2165 (not shown) is a series of tone bursts in 6 successive 10 dB steps. The tone bursts range from –66 dBu (0.5 mV rms) to –6 dBu (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 for CAVG and the input signal step size. The rate of release is approximately 240 dB/s for a CAVG = 2.2 µF, and 12 dB/s for a CAVG of 22 µF. VCAIN +1 VOUT VCA 100mV BUFFER 100 6dBV 90 LEVEL DETECTOR CONTROL SSM2165 GND AVG CAP C3 22mF + R1 25kV 66dBV COMPRESSION RATIO SET 10 85dBV 0% 100ms Figure 11. Functional Block Diagram and Typical Voice Application Figure 12. RMS Level Detector Performance with CAVG = 2.2 µ F The bandwidth of the SSM2165 is quite wide at all gain settings. The upper –3 dB point is approximately 300 kHz. The GBW plots are shown in Figure 6. 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 optimizing the usable dynamic range of the part. A photograph of the SSM2165’s wideband peak-to-peak output noise is illustrated in Figure 5. 100mV 100 90 The Level Detector 1s 6dBV 66dBV The SSM2165 incorporates a full-wave rectifier and a patentpending, true rms level detector circuit whose averaging time constant is set by an external capacitor connected to the AVG CAP pin (Pin 5). Capacitor values from 18 µF to 22 µF have been found to be more appropriate in voiceband applications, where capacitors on the low end of the range seem more appropriate for music program material. For optimal low frequency 10 0% 85dBV Figure 13. RMS Level Detector Performance with CAVG = 22 µ F –6– REV. A SSM2165 Control Circuitry Downward Expansion Threshold 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 10. The downward expansion threshold, or noise gate, is determined by a reference voltage internal to the control circuitry. The noise gate threshold is 500 µV for both versions of the SSM2165. Users requiring some other noise gate should consider using the SSM2166. High volume users may wish to consider a custom version of the SSM2165 with other noise gate thresholds or rotation points. Power-On/Power-Off Settling Time Cycling the power supply to the SSM2165 will result in quick settling times: the off-on settling time of the SSM2165 is less than 200 ms, while the on-off settling time is less than 1 ms. Note that transients may appear at the output of the device during power up and power down. A clickless mute function is available on the SSM2166 only. 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 14. The compression ratio can be set by connecting a resistor between the COMP RATIO pin (Pin 6) and GND. Lowering RCOMP gives smaller compression ratios as indicated in Figure 2, with values of about 5 kΩ or less resulting in a compression ratio of 1:1. AGC performance is achieved with compression ratios between 2:1 and 15:1, and is dependent on the application. A 200 kΩ potentiometer may be used to allow this parameter to be adjusted. PC Board Layout Considerations Since the SSM2165 is capable of wide bandwidth operation at high gain, special care must be exercised in the layout of the PC board which contains the IC and its associated components. The following applications hints should be considered and/or followed: 1. In some high system gain applications, the shielding of input wires to minimize possible feedback from the output of the SSM2165 back to the input circuit may be necessary. 15:1 5:1 OUTPUT – dB 2. A single-point (“star”) ground implementation is recommended in addition to maintaining short lead lengths and PC board runs. In systems where an analog ground and a digital ground are available, the SSM2165 and its surrounding circuitry should be connected to the analog ground. Wire-wrap board connections and grounding implementations are to be explicitly avoided. VCA GAIN 2:1 1:1 3. The internal buffer of the SSM2165 was designed to drive only the input of the internal VCA and its own feedback network. Stray capacitive loading to ground from either Pin 3 or Pin 2 in excess of 5 pF to 10 pF can cause excessive phase shift and can lead to circuit instability. 