SSM2166*

Microphone Preamplifier with

Variable Compression and Noise Gating

Information furnished by Analog Devices is believed to be accurate and
reliable. However, no responsibility is assumed by Analog Devices for its
use, nor for any infringements of patents or other rights of third parties
which may result from its use. No license is granted by implication or
otherwise under any patent or patent rights of Analog Devices.

FEATURES
Complete Microphone Conditioner in a 14-Lead Package
Single +5 V Operation
Adjustable Noise Gate Threshold
Compression Ratio Set by External Resistor
Automatic Limiting Feature--Prevents ADC Overload
Adjustable Release Time
Low Noise and Distortion
Power-Down Feature
20 kHz Bandwidth ( 1 dB)
Low Cost

APPLICATIONS
Microphone Preamplifier/Processor
Computer Sound Cards
Public Address/Paging Systems
Communication Headsets
Telephone Conferencing
Guitar Sustain Effects Generator
Computerized Voice Recognition
Surveillance Systems
Karaoke and DJ Mixers

20 dB; this gain is in addition to the variable gain in other com-
pression settings. The input buffer can also be configured for front-
end gains of 0 dB to 20 dB. A downward expander (noise gate)
prevents amplification of noise or hum. This results in opti-
mized signal levels prior to digitization, thereby eliminating the
need for additional gain or attenuation in the digital domain
that could add noise or impair accuracy of speech recognition
algorithms. The compression ratio and time constants are set
externally. A high degree of flexibility is provided by the VCA
Gain, Rotation Point, and Noise Gate adjustment pins.

The SSM2166 is an ideal companion product for audio codecs
used in computer systems, such as the AD1845 and AD1847.
The device is available in 14-lead SOIC and P-DIP packages,
and guaranteed for operation over the extended industrial tempera-
ture range of 40

C to +85

C. For similar features/performance

in an 8-lead package, please refer to the SSM2165.

INPUT dBu

70

60

50

40

30

20

10

OUTPUT dBu

30

40

50

60

10

20

RATIO = 10:1

RATIO = 2:1

RATIO = 1:1

Figure 1. SSM2166 Compression and Gating Characteris-
tics with 10 dB of Fixed Gain (The Gain Adjust Pin Can Be
Used to Vary This Fixed Gain Amount)

REV. A

*Patents pending.

Figure 2. Functional Block Diagram and Typical Speech Application

GENERAL DESCRIPTION

The SSM2166 integrates a complete and flexible solution for
conditioning microphone inputs in computer audio systems. It
is also excellent for improving vocal clarity in communications
and public address systems. A low noise voltage controlled
amplifier (VCA) provides a gain that is dynamically adjusted by
a control loop to maintain a set compression characteristic. The
compression ratio is set by a single resistor and can be varied
from 1:1 to over 15:1 relative to a user defined "rotation
point;" signals above the rotation point are limited to prevent
overload and eliminate "popping." In the 1:1 compression set-
ting the SSM2166 can be programmed with a fixed gain of up to

