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microphone preamplifier with variable compression and noise gating ssm2166 rev. d 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. specifications subject to change without notice. no license is granted by implication or otherwise under any patent or patent rights of analog devices. trademarks and registered trademarks are the property of their respective owners. one technology way, p.o. box 9106, norwood, ma 02062-9106, u.s.a. tel: 781.329.4700 www.analog.com fax: 781.461.3113 ?1996C2008 analog devices, inc. all rights reserved. features complete microphone conditione r in a 14-lead soic package single 5 v operation adjustable noise gate threshold compression ratio set by external resistor automatic limiting featureprevents adc overload adjustable release time low noise and distortion power-down feature 20 khz bandwidth (1 db) applications microphone preamplifiers/processors computer sound cards public address/paging systems communication headsets telephone conferencing guitar sustain effects generators computerized voice recognition surveillance systems karaoke and dj mixers general description 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 to eliminate popping. in the 1:1 compression setting, the ssm2166 can be programmed with a fixed gain of up to 20 db; this gain is in addition to the variable gain in other compression settings. the input buffer can also be configured for front-end 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 may 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 . the ssm2166 is available in a 14-lead soic package and is guaranteed for operation over the extended industrial temperature range of ?40c to +85c. 10 0 ?10 ?20 ?30 ?40 ?50 ?60 ?70 ?60 ?50 ?40 ?30 ?20 ?10 0 input (dbu) output (dbu) 00357-002 comp ratio = 10:1 comp ratio = 2:1 comp ratio = 1:1 figure 1. compression and gating characteristics with 10 db of fixed gain (the gain adjust pin can be used to vary this fixed gain amount) functional block diagram and typical speech application buffer 1k ? 1k? buf out vca in vca r v+ 4 3 5 14 level detector vca control 8 1 12 6 7 2 13 9 11 10 ssm2166 power down gnd avg cap comp ratio set gain adjust output noise gate set rotation set ?in audio +in 0.1f r2 10k? 1f + r1 10k? 10f 10f (optional) ++ + 25k? 22f 500k ? 2.3k ? 17k? v + 00357-001 figure 2.
ssm2166 rev. d | page 2 of 20 table of contents features .............................................................................................. 1 ? applications ....................................................................................... 1 ? general description ......................................................................... 1 ? functional block diagram and typical speech application ...... 1 ? revision history ............................................................................... 2 ? specifications ..................................................................................... 3 ? absolute maximum ratings ............................................................ 4 ? thermal resistance ...................................................................... 4 ? esd caution .................................................................................. 4 ? pin configuration and function descriptions ............................. 5 ? typical performance characteristics ............................................. 6 ? theory of operation ........................................................................ 8 ? applications information ............................................................ 8 ? signal path ......................................................................................8 ? level detector ................................................................................9 ? control circuitry ........................................................................ 10 ? power-down feature ................................................................. 12 ? pcb layout considerations ...................................................... 12 ? evaluation board ............................................................................ 13 ? evaluation board examples ...................................................... 14 ? evaluation board setup procedure .......................................... 15 ? test equipment setup ................................................................ 15 ? setup summary .......................................................................... 16 ? outline dimensions ....................................................................... 17 ? ordering guide .......................................................................... 17 ? revision history 7/08rev. c to rev. d changes to figure 4 through figure 9 ........................................... 6 changes to figure 11 and figure 12 ............................................... 7 changes to figure 19 ...................................................................... 10 changes to figure 26 ...................................................................... 13 added top branding revision reflecting die replacement table ................................................................................................. 17 5/08rev. b to rev. c updated format .................................................................. universal changes to features section and general description section ................................................................................................ 1 changes to table 1 ............................................................................ 3 changes to table 2 ............................................................................ 4 deleted tpc 3; renumbered sequentially .................................... 4 changes to table 4, pin 8 description column ........................... 5 changes to figure 5, figure 6, figure 8, and figure 9 ................. 6 change to figure 11 ......................................................................... 7 changes to signal path section ...................................................... 9 added figure 19 ............................................................................. 10 deleted figure 14 and figure 17 .................................................. 12 deleted other versions section ................................................... 13 changes to figure 26 ...................................................................... 13 changes to figure 27 ...................................................................... 14 changes to test equipment section ............................................ 15 added table 6 ................................................................................. 16 added table 7 ................................................................................. 16 updated outline dimensions ....................................................... 17 changes to ordering guide .......................................................... 17 3/03rev. a to rev. b deleted pdip package ....................................................... universal change to general description ....................................................... 1 changes to thermal characteristics ............................................... 2 changes to ordering guide ............................................................. 2 deleted 14-lead pdip, outline dimensions .............................. 15 updated 14-lead narrow-body soic, outline dimensions ... 15
ssm2166 rev. d | page 3 of 20 specifications v+ = 5 v, f = 1 khz, r l = 100 k, r gate = 600 k, r rot pt = 3 k, r comp = 0 , r1 = 0 , r2 = , t a = 25c, unless otherwise noted; v in = 300 mv rms. table 1. parameter symbol conditions min typ max unit audio signal path voltage noise density e n 15:1 compression 17 nv/hz noise 20 khz bandwidth, v in = gnd ?109 dbu 1 total harmonic distortion and noise thd + n second and third harmonics, v in = ?20 dbu, 22 khz low-pass filter 0.25 0.5 % input impedance z in 180 k output impedance z out 75 load drive resistive 5 k capacitive 2 nf buffer input voltage range 1% thd 1 v rms output voltage range 1% thd 1 v rms vca input voltage range 1% thd 1 v rms output voltage range 1% thd 1.4 v rms gain bandwidth product 1:1 compression, vca gain = 60 db 30 mhz control section vca dynamic gain range 60 db vca fixed gain range ?60 to +19 db compression ratio, minimum 1:1 compression ratio, maximum see figure 19 for r comp /r rot pt , rotation point = 100 mv rms 15:1 control feedthrough 15:1 compression, rotation point = ?10 dbu, r2 = 1.5 k 5 mv power supply supply voltage range v+ 4.5 5.5 v supply current i sy 7.5 10 ma quiescent output voltage level 2.2 v power supply rejection ratio psrr 50 db power down supply current pin 12 = v+ 2 10 100 a 1 0 dbu = 0.775 v rms. 2 normal operation for pin 12 is 0 v.
