SSM2166 datasheet(10/20 Pages) AD

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SSM2166

Data Sheet

Rev. E | Page 10 of 20

The performance of the rms level detector is illustrated for a

CAVG of 2.2 μF in Figure 17 and for a CAVG of 22 μF in Figure 18.

In each of these images, the input signal to the SSM2166 (not

shown) is a series of tone bursts in six successive 10 dB steps.

The tone bursts range from −66 dBV (0.5 mV rms) to −6 dBV

(0.5 V rms). As shown in Figure 17 and Figure 18, the attack

time of the rms level detector is dependent only on CAVG, but the

release times are linear ramps whose decay times are dependent

on both CAVG and the input signal step size. The rate of release is

approximately 240 dB/s for a CAVG of 2.2 μF and 12 dB/s for a

CAVG of 22 μF.

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100mV

100ms

–6dBV

–66dBV

–85dBV

100

Figure 17. RMS Level Detector Performance with CAVG = 2.2 μF

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100

100mV

–6dBV

–66dBV

–85dBV

Figure 18. 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 gain control of the VCA is logarithmic—a linear change

in the control signal causes a decibel 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 15.

Compression Ratio

Changing the scaling of the control signal fed to the VCA causes a

change in the circuit compression ratio, r. This effect is shown

in Figure 20. The compression ratio can be set by connecting a

resistor between the COMP RATIO SET pin (Pin 10) and GND.

Lowering RCOMP gives smaller compression ratios as shown in

Figure 19, with values of approximately 0.17 kΩ 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 kΩ potentiometer can be used to

allow this parameter to be adjusted. On the evaluation board

(see Figure 26), an optional resistor can be used to set the

compression equal to 1:1 when the wiper of the potentiometer

is at its full counterclockwise (CCW) position.

ROTATION POINT

100mV rms

300mV rms

1V rms

1:1

0.1

2:1

8.7

5:1

19.4

10:1

15:1

395

N/A

COMPRESSION

RATIO

TYPICAL RCOMP VALUES IN kΩ.

Figure 19. Compression Ratio vs. RCOMP (Pin 10 to GND)

INPUT (dB)

VRP

15:1

5:1

2:1

1:1

VCA GAIN

VDE

Figure 20. 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 Figure 9.

The effect on rotation point is shown in Figure 21. By varying a

resistor, RROT PT, connected between the positive supply and the

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

by approximately 20 mV rms to 1 V rms. From Figure 21, the

rotation point is inversely proportional to RROTPT. For example, a

1 kΩ resistor would typically set the rotation point at 1 V rms,

whereas a 55 kΩ resistor would typically set the rotation point

at approximately 30 mV rms.

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