Datasheet SSM2166 (Analog Devices) - 9

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Data Sheet SSM2166
The gain of the VCA at the rotation point is set by the value of a
LEVEL DETECTOR
resistor, RGAIN, connected between Pin 2 and GND. The relationship The SSM2166 incorporates a full-wave rectifier and true rms between the VCA gain and RGAIN is shown in Figure 6. The AGC level detector circuit whose averaging time constant is set by range can be as high as 60 dB. The VCAIN pin (Pin 3) is the non- an external capacitor connected to the AVG CAP pin (Pin 8). inverting input terminal to the VCA. The inverting input of the For optimal low frequency operation of the level detector VCA is available at the VCAR pin (Pin 4) and exhibits an input down to 10 Hz, the value of the capacitor must be 2.2 μF. Some impedance of 1 kΩ, as well. As a result, this pin can be used for experimentation with larger values for the AVG CAP may be differential inputs or for the elimination of grounding problems necessary to reduce the effects of excessive low frequency ambient by connecting a capacitor whose value equals that used in series background noise. The value of the averaging capacitor affects with the VCAIN pin to ground. sound quality: too small a value for this capacitor may cause a The output impedance of the SSM2166 is typically less than 75 Ω, pumping effect for some signals, while too large a value may and the external load on Pin 13 must be >5 kΩ. The nominal result in slow response times to signal dynamics. Electrolytic output dc voltage of the device is approximately 2.2 V. Use a capacitors are recommended for lowest cost and must be in the blocking capacitor for grounded loads. range of 2 μF to 47 μF. Capacitor values from 18 μF to 22 μF have The bandwidth of the SSM2166 is quite wide at all gain settings. been found to be more appropriate in voice-band applications The upper 3 dB point is approximately 30 kHz at gains as high as where capacitors on the low end of the range seem more 60 dB (using the input buffer for additional gain, circuit bandwidth appropriate for music program material. is unaffected). The gain bandwidth (GBW) plots are shown in The rms detector filter time constant is approximately given by Figure 11. The lower 3 dB cutoff frequency of the SSM2166 is 10 × CAVG milliseconds, where CAVG is in μF. This time constant set by the input impedance of the VCA (1 kΩ) and C6. While the controls both the steady-state averaging in the rms detector as noise of the input buffer is fixed, the input referred noise of the well as the release time for compression; that is, the time it takes VCA is a function of gain. The VCA input noise is designed to for the system gain to react when a large input is followed by be a minimum when the gain is at a maximum, thereby optimizing a small signal. The attack time, the time it takes for the gain to the usable dynamic range of the device. A plot of wideband be reduced when a small signal is followed by a large signal, is peak-to-peak output noise is shown in Figure 10. 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.
C6 C7 10µF 10µF (OPTIONAL) + + V+ BUF OUT VCAIN VCAR R1 14 5 3 4 10kΩ BUFFER 1kΩ 1kΩ –IN 6 OUTPUT VCA 13 VOUT AUDIO +IN 7 GAIN ADJUST C1 V+ 2 R2 0.1µF 10kΩ SSM2166 RGAIN + RGATE 1µF RMS CONTROL NOISE GATE SET LEVEL 9 CIRCUIT DETECTOR RROT PT ROTATION SET 11 POWER DOWN 12 1 8 10 GND AVG CAP COMP RATIO SET + CAVG R
6
COMP 2.2µF
01 7- 35 00 Figure 16. Functional Block Diagram and Typical Application Rev. F | Page 9 of 13 Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION FUNCTIONAL BLOCK DIAGRAM AND TYPICAL SPEECH APPLICATION TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS THERMAL RESISTANCE ESD CAUTION PIN CONFIGURATION AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION APPLICATIONS INFORMATION SIGNAL PATH LEVEL DETECTOR CONTROL CIRCUITRY Compression Ratio Rotation Point VCA Gain Setting and Muting Downward Expansion Threshold POWER-DOWN FEATURE PCB LAYOUT CONSIDERATIONS OUTLINE DIMENSIONS ORDERING GUIDE