ssm2166 rev. d | page 4 of 20 absolute maximum ratings table 2. parameter rating supply voltage 10 v audio input voltage supply voltage operating temperature range ?40c to +85c storage temperature range ?65c to +150c junction temperature (t j ) 150c lead temperature (soldering, 60 sec) 300c 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 indicated in the operational section of this specification is not implied. exposure to absolute maximum rating conditions for extended periods may affect device reliability. thermal resistance table 3. package type ja jc unit 14-lead soic 120 36 c/w esd caution
ssm2166 rev. d | page 5 of 20 pin configuration and fu nction descriptions 00357-003 gnd 1 g ain adjust 2 vca in 3 vca r 4 v+ 14 output 13 power down 12 rotation set 11 buf out 5 comp ratio set 10 ?in 6 noise gate set 9 audio +in 7 avg cap 8 ssm2166 top view (not to scale) figure 3. pin configuration table 4. pin function descriptions pin o. nemonic description 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, make sure the compression ratio set pin (pin 10) is 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 k. for 10 db of fixed gain, the resistor va lue is approximately 2 k to 3 k. for resistor values <1 k, the vca can attenuate or mute (see figure 6). 3 vca in vca input pin. a typical connection is a 10 f capacito r from the buffer output pin (pin 5) to this pin. 4 vca r inverting input to the vca. this inp ut can be used as a nonground referenc e for the audio input signal (see the applications information section). 5 buf out input buffer amplifier output pin. this pin must not be loaded by capacitance to ground. 6 ?in inverting input to the buffer. a 10 k feedback resistor, r1, from the buffer output (pin 5) to this input pin and a resistor, r2, from this pin through a 1 f capacitor to ground give gains of 6 db to 20 db for r2 = 10 k to 1.1 k. 7 audio +in input audio signal. the input signal should be ac-coupled (0.1 f typical) into this pin. 8 avg cap detector averaging capacitor. a capacitor, 1 f to 22 f , to ground from this pin is the averaging capacitor for the detector circuit. 9 noise gate set 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 k. increasing the resistor value reduces the threshold (see figure 5). 10 comp ratio set compression ratio set pin. a resistor to ground from this pin sets the compression ratio, as shown in figure 2. figure 19 gives resistor values for various rotation points. 11 rotation set rotation point set pin. this pin is set by adding a resist or to the positive supply. this resistor together with the gain adjust pin determines the onset of limiting. a typical value for this resistor is 17 k for a 100 mv rotation point. increasing the resistor value reduces the level at which limiting occurs (see figure 9). 12 power down power-down pin. connect this pin to ground for normal operation. connect this pin to the positive supply for power-down mode. 13 output output signal. 14 v+ positive supply, 5 v nominal.
