SSM2166

REV. A

–8–

10

0%

100

90

100mV

1S

6dBV

66dBV

85dBV

Figure 15b. RMS Level Detector Performance with

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 13.

Compression Ratio. Changing the scaling of the control sig-

nal fed to the VCA causes a change in the circuit’s compression

ratio, “r.” This effect is shown in Figure 16. The compression

ratio can be set by connecting a resistor between the COMP

compression ratios as indicated in Figure 5, with values of about

17 k

Ω or less resulting in a compression ratio of 1:1. AGC per-

formance is achieved with compression ratios between 2:1 and

15:1, and is dependent on the application. A 100 k

Ω potentiom-

eter may be used to allow this parameter to be adjusted. On the

evaluation board (Figure 22), an optional resistor can be used to

set the compression equal to 1:1 when the wiper of the potenti-

ometer is at its full CCW position.

INPUT – dB

15:1

5:1

2:1

1:1

1

1

VCA GAIN

Figure 16. 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 il-

lustrated in Figure 9. The effect on rotation point is shown in

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 propor-

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

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 in-

put signal range. Occasional larger signal transients will then be

attenuated by the action of the limiter.

INPUT – dB

VCA GAIN

1

1

r:1

Figure 17. Effect of Varying the Rotation Point

VCA Gain Setting and Muting. The maximum gain of the

This resistor, with a range between 1 k

Ω and 20 kΩ, will cause

the nominal VCA gain to vary from 0 dB to approximately

20 dB, respectively. To set the VCA gain to its maximum can

also be achieved by leaving the GAIN ADJUST pin in an

OPEN condition (no connect). Figure 18 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.

INPUT – dB

VCA GAIN

1

1

r:1

Figure 18. Effect of Varying the VCA Gain Setting