Data Sheet

SSM2166

Rev. E | Page 11 of 20

Because limiting occurs for signals larger than the rotation point

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

INPUT (dB)

r:1

1

1

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

20 kΩ, 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 kΩ resistor.

VCA GAIN

INPUT (dB)

r:1

1

1

Figure 22. Effect of Varying the VCA Gain Setting

The gain of the VCA can be reduced below 0 dB by making

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 µF

capacitor is a good starting value.

C5

330Ω

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

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 µV 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 MΩ sets the threshold at approximately

250 µV rms, whereas a 10 kΩ 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.

INPUT (dB)

1

1

r:1

VCA GAIN

Figure 24. Effect of Varying the Downward Expansion (Noise Gate) Threshold