ssm2166 rev. d | page 6 of 20 typical performance characteristics 0 ?10 ?20 ?30 ?40 ?50 ?60 ?70 ?80 ?70 ?80 ?60 ?50 ?40 ?30 ?20 ?10 0 input (dbu) output (dbu) 00357-004 comp ratio = 1:1 comp ratio = 15:1 comp ratio = 10:1 comp ratio = 5:1 comp ratio = 2:1 t a = 25c v+ = 5v v in = 300mv rms @ 1khz r l = 100k ? noise gate setting 550v rms rotation set 300mv rms gain adjust = 1.25k ? figure 4. output vs. input characteristics 100 10 1 0.1 0 50 100 150 200 250 300 350 400 450 500 550 600 650 r gate (k ? ) noise gate (mv rms) 00357-005 t a = 25c v+ = 5v r l = 100k ? compression ratio = 2:1 rotation set 700mv rms gain adjust = 1.56k ? figure 5. noise gate vs. r gate (pin 9 to v+) 20 18 16 14 12 10 8 6 4 2 0 0 2 4 6 8 1012141618202224262830 gain adjust resistor (k ? ) vca gain (db) 00357-006 t a = 25c v+ = 5v v in = 77.5mv rms @ 1khz r l = 100k ? noise gate setting 550v rms rotation set 1v rms compression ratio = 1:1 figure 6. vca gain vs. r gain (pin 2 to gnd) 5 1 0.1 0.05 0.01 0.1 1 input voltage (v rms) thd + n (%) 00357-007 r l = 100k ? r l = 10k ? t a = 25c v+ = 5v compression ratio = 1:1 noise gate setting 550v rms rotation set 1v rms gain adjust = 1.25k ? v in frequency = 1khz figure 7. thd + n (%) vs. input (v rms) 10 1 0.1 0.01 10 100 1k 10k 30k frequency (hz) thd + n (%) 00357-008 t a = 25c v+ = 5v v in = 77.5mv rms @ 1khz compression ratio = 1:1 noise gate setting 550v rms rotation set 1v rms gain adjust = 156k ? measurement filter bw: 20hz to 20khz figure 8. thd + n (%) vs. frequency (hz) 1 0.1 0.01 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 r rot pt resistor (k ? ) rotation point (v rms) 00357-009 figure 9. rotation point vs. r rot pt (pin 11 to v+)
ssm2166 rev. d | page 7 of 20 00357-010 ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? t a = 25c compression ratio = 15:1 noise bw = 20khz 5v 1s 100 90 10 0% figure 10. wideband peak-to-peak output noise 80 60 40 20 0 ?20 ?40 1k 10k 100k 1m frequency (hz) vga gain (db) 00357-011 g = 60db g = 40db g = 20db rotation set 1v rms r comp = 30k ? noise gate setting 550v rms v in = 1mv rms figure 11. vca gain bandwidth curves vs. frequency ? 10 ?20 ?30 ?40 ?50 ?60 ?70 ?80 20 100 1k 10k 30k frequency (hz) psrr (db) 00357-012 v+ = 5v 1v p-p v+ = 5v 0.5v p-p r comp = 0 ? r gain = 1.24k ? r gate = 500k ? r rot pt = 1.74k ? figure 12. psrr vs. frequency 00357-013 ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? 100 90 10 t a = 25c c avg = 2.2f system gain = 0db r l = 10k ? compression ratio = 1:1 20mv 10s 0% figure 13. small signal transient response 00357-014 ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? 100 90 10 t a = 25c c avg = 2.2f system gain = 0db r l = 10k ? compression ratio = 1:1 200mv 10s 0% figure 14. large signal transient response
ssm2166 rev. d | page 8 of 20 theory of operation figure 15 illustrates a typical transfer characteristic for the ssm2166 where the output level in decibels is plotted as a function of the input level in decibels. the dotted line indicates the transfer characteristic for a unity-gain amplifier. for input signals in the range of v de (downward expansion) to v rp (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 can be varied from 1:1 (no compression) to over 15:1 via a single resistor, r comp . input signals above v rp are compressed with a fixed compression ratio of approximately 15:1. this region of operation is the limiting region. varying the compression ratio has no effect on the limiting region. the break- point between the compression region and the limiting region is referred to as the limiting threshold or the rotation point and is user specified in the ssm2166. 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 v rp is fixed by r gain , regardless of the compression ratio, and is the nominal gain of the system. the nominal gain of the system can be increased by the user via the on-board vca by up to 20 db. additionally, the input buffer of the ssm2166 can be configured to provide fixed gains of 0 db to 20 db with r1 and r2. input signals below v de are downward expanded; that is, a ?1 db change in the input signal level causes approximately a ?3 db change in the output level. as a result, the gain of the system is small for very small input signal levels, even though it may be quite large for small input signals above v de . the down- ward expansion threshold, v de , is set externally by the user via r gate at pin 9 (noise gate set). the ssm2166 provides an active high, cmos-compatible digital input whereby a power- down feature reduces the device supply current to less than 100 a. limiting region limitin g threshold (rotation point) compression region 1 r 1 1 downward expansion threshold (noise gate) downward expansion region vca gain v rp output (db) input (db) 00357-015 v de figure 15. general input/ output characteristics applications information the ssm2166 is a complete microphone signal conditioning system on a single integrated circuit. designed primarily for voice- band applications, this integrated circuit provides amplification, rms detection, limiting, variable compression, and downward expansion. an integral voltage-controlled amplifier (vca) provides up to 60 db of gain in the signal path with approximately 30 khz bandwidth. additional gain is provided by an input buffer, op amp circuit that can be set anywhere from 0 db to 20 db for a total signal path gain of up to 80 db. the device operates on a single 5 v supply, accepts input signals up to 1 v rms, and produces output signal levels >1 v rms (3 v p-p) into loads >5 k. the internal rms detector has a time constant set by an external capacitor. the ssm2166 contains an input buffer and automatic gain control (agc) circuit for audio-band and voice-band signals. circuit operation is optimized by providing a user-adjustable time constant and compression ratio. a downward expansion (noise gating) feature eliminates circuit noise in the absence of an input signal. the ssm2166 allows the user to set the downward expansion threshold, the limiting threshold (rotation point), the input buffer fixed gain, and the internal vca nominal gain at the rotation point. the ssm2166 also features a power-down mode and muting capability. signal path figure 16 illustrates the block diagram of the ssm2166. the audio input signal is processed by the input buffer and then by the vca. the input buffer presents an input impedance of approximately 180 k to the source. a dc voltage of approximately 1.5 v is present at audio +in (pin 7), requiring the use of a blocking capacitor (c1) for ground referenced sources. a 0.1 f capacitor is a good choice for most audio applications. the input buffer is a unity-gain stable amplifier that can drive the low impedance input of the vca. the vca is a low distortion, variable-gain amplifier (vga) whose gain is set by the side-chain control circuitry. the input to the vca is a virtual ground in series with approximately 1 k. an external blocking capacitor (c6) must be used between the buffer output and the vca input. the 1 k impedance between amplifiers determines the value of this capacitor, which is typically between 1 f and 10 f. 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 output pin (pin 13). the net gain from input to output can be as high as 60 db (without additional buffer gain), depending on the gain set by the control circuitry.