1 1 VDE INPUT – dB VRP 4. When using high impedance sources, it can be advantageous to shunt the source with a capacitor to ground at the input pin of the IC (Pin 4) to lower the source impedance at high frequencies, as shown in Figure 15. A capacitor with a value of 1000 pF is a good starting value and sets a low pass corner at 31 kHz for 5 kΩ sources. Figure 14. Effect of Varying the Compression Ratio Rotation Point An internal dc reference voltage in the control circuitry sets the rotation point. The rotation point determines the output level above which limiting occurs. That is, in the limiting region, a 10 dB change of input results in a 1 dB change of output. The rotation point is set to 40 mV (–26 dBu) for the SSM2165-1 and 100 mV (–18 dBu) for the SSM2165-2. In the SSM2165, limiting is compression at a fixed compression ratio of approximately 15:1. The fixed gain in the VCA is 18 dB for the SSM2165-1 and 8 dB for the SSM2165-2. The output signals at limiting are, therefore, 320 mV and 250 mV respectively. These are summarized in Table I. C1 0.1mF AUDIO IN (RS > 5kV) Maximum Output Since limiting occurs for signals larger than the rotation point (VIN > VRP), the rotation point effectively sets the maximum output signal level. The application will determine which version of the SSM2165 should be selected. The output level should match the maximum input allowed by the following stage. Occasional larger signal transients will then be attenuated by the action of the limiter. REV. A –7– 4 +IN CX 1000pF SSM2165 NOTE: ADDITIONAL CIRCUIT DETAILS OMITTED FOR CLARITY. Figure 15. Circuit Configuration for Use with High Impedance Signal Sources SSM2165 C2 10mF + +5V GENERATOR AND AC VOLTMETER BUFOUT V+ C1 0.1mF VCAIN +1 AC VOLTMETER AND OSCILLOSCOPE VCA BUFFER LEVEL DETECTOR +2V CONTROL HEADPHONES SSM2165-1 2kV + MICROPHONE (ELECTRET) GND AVG CAP C3 + 22mF 1:1 R1 – COMPRESSION 15:1 RATIO SET 200kV CW Figure 16. Electret Microphone Preamp Example Compression Adjustment—A Practical Example STEP 1. Initialize Potentiometer To illustrate how to set the compression ratio of the SSM2165, we will take a practical example. The SSM2165 will be used interface an electret-type microphone to a post-amplifier, as shown in Figure 16. The signal from the microphone was measured under actual conditions to vary from 2 mV to 30 mV. The post-amplifier requires no more than 350 mV at its input. We will therefore choose the SSM2165-1, whose “rotation” point is 40 mV and whose VCA fixed gain is 18 dB (×8), thus giving 320 mV at limiting. From prior listening experience, we will use a 2:1 compression ratio. The noise gate threshold of the SSM2165-1 will operate when the input signal falls below 500 µV. These objectives are summarized in Table II. The transfer characteristic we will implement is illustrated in Figure 18. With power off, preset R1—Compression Ratio potentiometer to zero ohms. STEP 2. Check Setup With power on, adjust the generator for an input level of 50 mV (–24 dBu), 1 kHz. The output meter should indicate approximately 350 mV (–6.9 dBu). If not, check your setup. STEP 3. Find the Rotation Point Set the input level to 50 mV (–24 dBu), and observe the output on the oscilloscope. The output will be in the limiting range of operation. Slowly reduce the input signal level until the output level just begins to stop limiting and follows the input down. Increase the input so that the output is 320 mV (–7.7 dBu). You have located the knee of the rotation point. Table II. Objective Specification of Example Input Range Output Range Limiting Level Compression Gain Noise Gate STEP 4. Adjust the Compression Ratio With the input set as in Step 3, note the exact value of the input signal level just below the knee (around 40 mV (–26 dBu)). Next, reduce the input to 1/4 the value noted, (around 10 mV (–38 dBu)), for a change of –12 dB. Next, increase the RCOMP potentiometer resistance so the output is 160 mV (–13.7 dBu) for an output change of –6 dB. You have now set the compression, which is the ratio of input change to output change, in dB, to 2:1. 