REV. B

SSM2166

–7–

bandwidth is unaffected). The GBW plots are shown in TPC 8b.

The lower 3 dB cutoff frequency of the SSM2166 is set by the

input impedance of the VCA (1 k

Ω) and C6. While the noise of

the input buffer is fixed, the input referred noise of the VCA is a

function of gain. The VCA input noise is designed to be a mini-

mum when the gain is at a maximum, thereby optimizing the usable

dynamic range of the part. An image of the SSM2166’s wideband

peak-to-peak output noise is illustrated in TPC 8a.

Level Detector

The SSM2166 incorporates a full-wave rectifier and a patent-

pending, true rms level detector circuit whose averaging time

constant is set by an external capacitor connected to the AVG

CAP pin (Pin 8). For optimal low frequency operation of the

level detector down to 10 Hz, the value of the capacitor should

be 2.2

µF. Some experimentation with larger values for the

AVG CAP may be necessary to reduce the effects of excessive

low frequency ambient background noise. The value of the

averaging capacitor affects sound quality: too small a value for

this capacitor may cause a “pumping effect” for some signals,

while too large a value can result in slow response times to

signal dynamics. Electrolytic capacitors are recommended here

for lowest cost and should be in the range of 2

µF to 47 µF.

Capacitor values from 18

µF to 22 µF have been found to be

more appropriate in voice-band applications, where capacitors

on the low end of the range seem more appropriate for music

program material.

The rms detector filter time constant is approximately given by

10

µF. This time constant

controls both the steady-state averaging in the rms detector as

well as the release time for compression; that is, the time it takes

for the system gain to react when a large input is followed by a

small signal. The attack time, the time it takes for the gain to be

reduced when a small signal is followed by a large signal, is

controlled partly by the AVG CAP value, but is mainly controlled

by internal circuitry that speeds up the attack for large level

changes. This limits overload time to under 1 ms in most cases.

The performance of the rms level detector is illustrated in Figure 5

µF (Figure 5a) and 22 µF (Figure 5b). 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 illustrated in the images, the attack time of the rms level

and the input signal step size. The rate of release is approximately

µF, and 12 dB/s for a C

µF.

10

0%

100

90

100mV

100ms

–6dBV

–66dBV

–85dBV

Figure 5a. RMS Level Detector Performance with

µF

AUDIO

+IN

BUFOUT

53

2

9

NOISE

GATE

11

ROTATION

POINT

ADJUST

10

1

GND

13

7

RMS

LEVEL

DETECTOR

INPUT

BUFFER

VCA

SSM2166

R2 = 10k

1 F

CONTROL

CIRCUITRY

12

POWER

DOWN

POWER

DOWN

COMPRESSION

RATIO SET

8

2.2 F

AVG

CAP

GAIN

ADJUST

OUTPUT

1k

4

1k

C6

10 F

V+

V+

GND

C7

*

10 F

0.1 F

R1 = 10k

+

14

6

–IN

Figure 4. Functional Block Diagram and Typical Application