ssm2166 rev. d | page 9 of 20 the gain of the vca at the rotation point is set by the value of a resistor, r gain , connected between pin 2 and gnd. the relationship between the vca gain and r gain is shown in figure 6. the agc range can be as high as 60 db. the vca in pin (pin 3) is the non- inverting input terminal to the vca. the inverting input of the vca is available at the vca r pin (pin 4) and exhibits an input impedance of 1 k, as well. as a result, this pin can be used for differential inputs or for the elimination of grounding problems by connecting a capacitor whose value equals that used in series with the vca in pin to ground (see figure 26 for more details). the output impedance of the ssm2166 is typically less than 75 , and the external load on pin 13 should be >5 k. the nominal output dc voltage of the device is approximately 2.2 v. use a blocking capacitor for grounded loads. the bandwidth of the ssm2166 is quite wide at all gain settings. the upper 3 db point is approximately 30 khz at gains as high as 60 db (using the input buffer for additional gain, circuit bandwidth is unaffected). the gain bandwidth (gbw) plots are shown in figure 11. the lower 3 db cutoff frequency of the ssm2166 is set by the input impedance of the vca (1 k) and c6. 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 plot of wideband peak- to-peak output noise is shown in figure 10. level detector the ssm2166 incorporates a full-wave rectifier and true rms level detector circuit whose averaging time constant is set by an external capacitor connected to the avg cap pin (pin 8). for optimal low frequency operation of the level detector down to 10 hz, the value of the capacitor should be 2.2 f. 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 may result in slow response times to signal dynamics. electrolytic capacitors are recommended for lowest cost and should be in the range of 2 f to 47 f. capacitor values from 18 f to 22 f have been found to be more appropriate in voice-band applications where capacitors on the low end of the range seem more appropriate for music program material. the rms detector filter time constant is approximately given by 10 c avg milliseconds, where c avg is in f. 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 controlled partly by the avg cap value but is mainly controlled by internal circuitry that speeds up the attack for large level changes. this limits overload time to less than 1 ms in most cases. buffer 1k ? 1k? buf out vca in v out vca r 4 3 5 14 rms level detector vca 1 2 13 9 11 ssm2166 power down gnd avg cap comp ratio set gain adjust output noise gate set rotation set ?in audio +in c1 0.1f r2 10k ? r1 10k ? 1f + c6 10f c7 10f (optional) + + + r comp c avg 2.2f v+ 00357-016 8 10 12 control circuit 6 7 r gain r gate r rot pt v+ figure 16. functional block diagram and typical application
ssm2166 rev. d | page 10 of 20 the performance of the rms level detector is illustrated for a c avg of 2.2 f in figure 17 and for a c avg of 22 f in figure 18. in each of these images, the input signal to the ssm2166 (not shown) is a series of tone bursts in six successive 10 db steps. the tone bursts range from ?66 dbv (0.5 mv rms) to ?6 dbv (0.5 v rms). as shown in figure 17 and figure 18, the attack time of the rms level detector is dependent only on c avg , but the release times are linear ramps whose decay times are dependent on both c avg and the input signal step size. the rate of release is approximately 240 db/s for a c avg of 2.2 f and 12 db/s for a c avg of 22 f. 00357-017 ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? 100mv 100ms ?6dbv ?66dbv ?85dbv 100 90 10 0% figure 17. rms level detector performance with c avg = 2.2 f 00357-018 ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? 100 90 10 100mv 1s ?6dbv ?66dbv ?85dbv 0% figure 18. rms level detector performance with c avg = 22 f control circuitry 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 gain control of the vca is logarithmica linear change in the control signal causes a decibel 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 15. compression ratio changing the scaling of the control signal fed to the vca causes a change in the circuit compression ratio, r. this effect is shown in figure 20. the compression ratio can be set by connecting a resistor between the comp ratio set pin (pin 10) and gnd. lowering r comp gives smaller compression ratios as shown in figure 19, with values of approximately 17 k 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 100 k potentiometer can be used to allow this parameter to be adjusted. on the evaluation board (see figure 26), an optional resistor can be used to set the compression equal to 1:1 when the wiper of the potentiometer is at its full counterclockwise (ccw) position. 00357-031 rotation point 100mv rms 300mv rms 1v rms 1:1 0.1 0.1 0.1 2:1 8.7 8.7 8.7 5:1 19.4 19.4 19.4 10:1 45 45 45 15:1 395 n/a n/a compression ratio typical r comp values in k ? . figure 19. compression ratio vs. r comp (pin 10 to gnd) output (db) input (db) v rp 15:1 5:1 2:1 1:1 1 1 vca gain 00357-019 v de figure 20. effect of varying the compression ratio rotation point an internal dc reference voltage in the control circuitry, used to set the rotation point, is user specified, as illustrated in figure 9. the effect on rotation point is shown in figure 21. by varying a resistor, r rot pt , connected between the positive supply and the rotation set pin (pin 11), the rotation point may be varied by approximately 20 mv rms to 1 v rms. from figure 21, the rotation point is inversely proportional to r rot pt . for example, a 1 k resistor would typically set the rotation point at 1 v rms, whereas a 55 k resistor would typically set the rotation point at approximately 30 mv rms.