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781/329-4700

World Wide Web Site: http://www.analog.com

Fax: 781/326-8703

© Analog Devices, Inc., 1999

CONTROL

AUDIO

+IN

0.1 F

BUF

OUT

10 F

OUT

NOISE GATE

SET

V+

ROTATION
POINT SET

GND

22 F

COMPRESSION
RATIO
SET

25k

2.3k

500k

17k

VCA

LEVEL

DETECTOR

BUFFER

VCA

SSM2166

10 F

R1 = 10k

R2 = 10k

1 F

POWER

DOWN

AVG
CAP

OPTIONAL

V+

VCA GAIN

ADJ

Parameter

Symbol

Conditions

Min

Typ

Max

Units

AUDIO SIGNAL PATH

Voltage Noise Density

15:1 Compression

nV/

Noise

20 kHz Bandwidth, V

= GND

109

dBu

Total Harmonic Distortion

THD+N

2nd and 3rd Harmonics, V

= 20 dBu

0.25

0.5

22 kHz Low-Pass Filter

Input Impedance

180

Output Impedance

OUT

Load Drive

Resistive

Capacitive

Buffer

Input Voltage Range

1% THD

V rms

Output Voltage Range

1% THD

V rms

VCA

Input Voltage Range

1% THD

V rms

Output Voltage Range

1% THD

1.4

V rms

Gain Bandwidth Product

1:1 Compression, VCA G = 60 dB

MHz

CONTROL SECTION

VCA Dynamic Gain Range

VCA Fixed Gain Range

60 to +19

Compression Ratio, Min

1:1

Compression Ratio, Max

See Figure 5 for R

COMP

ROT

15:1

Control Feedthrough

15:1 Compression, Rotation Point = 10 dBu

POWER SUPPLY

Supply Voltage Range

4.5

5.5

Supply Current

7.5

Quiescent Output Voltage Level

2.2

Power Supply Rejection Ratio

PSRR

POWER DOWN

Supply Current

Pin 12 = V+

100

NOTES

0 dBu = 0.775 V rms.

Normal operation: Pin 12 = 0 V.

Specifications subject to change without notice.

REV. A

CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily
accumulate on the human body and test equipment and can discharge without detection.
Although the SSM2166 features proprietary ESD protection circuitry, permanent damage may
occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD
precautions are recommended to avoid performance degradation or loss of functionality.

SSM2166SPECIFICATIONS

ORDERING GUIDE

Temperature

Package

Model

Range

Description

Option

SSM2166P

C to +85

Plastic DIP

N-14

SSM2166S

C to +85

Narrow SOIC

SO-14

ABSOLUTE MAXIMUM RATINGS

Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +10 V
Audio Input Voltage . . . . . . . . . . . . . . . . . . . . . Supply Voltage
Operating Temperature Range . . . . . . . . . . . . 40

C to +85

Storage Temperature Range . . . . . . . . . . . . . 65

C to +150

Junction Temperature (T

) . . . . . . . . . . . . . . . . . . . . . +150

Lead Temperature (Soldering, 60 sec) . . . . . . . . . . . . +300

ESD RATINGS

883 (Human Body) Model . . . . . . . . . . . . . . . . . . . . . . 2.0 kV

THERMAL CHARACTERISTICS

Thermal Resistance

14-Lead Plastic DIP

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

C/W

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

C/W

14-Lead SOIC

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120

C/W

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

C/W

(V+ = +5 V, f = 1 kHz, R

= 100 k

, R

GATE

= 600 k

, R

ROTATION

= 3 k

, R

COMP

= 0

R1 = 0

, R2

, T

= +25 C, unless otherwise noted, V

= 300 mV rms.)

WARNING!

ESD SENSITIVE DEVICE

SSM2166

REV. A

PIN DESCRIPTION

Pin #

Mnemonic

Function

GND

Ground

GAIN ADJUST

VCA Gain Adjust Pin. A resistor from this pin to ground sets the fixed gain of the VCA. To
check the setting of this pin the compression pin (Pin 10) should be grounded for no com-
pression. The gain can be varied from 0 dB to 20 dB. For 20 dB leave the pin open. For
0 dB of fixed gain, a typical resistor value is approximately 1 k

. For 10 dB of fixed gain, the

resistor value is approximately 2 k

3 k

. For resistor values < 1 k

, the VCA can attenu-

ate or mute. Refer to Figure 6.

VCA

VCA Input Pin. A typical connection is a 10

F capacitor from the buffer output pin (Pin 5)

to this pin.

VCA

Inverting Input to the VCA. This input can be used as a nonground reference for the audio
input signal (see application notes).

BUF OUT

Input Buffer Amplifier Output Pin. Must not be loaded by capacitance to ground.

