REV. A

SSM2165

–6–

operation of the level detector down to 10 Hz, the value of the

capacitor should be around 22

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 re-

sponse times to signal dynamics. Electrolytic capacitors are

recommended here for lowest cost.

The rms detector filter time constant is approximately given by

10

× C

stant controls both the steady-state averaging in the rms detec-

tor 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 fol-

lowed 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 mainly controlled by internal circuitry that speeds up

the attack for large level changes, and controlled partly by the

AVG CAP value. This limits overload time to under 1 ms in

most cases.

The performance of the rms level detector is illustrated in Fig-

each of these photographs, the input signal to the SSM2165

(not shown) is a series of tone bursts in 6 successive 10 dB

steps. The tone bursts range from –66 dBu (0.5 mV rms) to

–6 dBu (0.5 V rms). As illustrated in the photographs, the

but the release times are linear ramps whose decay times are

10

0%

100

90

100mV

100ms

6dBV

66dBV

85dBV

Figure 12. RMS Level Detector Performance with

10

0%

100

90

100mV

1s

6dBV

66dBV

85dBV

Figure 13. RMS Level Detector Performance with

The SSM2165 Signal Path

Figure 11 illustrates the block diagram of the SSM2165. The

audio input signal is processed by the unity gain input buffer

and then by the VCA. The buffer presents an input impedance

of approximately 180 k

Ω to the source. A dc voltage of approxi-

mately 1.5 V is present at AUDIO +IN (Pin 4), requiring the

use of a blocking capacitor (C1) for ground-referenced sources.

A 0.1

µF capacitor is a good choice for most audio applications.

The buffer is designed to drive only the low impedance input of

the VCA, and must not be loaded by capacitance to ground.

The VCA is a low distortion, variable-gain amplifier whose gain

is set by the internal control circuitry. The input to the VCA is

a virtual ground in series with 500

Ω. An external blocking

capacitor (C2) must be used between the buffer’s output and

the VCA input. The desired low frequency response and the

total of 1 k

Ω impedance between amplifiers determines the

value of this capacitor. For music applications, 10

µF will give

1

µF will give f

is an economical choice. The VCA amplifies the input signal

current flowing through C6 and converts this current to a

voltage at the SSM2165’s output (Pin 7). The net gain from

input to output can be as high as 40 dB for high compression

ratios and depending on the gain set by the control circuitry.

The output impedance of the SSM2165 is typically less than

75

Ω, and the external load on Pin 7 should be >5 kΩ. The

nominal output dc voltage of the device is approximately 2.2 V.

Use a dc blocking capacitor for grounded loads.

VCA

+1

V+

V+

+

C2

10 F

BUFFER

LEVEL

DETECTOR

CONTROL

SSM2165

C1

0.1 F

AUDIO

IN+

R1

25k

C3

22 F

AVG CAP

COMPRESSION

RATIO SET

GND

+

500

500

Figure 11. Functional Block Diagram and Typical Voice

Application

The bandwidth of the SSM2165 is quite wide at all gain set-

tings. The upper –3 dB point is approximately 300 kHz. The

GBW plots are shown in Figure 6. 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 minimum

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

dynamic range of the part. A photograph of the SSM2165’s

wideband peak-to-peak output noise is illustrated in Figure 5.

The Level Detector

The SSM2165 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 5). Capacitor values from 18

µF to 22 µF have

been found to be more appropriate in voiceband applications,

where capacitors on the low end of the range seem more appro-

priate for music program material. For optimal low frequency