ssm2166 rev. d | page 11 of 20 because limiting occurs for signals larger than the rotation point (v in > v rp ), the rotation point effectively sets the maximum output signal level. it is recommended that the rotation point be set at the upper extreme of the range of typical input signals so that the compression region covers the entire desired input signal range. occasional larger signal transients are then attenuated by the action of the limiter. vca gain vca gain vca gain output (db) input (db) v rp1 r:1 1 1 0 0357-020 v de v rp2 v rp3 figure 21. effect of varying the rotation point vca gain setting and muting the maximum gain of the ssm2166 is set by the gain adjust pin (pin 2) via r gain . this resistor, with a range of 1 k to 20 k, causes the nominal vca gain to vary from 0 db to approximately 20 db, respectively. setting the vca gain to its maximum can also be achieved by leaving the gain adjust pin in an open condition (no connect). figure 22 illustrates the effect on the transfer characteristic by varying this parameter. for low level signal sources, the vca should be set to maximum gain using a 20 k resistor. vca gain output (db) input (db) v rp r:1 1 1 00357-021 v de figure 22. effect of varying the vca gain setting the gain of the vca can be reduced below 0 db by making r gain smaller than 1 k. switching pin 2 through 330 or less to gnd mutes the output. either a switch connected to ground or a transistor can be used, as shown in figure 23. to avoid audible clicks when using the mute feature, a capacitor (c5) can be connected from pin 2 to gnd. the value of the capacitor is arbitrary and should be determined empirically, but a 0.01 f capacitor is a good starting value. 00357-022 r gain c5 330? 2 gain adjust ssm2166 notes 1. additional circuit details omitted mute (closed switch) for clarity. figure 23. details of mute option downward expansion threshold the downward expansion threshold, or noise gate, is deter- mined via a second reference voltage internal to the control circuitry. this second reference can be varied in the ssm2166 using a resistor, r gate , connected between the positive supply and the noise gate set pin (pin 9). the effect of varying this threshold is shown in figure 24. the downward expansion threshold can be set between 300 v rms and 20 mv rms by varying the resistance value between pin 9 and the supply voltage. like the rotation set pin, the downward expansion threshold is inversely proportional to the value of this resistance: setting this resistance to 1 m sets the threshold at approximately 250 v rms, whereas a 10 k resistance sets the threshold at approximately 20 mv rms. this relationship is illustrated in figure 5. a potentiometer network is provided on the evaluation board for this adjustment. in general, the downward expansion threshold should be set at the lower extreme of the desired range of the input signals so that signals below this level are attenuated. output (db) input (db) 1 1 v rp v de1 r:1 vca gain 00357-023 v de3 v de2 figure 24. effect of varying the down ward expansion (noi se gate) threshold
ssm2166 rev. d | page 12 of 20 power-down feature the supply current of the ssm2166 can be reduced to less than 100 a by applying an active high, 5 v cmos-compatible input to the power down pin (pin 12). in this state, the input and output circuitry of the ssm2166 assumes a high impedance state; as such, the potentials at the input pin and the output pin are determined by the external circuitry connected to the ssm2166. the ssm2166 takes approximately 200 ms to settle from a power- down to power-on command. for power-on to power-down, the ssm2166 requires more time, typically less than 1 second. cycling the power supply to the ssm2166 can result in quicker settling times: the off-to-on settling time of the ssm2166 is less than 200 ms, while the on-to-off settling time is less than 1 ms. in either implementation, transients may appear at the output of the device. to avoid these output transients, use mute control of the vca gain as previously mentioned. pcb layout considerations because the ssm2166 is capable of wide bandwidth operation and can be configured for as much as 80 db of gain, special care must be exercised in the layout of the pcb that contains the ic and its associated components. the following recommendations should be considered and/or followed: ? in some high system gain applications, the shielding of input wires to minimize possible feedback from the output of the ssm2166 back to the input circuit may be necessary. ? a single-point (star) ground implementation is recom- mended in addition to maintaining short lead lengths and pcb runs. the evaluation board layout shown in figure 27, figure 28, and figure 29 demonstrates the single-point grounding scheme. in applications where an analog ground and a digital ground are available, the ssm2166 and its surrounding circuitry should be connected to the analog ground of the system. because of these recommendations, wire-wrap board connections and grounding implementations should be avoided. ? the internal buffer of the ssm2166 was designed to drive only the input of the internal vca and its own feedback network. stray capacitive loading to ground from the buf out pin in excess of 5 pf to 10 pf can cause excessive phase shift and can lead to circuit instability. ? when using high impedance sources (5 k), system gains in excess of 60 