2 mV–30 mV To 350 mV 320 mV 2:1 18 dB 500 µV Test Equipment Setup STEP 5. Confirm the Noise Gate Threshold The recommended equipment and configuration is shown in Figure 17. A low noise audio generator with a smooth output adjustment range of 100 µV to 25 mV is a suitable signal source. The output voltmeter should go up to 2 volts. The oscilloscope is used to verify that the output is sinusoidal, that no clipping is occurring in the buffer, and to observe the limiting and noise gating “knees.” Set the input to 1 mV, and observe the output on the oscilloscope. A 20 dB pad between generator and input may facilitate this measurement. Reduce the input gradually until the output falls off more rapidly. This point is the noise gate threshold, and should be approximately 500 µV (–64 dBu). The noise gate threshold on the SSM2165 is fixed at 500 µV, a practical value for many microphones. Should you require a different noise gate threshold, consider using the SSM2166. Breadboard Considerations When building your breadboard, keep the leads to Pins 2 and 3 as short as possible. Use a central analog ground and decouple power supply connections adequately. SIGNAL GENERATOR AC VOLTMETER SSM2165-1 STEP 6. Listen At this time, you may replace the signal generator with a properly powered electret microphone and listen to the results through a set of headphones. The microphone’s internal FET usually requires around +2 V through a 2 kΩ resistor; this varies with the manufacturer. Experiment with the compression ratio value and averaging capacitor size. More compression will keep the output steady over a wider range of microphone-to-source distance. Varying the averaging capacitor, CAVG, changes the AC VOLTMETER OSCILLOSCOPE Figure 17. Test Equipment Setup –8– REV. A SSM2165 SUMMARY rms detector averaging time, and the decay time of the gate. Both compression ratio and decay time are usually determined by critical listening to the intended audio input. We have implemented the transfer characteristic of Figure 18. For inputs below the 500 µV noise gate threshold, circuit and background noise will be downward expanded (gain-reduced) at a ratio of approximately 1:3. That is, a –1 dB change in the noise will result in –3 dB decrease at the output. Above threshold, the signal will increase at a rate of 1 dB for each 2 dB input increase, until the rotation point is reached at an input of approximately 40 mV. In the limiting region, the compression ratio increases to approximately 15:1. That is, a 15 dB increase in input will produce a 1 dB increase at the output, so there will be little further increase for higher level inputs. STEP 7. Record Values OUTPUT – mV With the power removed from the test fixture, measure and record the values of the RCOMP and CAVG. 300 COMPRESSION REGION LIMITING REGION Other Versions 45 The SSM2165 is an 8-lead version of the 14-lead SSM2166 which is recommended for applications requiring more versatility. The SSM2166 allows selection of noise gate threshold and rotation point, and allows the buffer to provide up to 20 dB of gain. Power-down and mute functions are also built in. Customized versions of the SSM2165 are available for large volume users. The wide dynamic range of the SSM2165 makes it useful in many applications other than microphone signal conditioning such as a sustain generator for guitars. For further information, contact your Analog Devices representative. NOISE GATING REGION 0.5 2 30 40 INPUT – mV Figure 18. Transfer Characteristic REV. A –9– SSM2165 OUTLINE DIMENSIONS Dimensions shown in inches and (mm). C2178a–0–6/99 8-Lead Plastic DIP (N-8) 0.430 (10.92) 0.348 (8.84) 8 5 0.280 (7.11) 0.240 (6.10) 1 4 0.325 (8.25) 0.300 (7.62) PIN 1 0.100 (2.54) BSC 0.210 (5.33) MAX 0.060 (1.52) 0.015 (0.38) 0.195 (4.95) 0.115 (2.93) 0.130 (3.30) MIN 0.160 (4.06) 0.115 (2.93) 0.022 (0.558) 0.070 (1.77) SEATING 0.014 (0.356) 0.045 (1.15) PLANE 0.015 (0.381) 0.008 (0.204) 8-Lead Narrow-Body SOIC (SO-8) 0.1968 (5.00) 0.1890 (4.80) 0.1574 (4.00) 0.1497 (3.80) 8 5 1 4 0.2440 (6.20) 0.2284 (5.80) PIN 1 0.0196 (0.50) 3 458 0.0099 (0.25) 0.0500 (1.27) BSC SEATING PLANE 0.0688 (1.75) 0.0532 (1.35) 88 0.0500 (1.27) 0.0098 (0.25) 08 0.0160 (0.41) 0.0075 (0.19) 0.0192 (0.49) 0.0138 (0.35) PRINTED IN U.S.A. 0.0098 (0.25) 0.0040 (0.10) –10– REV. A