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SSM2166 Datasheet(PDF) 8 Page - Analog Devices |
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SSM2166 Datasheet(HTML) 8 Page - Analog Devices |
8 / 16 page REV. B –8– SSM2166 10 0% 100 90 100mV 1S –6dBV –66dBV –85dBV Figure 5b. RMS Level Detector Performance with CAVG = 22 µF 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 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 3. 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 6. The compression ratio can be set by con- necting a resistor between the COMP RATIO pin (Pin 10) and GND. Lowering RCOMP gives smaller compression ratios as indicated in TPC 3, with values of about 17 k Ω 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 Ω potentiometer may be used to allow this parameter to be adjusted. On the evaluation board (Figure 12), an optional resistor can be used to set the compres- sion equal to 1:1 when the wiper of the potentiometer is at its full CCW position. INPUT – dB VDE VRP 15:1 5:1 2:1 1:1 1 1 VCA GAIN Figure 6. 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 TPC 7. The effect on rotation point is shown in Figure 7. By varying a resistor, RROT PT, connected between the positive supply and the ROTATION POINT SET pin (Pin 11), the rotation point may be varied from approximately 20 mV rms to 1 V rms. From the figure, the rotation point is inversely proportional to RROT PT. For example, a 1 k Ω resistor would typically set the rotation point at 1 V rms, whereas a 55 k Ω resistor would typically set the rotation point at approximately 30 mV rms. Since limiting occurs for signals larger than the rotation point (VIN > VRP), 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 will cover the entire desired input signal range. Occasional larger signal transients will then be attenuated by the action of the limiter. VDE VRP1 INPUT – dB VRP2 VRP3 VCA GAIN 1 1 r:1 Figure 7. 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 RGAIN. This resistor, with a range between 1 k Ω and 20 k Ω, will cause 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 8 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 Ω resistor. |
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