db are not recommended. this configuration is rarely appropriate because virtually all high impedance inputs provide larger amplitude signals that do not require as much amplification. when using high impedance sources, however, it can be advantageous to shunt the source with a capacitor to ground at the input pin of the ic (pin 7) to lower the source impedance at high frequencies, as shown in figure 25. a capacitor with a value of 1000 pf is a good starting value and sets a low-pass corner at 31 khz for 5 k sources. in applications where the source ground is not as clean as would be desirable, a capacitor (illustrated as c7 on the evaluation board) from the vca r input to the source ground may prove beneficial. this capacitor is used in addition to the grounded capacitor (illustrated as c2 on the evaluation board) used in the feedback around the buffer, assuming that the buffer is configured for gain. 00357-024 notes 1. additional circuit details omitted c x 1000pf a udio +in (r s > 5k ? ) audio +in ssm2166 c1 0.1f 7 for clarity. figure 25. circuit configuration for use with high impedance signal sources the value of c7 should be the same as c6, which is the capacitor value used between buf out and vca in . this connection makes the source ground noise appear as a common-mode signal to the vca, allowing the common-mode noise to be rejected by the vca differential input circuitry. c7 can also be useful in reducing ground loop problems and in reducing noise coupling from the power supply by balancing the impedances connected to the inputs of the internal vca.
ssm2166 rev. d | page 13 of 20 evaluation board a schematic diagram of the ssm2166 evaluation board is illustrated in figure 26. as a design aid, the layouts for the topside silkscreen and the topside and backside metallization layers are shown in figure 27, figure 28, and figure 29. although not shown to scale, the finished dimension of the evaluation board is 3.5 inches by 3.5 inches and comes complete with pin sockets and a sample of the ssm2166. signal sources are connected to the ssm2166 through a 1/8-inch phone jack where a 0.1 f capacitor couples the input signal to the audio +in pin (pin 7). as shown in figure 26 and in microphone applications, the phone jack shield can be optionally connected to the ground plane of the board (jumper j1 inserted into the board socket for pin 1 and pin 2) or to the vca r input at pin 4 (j1 inserted into the board socket for pin 1 and pin 3). if the signal source is a waveform or function generator, the phone jack shield should be connected to ground. for ease in making adjustments for all configuration parameters, single-turn potentiometers are used throughout. optional jumper j2 connects the comp ratio set pin to ground and sets the ssm2166 for no compression (that is, compression ratio = 1:1). optional jumper j3 connects the power down (pin 12) input to ground for normal operation. j3 can be replaced by an open-drain logic buffer for a digitally controlled shutdown function. an output signal mute function can be implemented on the ssm2166 by connecting the gain adjust pin (pin 2) through a 330 resistance to ground. this is provided on the evaluation board via r11 and s1. capacitor c5, connected between pin 2 and ground and provided on the evaluation board, can be used to avoid audible clicks when using the mute function. to configure the ssm2166 input buffer for gain, provisions for r1, r2, and c2 have been included. to configure the input buffer for unity-gain operation, r1 and r2 are removed and a direct connection is made between the ?in pin (pin 6) and the buf out pin (pin 5). the output stage of the ssm2166 is capable of driving >1 v rms (3 v p-p) into >5 k loads and is externally available through an rca phono jack provided on the board. if the output of the ssm2166 is required to drive a lower load resistance or an audio cable, the on-board op113 can be used. to use the op113 buffer, insert jumper j4 into the board socket for pin 4 and pin 5 and insert jumper j5 into the board socket for pin 6 and pin 7. if the output buffer is not required, remove jumper j5 and insert jumper j4 into board socket pin 5 and pin 7. there are no blocking capacitors either on the input or at the output of the buffer. as a result, the output dc level of the buffer matches the output dc level of the ssm2166, which is approximately 2.3 v. a dc blocking capacitor can be inserted at pin 6 and pin 7. an evaluation board and setup procedure is available from a analog devices, inc., sales representative. ssm2166 00357-025 comp ratio adj cw r6 100k? 5 4 6 7 output jack rca phono op113 r9 1k ? r11 330? r10 20k ? cw gain adj mute switch c5 0.01f c4 22f c7 10f c1 0.1f + + 3 1 2 input jack 1/8" phone mic pwr c2 1f r2 10k ? c6 10f c3 0.1f r1 10k ? + r3 50k? cw r7 1m ? r8 500 ? r4 500 ? ccw rotation pt adj + v r12 100k ? j3 noise gate adj 4 8 7 2 13 10 1 6 power down noise gate set v+ rotation set vca in buf out avg cap audio +in gain adjust comp ratio set 3 5 14 12 9 11 ?in vca r output gnd figure 26. evaluation board schematic
