REV. B

–8–

SSM2166

10

0%

100

90

100mV

1S

–6dBV

–66dBV

–85dBV

Figure 5b. RMS Level Detector Performance

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

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

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

ROTATION POINT SET pin (Pin 11), the rotation point may

be varied from approximately 20 mV rms to 1 V rms. From the

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

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.

INPUT – dB

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

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.