Inverting Input to the Buffer. A 10 k

feedback resistor R1 from the buffer output Pin 5 to

this input pin, and a resistor R2, from this pin through a 1

F to ground gives gains of 6 dB

to 20 dB for R2 = 10 k

to 1.1 k

AUDIO +IN

Input Audio Signal. The input signal should be ac-coupled (0.1

F typical) into this pin.

AVG CAP

Detector Averaging Capacitor. A capacitor, 2.2

F22

F, to ground from this pin is the

averaging capacitor for the detector circuit.

NOISE GATE SET

Noise Gate Threshold Set Point. A resistor to V+ sets the level below which input signals are
downward-expanded. For a 0.7 mV threshold, the resistor value is approximately 380 k

Increasing the resistor value reduces the threshold. See Figure 4.

COMP RATIO SET

Compression Ratio Set Pin. A resistor to ground from this pin sets the compression ratio as
shown in Figure 1. Figure 5 gives resistor values for various rotation points.

ROTATION SET

Rotation Point Set Pin. This is set by a resistor to the positive supply. This resistor together
with the gain adjust pin determines the onset of limiting. A typical value for this resistor is
17K for a 100 mV "rotation point." Increasing the resistor value reduces the level at which
limiting occurs. Refer to Figure 9.

POWER DOWN

Power-Down Pin. Connect to ground for normal operation. Connect to positive supply for
power-down mode.

OUTPUT

Output Signal.

V+

Positive Supply, +5 V Nominal.

PIN CONFIGURATION

TOP VIEW

(Not to Scale)

GND

ROTATION SET

POWER DOWN

OUTPUT

V+

GAIN ADJUST

VCA

SSM2166

AVG CAP

NOISE GATE SET

COMP RATIO SET

BUF OUT

IN

AUDIO +IN

SSM2166

REV. A

80

70

60

50

40

30

20

10

40

50

60

70

20

30

= +25

V+ = 5V
V

= 300mV rms @ 1kHz

= 100k

NOISE GATE SETTING 550µV rms
ROTATION POINT 300mV rms
GAIN ADJUST (PIN 2) = 1.25k

COMP RATIO = 1:1

COMP RATIO = 2:1

COMP RATIO = 5:1

COMP RATIO = 10:1

COMP RATIO = 15:1

INPUT dBu

OUTPUT dBu

INPUT dBu

Figure 3. Output vs. Input Characteristics

GATE

 k

NOISE GATE mV rms

0.1

450

50 100

350 400

150 200 250 300

500 550 600 650

100

= +25

V+ = 5V
R

= 100k

COMPRESSION RATIO = 2:1
ROTATION POINT 1V rms
GAIN ADJUST (PIN 2) = 1.25k

Figure 4. Noise Gate vs. R

GATE

(Pin 9 to V+)

COMPRESSION

RATIO

ROTATION POINT

1:1

2:1

5:1

10:1

15:1

100mV rms

300mV rms

1V rms

12.5

215

395

COMP

 k , TYPICAL

Figure 5. Compression Ratio vs. R

COMP

(Pin 10 to GND)

GAIN dB

GAIN ADJUST RESISTOR k

10 12 14

16 18

20 22 24

= +25

V+ = 5V
R

= 100k

= 100mV rms @ 1kHz

NOISE GATE SETTING 550 V rms
ROTATION POINT (PIN 11) 1V rms
COMPRESSION RATIO = 1:1

Figure 6. VCA Gain vs. R

GAIN

(Pin 2 to GND)

INPUT VOLTAGE V rms

THD + N %

0.1

0.01

0.1

0.05

= 10k

= 100k

= +25

V+ = 5V
COMPRESSION RATIO = 1:1
NOISE GATE SETTING 550 V rms
ROTATION POINT 1V rms
GAIN ADJUST (PIN 2) = 1.25k
V

FREQUENCY = 1kHz

Figure 7. THD+N (%) vs. Input (V rms)