ssm2166 rev. d | page 14 of 20 00357-026 ic2 ic1 figure 27. evaluation board topside silkscreen (not to scale) 0 0357-027 figure 28. evaluation board topside metallization (not to scale) 00357-028 figure 29. evaluation board backside metallization (not to scale) evaluation board examples to illustrate how easy it is to program the ssm2166, a practical example is provided. the ssm2166 was used to interface an electret-type microphone to a postamplifier. the evaluation board or the circuit configuration shown in figure 26 can be used. the signal from the microphone was measured under actual conditions to vary from 1 mv to 15 mv. the postamplifier requires no more than 500 mv at its input. the required gain from the ssm2166 is, therefore g total = 20 log(500/15) = 30 db the input buffer gain is set to 20 db, and the vca gain is adjusted to 10 db. the limiting or rotation point is set at 500 mv output. a 2:1 compression ratio and a noise gate threshold that operates below 100 v is also used. these objectives are summarized in table 5. the transfer characteristic implemented is illustrated in figure 30. table 5. objective specifications parameter value input range 1 mv to 15 mv output range to 500 mv limiting level 500 mv compression 2:1 buffer gain 20 db vca gain 10 db noise gate 100 v
ssm2166 rev. d | page 15 of 20 note that the ssm2166 processes the output of the buffer, which in the previous example is 20 db or 10 times the input level. use the oscilloscope to verify that the buffer is not being driven into clipping with excessive input signals. in the application, take the minimum gain in the buffer consistent with the average source level as well as the crest factor (ratio of peak to rms). 00357-029 rotation point compression region gate threshold 1 2 limiting region 0.1 1.0 10 15 500 40 output (mv) input (mv) figure 30. transfer characteristic evaluation board setup procedure when building a breadboard, keep the leads to pin 3, pin 4, and pin 5 short. an evaluation board is available from an analog devices sales representative. the r and c designations refer to the demonstration board schematic of figure 26 and the parts list in table 7. test equipment setup the recommended equipment and configuration are shown in figure 31. a low noise audio generator with a smooth output adjustment range of 50 v to 50 mv is a suitable signal source. a 40 db pad is useful to reduce the level of most generators by 100 to simulate the microphone levels. the input voltmeter can be connected before the pad and need only go down to 10 mv. the output voltmeter should go up to 2 v. the oscilloscope is used to verify that the output is sinusoidal and that no clipping occurs in the buffer, and to set the limiting and noise gating knees. 00357-030 ac voltmeter ac voltmeter ssm2166 evaluation board signal generator oscilloscope figure 31. test equipment setup step 1: configure the buffer the ssm2166 has an input buffer that can be used when the overall gain required exceeds 20 db, the maximum user-selectable gain of the vca. in the example, the desired output is 500 mv for an input of ~15 mv, requiring a total gain of 30 db. set the buffer gain at 20 db and adjust the vca for 10 db. in the socket pins provided on the evaluation board, insert r1 = 100 k and r2 = 11 k. the buffer gain is set to 20 db (10). step 2: initializing potentiometers with the power off, preset the potentiometers per table 6. step 3: testing setup with the power on, adjust the generator for an input level of 15 mv, 1 khz. the output meter should indicate approximately 100 mv; if not, check the setup. step 4: adjusting the rotation point set the input level to 15 mv and observe the output on the oscilloscope. adjust r3, rotation pt adj, until the output level just begins to drop, then reverse so that the output is 500 mv. the limiting has been set to 500 mv. step 5: adjusting the vca gain set the input level to 15 mv. adjust r10, gain adj, clockwise (cw) for an output level of 500 mv. the vca gain has been set to 10 db. step 6: adjusting the compression ratio set the input signal for an output of 500 mv but not in limiting. note the value (around 15 mv). next, reduce the input to 1/10 of the value noted (around 1.5 mv) for a change of ?20 db. next, adjust r6, comp ratio adj, cw until the output is 160 mv for an output change of ?10 db. the compression, which is the ratio of the output change to the input change, in decibels (db), has been set to 2:1. step 7: setting the noise gate with the input set at 100 v, observe the output on the oscilloscope and adjust r7, noise gate adj, ccw until the output drops rapidly. rock the control back and forth to find the knee. the noise gate has been set to 100 v. the range of the noise gate is from 0.3 mv to over 0.5 mv relative to the output of the buffer. to fit this range to the application, it may be necessary to attenuate the input or apportion the buffer gain and vca gain differently. step 8: listening at this time, it may be desirable to connect an electret micro- phone to the ssm2166 and listen to the results. be sure to include the proper power for the internal fet of the microphone (usually 2 v dc to 5 v dc through a 2.2 k resistor). experiment with the settings to hear how the results change. varying the averaging capacitor, c4, changes the attack and decay times, which are best determined empirically. the compression ratio keeps the output steady over a range of microphone to speaker distances, and the noise gate keeps the background sounds subdued. step 9: recording values with the power removed from the test fixture, measure and record the values of all potentiometers, including any fixed resistance in series with them. if the averaging capacitor, c4, changes, also note its value.
