REV. A

SSM2167

–6–

APPLICATIONS INFORMATION

The SSM2167 is a complete microphone signal conditioning

system on a single integrated circuit. Designed primarily for

voice band applications, this integrated circuit provides amplifi-

cation, limiting, variable compression, and noise gate. User

adjustable compression ratio, noise gate threshold, and two

different fixed gains optimize circuit operation for a variety of

applications. The SSM2167 also features a low power shutdown

mode for battery-powered applications.

10 F

GND

SHUTDOWN

OUTPUT

0.1 F

INPUT

10 F

+

10 F

+

100k

GND

10 F

+

500k

SSM2167

Figure 2. Typical Application Circuit

LEVEL

DETECTOR

CONTROL

C3

10 F

+

NOISE GATE AND

COMPRESSION

SETTINGS

GND

SHUTDOWN

VCA

+1

INPUT

C1

0.1 F

BUFFER

C2

10 F

OUTPUT

+

1k

1k

Figure 3. Functional Block Diagram

Theory of Operation

The typical transfer characteristic for the SSM2167 is shown in

Figure 1 where the output level in dB is plotted as a function of

the input level in dB. The dotted line indicates the transfer

characteristic for a unity-gain amplifier. For input signals in the

an “r” dB change in the input level causes a 1 dB change in the

output level. Here, “r” is defined as the “compression ratio.”

The compression ratio may be varied from 1:1 (no compression)

compressed with a fixed compression ratio of approximately

10:1. This region of operation is the “limiting region.” Varying

the compression ratio has no effect on the limiting region. The

breakpoint between the compression region and the limiting

region is referred to as the “limiting threshold” or the “rotation

point.” The term “rotation point” derives from the observation

that the straight line in the compression region “rotates” about

this point on the input/output characteristic as the compression

ratio is changed.

“fixed gain,” 18 dBV for the SSM2167-1 and 8 dBV for the

SSM2167-2, regardless of the compression ratio.

change in the input signal level causes approximately a –3 dB

change in the output level. As a result, the gain of the system is

small for very small input signal levels, even though it may be

Finally, the SSM2167 provides an active low, CMOS-compatible

digital power-down feature that will reduce device supply current

to typically less than 2

A.

SSM2167 Signal Path

Figure 3 illustrates the block diagram of the SSM2167. The audio

input signal is processed by the input buffer and then by the VCA.

The input buffer presents an input impedance of approximately

100 k

Ω to the source. A dc voltage of approximately 1.5 V is

present at INPUT (Pin 5 of the SSM2167), 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 input

buffer is a unity-gain stable amplifier that can drive the low imped-

ance input of the VCA and an internal rms detector.

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

is set by the side-chain control circuitry. An external blocking

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

the VCA input. The 1 k

Ω impedance between amplifiers determines

the value of this capacitor, which is typically between 4.7

µF and

10

µF. An aluminum electrolytic capacitor is an economical choice.

The VCA amplifies the input signal current flowing through C2

and converts this current to a voltage at the SSM2167’s output pin

(Pin 9). The net gain from input to output can be as high as 40 dB

for the SSM2167-1 and 30 dB for the SSM2167-2, depending on

the gain set by the control circuitry.

The output impedance of the SSM2167 is typically less than 145

Ω,

and the external load on Pin 9 should be > 5 k

Ω. The nominal

output dc voltage of the device is approximately 1.4 V, so a blocking

capacitor for grounded loads must be used.

TIME – 500ms/DIV

0

0

00

0

00

00

00

0

0

0

0

0

0

0

85dBV

66dBV

6dBV

TPC 13. SSM2167-1 RMS Level Detector Performance

µF