FREQUENCY Hz

THD + N %

0.1

30k

100

0.05

10k

= +25

V+ = 5V
V

= 77.5mV rms @ 1kHz

COMPRESSION RATIO = 1:1
NOISE GATE SETTING 550 V rms
ROTATION POINT 1V rms
GAIN ADJUST (PIN 2) = 1.2k
MEASUREMENT FILTER BW : 22Hz TO 30kHz

Figure 8. THD+N (%) vs. Frequency (Hz)

SSM2166

REV. A

ROTATION POINT V rms

ROT PT

RESISTOR k

1.0

0.1

0.01

5 10

35 40

15 20 25 30

50 55

= +25°C

V+ = 5V

= 100k

COMPRESSION RATIO = 1:1

NOISE GATE SETTING 550 V rms
GAIN ADJUST (PIN 2) = 1.25k

60 65 70 75 80 85 90

Figure 9. Rotation Point vs. R

ROT PT

(Pin 11 to V+)

100

5 V

= +25

COMPRESSION RATIO = 15:1
NOISE BW = 20kHz

Figure 10a. Wideband Output Noise

FREQUENCY Hz

GAIN dB

20

10k

100k

10

ROTATION POINT = 1.13V rms
NOISE GATE SETTING = 336 V rms
R

COMP

= 40k

= 400 V rms

G = 60dB

G = 40dB

G = 20dB

Figure 10b. GBW Curves vs. VCA Gain

FREQUENCY Hz

10

80

30k

100

20

30

40

50

60

70

COMP

= 0

GAIN

= 1.24k

GATE

= 500k

ROT

= 1.74k

10k

V+ = 5±1V p-p

V+ = 5±0.5V p-p

PSRR dB

Figure 10c. PSRR vs. Frequency

100

20mV

10 s

= +25

AVG

= 2.2 F

SYSTEM GAIN = 0dB

= 10k

COMP RATIO = 1:1

Figure 11. Small Signal Transient Response

100

10 s

200mV

= +25

AVG

= 2.2 F

SYSTEM GAIN = 0dB
R

= 10k

COMP RATIO = 1:1

Figure 12. Large Signal Transient Response

SSM2166

REV. A

APPLICATIONS INFORMATION

The SSM2166 is a complete microphone signal conditioning
system on a single integrated circuit. Designed primarily for
voiceband applications, this integrated circuit provides amplifi-
cation, rms detection, limiting, variable compression, and down-
ward expansion. An integral voltage-controlled amplifier (VCA)
provides up to 60 dB of gain in the signal path with approxi-
mately 30 kHz bandwidth. Additional gain is provided by an
input buffer op amp circuit that can be set anywhere from 0 dB
to 20 dB, for a total signal path gain of up to 80 dB. The device
operates on a single +5 V supply, accepts input signals up to
1 V rms, and produces output signal levels > 1 V rms (3 V p-p)
into loads > 5 k

. The internal rms detector has a time con-

stant set by an external capacitor.

The SSM2166 contains an input buffer and automatic gain con-
trol (AGC) circuit for audio- and voiceband signals. Circuit
operation is optimized by providing a user-adjustable time con-
stant and compression ratio. A downward expansion (noise gat-
ing) feature eliminates circuit noise in the absence of an input
signal. The SSM2166 allows the user to set the downward ex-
pansion threshold, the limiting threshold (rotation point), input
buffer fixed gain, and the internal VCA's nominal gain at the ro-
tation point. The SSM2166 also features a power-down mode
and muting capability.

Theory of Operation

Figure 13 illustrates a typical transfer characteristic for the
SSM2166 where the output level in dB is plotted as a func-
tion of the input level in dB. The dotted line indicates the
transfer characteristic for a unity-gain amplifier. For input
signals in the range of V

(Downward Expansion) to V

(Rotation Point) 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) to over 15:1 via a single resistor,
R

COMP

. Input signals above V

are compressed with a fixed

compression ratio of approximately 15:1. This region of opera-
tion 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," and is user-specified
in the SSM2166. 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.