ssm2166 rev. d | page 16 of 20 setup summary the transfer condition of figure 2 has been implemented. for inputs below the 100 v noise gate threshold, circuit and back- ground noise is minimized. above it, the output increases at a rate of 1 db for each 2 db input increase until the 500 mv rotation point is reached at an input of approximately 15 mv. for higher inputs that drive the output beyond 500 mv, limiting occurs and there is little further increase. the ssm2166 processes the output of the buffer, which in the previous example is 20 db, or 10 the input level. use the oscilloscope to ensure that the buffer is not being driven into clipping with the highest expected input peaks. always take the minimum gain in the buffer consis- tent with the average source level and crest factor (ratio of peak to rms). the wide program range of the ssm2166 makes it useful in many applications other than microphone signal conditioning. table 6. initial potentiometer settings function potentiometer range initial position initial resistance effect of change gain adjust (vca) r10 0 k to 20 k ccw 0 0 db; cw to increase vca gain rotation point r3 0 k to 50 k ccw 0 1 v; cw to reduce rotation point compression point r6 0 k to 100 k ccw 0 1:1; cw to increase compression noise gate r7 0 k to 1 m cw 1 m 300 v; ccw to increase threshold table 7. demonstration board parts list component value description r1 10 k resistor feedback r2 10 k resistor input r3 50 k potentiometer rotation point, adjust r4 500 resistor rotation point, fixed r5 0 resistor compression ratio, fixed r6 100 k potentiometer compression ratio, adjust r7 1 m potentiometer noise gate, adjust r8 500 resistor noise gate, fixed r9 1 k resistor gain adjust, fixed r10 20 k potentiometer gain adjust r11 330 resistor mute r12 100 k resistor power-down, pull-up c1 0.1 f capacitor input dc block c2 1 f capacitor buffer low f, g = 1 c3 0.1 f capacitor +v bypass c4 2.2 f to 22 f capacitor average capacitor c5 0.01 f capacitor mute click suppress c6 10 f capacitor coupling c7 10 f capacitor vca noise/dc balance ic1 ssm2166 mic preamp ic2 op113, ic operational amplifier, output buffer s1 spst, switch mute j1 1/8-inch mini phone plug jumper mic input j2 rca female jumper output jack
ssm2166 rev. d | page 17 of 20 outline dimensions controlling dimensions are in millimeters; inch dimensions (in parentheses) are rounded-off millimeter equivalents for reference only and are not appropriate for use in design. compliant to jedec standards ms-012-ab 060606-a 14 8 7 1 6.20 (0.2441) 5.80 (0.2283) 4.00 (0.1575) 3.80 (0.1496) 8.75 (0.3445) 8.55 (0.3366) 1.27 (0.0500) bsc seating plane 0.25 (0.0098) 0.10 (0.0039) 0.51 (0.0201) 0.31 (0.0122) 1.75 (0.0689) 1.35 (0.0531) 0.50 (0.0197) 0.25 (0.0098) 1.27 (0.0500) 0.40 (0.0157) 0.25 (0.0098) 0.17 (0.0067) coplanarity 0.10 8 0 45 figure 32. 14-lead standard small outline package [soic_n] narrow body (r-14) dimensions shown in millimeters and (inches) ordering guide model temperature range package description package option ssm2166s ?40c to +85c 14-lead soic_n r-14 ssm2166s-reel ?40c to +85c 14-lead soic_n r-14 SSM2166S-REEL7 ?40c to +85c 14-lead soic_n r-14 ssm2166sz 1 ?40c to +85c 14-lead soic_n r-14 ssm2166sz-reel 1 ?40c to +85c 14-lead soic_n r-14 ssm2166sz-reel7 1 ?40c to +85c 14-lead soic_n r-14 1 z = rohs compliant part. top branding revision reflecting die replacement version original die revision (prior to rev. c of data sheet) new die revision (rev. c to current revision of data sheet) pb-free (rohs) version top line 1: ssm top line 1: ssm top line 2: 2166 top line 2: 2166a 1 top line 3: # xxxx 2 top line 3: # xxxx 2 snpb lead finish version top line 1: ssm top line 1: ssm top line 2: 2166 top line 2: 2166a 1 top line 3: xxxx top line 3: xxxx 1 letter a designates new die revi sion; refer to revised external component values in figure 5, figure 6, figure 9, and figure 1 9. 2 # designates rohs version.
ssm2166 rev. d | page 18 of 20 notes
ssm2166 rev. d | page 19 of 20 notes
ssm2166 rev. d | page 20 of 20 notes ?1996C2008 analog devices, inc. all rights reserved. trademarks and registered trademarks are the prop erty of their respective owners. d00357-0-7/08(d)

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