The gain of the system with an input signal level of V

is fixed

by R

GAIN

regardless of the compression ratio, and is the "nomi-

nal gain" of the system. The nominal gain of the system may be
increased by the user via the onboard VCA by up to 20 dB. Ad-
ditionally, the input buffer of the SSM2166 can be configured
to provide fixed gains of 0 dB to 20 dB with R1 and R2.

Input signals below V

are downward expanded; that is, a 1 dB

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
quite large for small input signals above of V

. The downward

expansion threshold, V

, is set externally by the user via R

GATE

at Pin 9 (NOISE GATE). Finally, the SSM2166 provides an
active HIGH, CMOS-compatible digital input whereby a
power-down feature will reduce device supply current to less
than 100

INPUT dB

OUTPUT dB

LIMITING

REGION

LIMITING

THRESHOLD

(ROTATION POINT)

COMPRESSION

REGION

DOWNWARD

EXPANSION

THRESHOLD

(NOISE GATE)

DOWNWARD

EXPANSION

REGION

VCA GAIN

Figure 13. General Input/Output Characteristics of the
SSM2166

The SSM2166 Signal Path

Figure 14 illustrates the block diagram of the SSM2166. The
audio input signal is processed by the input buffer and then
by the VCA. The input buffer presents an input impedance
of approximately 180 k

to the source. A dc voltage of approxi-

mately 1.5 V is present at AUDIO +IN (Pin 7 of the SSM2166),
requiring the use of a blocking capacitor (C1) for ground-
referenced sources. A 0.1

F capacitor is a good choice for most

audio applications. The input buffer is a unity-gain stable ampli-
fier that can drive the low impedance input of the VCA.

The VCA is a low distortion, variable-gain amplifier whose gain
is set by the side-chain control circuitry. The input to the VCA
is a virtual ground in series with approximately 1 k

. An exter-

nal blocking capacitor (C6) must be used between the buffer's
output and the VCA input. The 1 k

impedance between am-

plifiers determines the value of this capacitor which is typically
between 4.7

F and 10

F. An aluminum electrolytic capacitor

is an economical choice. The VCA amplifies the input signal
current flowing through C6 and converts this current to a volt-
age at the SSM2166's output pin (Pin 13). The net gain from
input to output can be as high as 60 dB (without additional
buffer gain), depending on the gain set by the control circuitry.

The gain of the VCA at the rotation point is set by the value of a
resistor connected between Pin 2 and GND, R

GAIN

. The rela-

tionship between the VCA gain and R

GAIN

is shown in Figure 6.

The AGC range of the SSM2166 can be as high as 60 dB. The
VCA

pin (Pin 3) on the SSM2166 is the noninverting input

terminal to the VCA. The inverting input of the VCA is also
available on the SSM2166's Pin 4 (VCA

) and exhibits an input

impedance of 1 k

, as well. As a result, this pin can be used for

differential inputs or for the elimination of grounding problems
by connecting a capacitor whose value equals that used in series
with the VCA

pin, to ground. See Figure 22, SSM2166

Evaluation Board for more details.

The output impedance of the SSM2166 is typically less that
75

, and the external load on Pin 13 should be >5 k

. The

nominal output dc voltage of the device is approximately 2.2 V.
Use a blocking capacitor for grounded loads.

The bandwidth of the SSM2166 is quite wide at all gain set-
tings. The upper 3 dB point is approximately 30 kHz at gains as
high as 60 dB (using the input buffer for additional gain, circuit

SSM2166

REV. A

AUDIO

+IN

BUF

OUT

NOISE

GATE

ROTATION

POINT

ADJUST

GND

VCA

RMS

LEVEL

DETECTOR

INPUT

BUFFER

VCA

SSM2166

R2 = 10k

1 F

CONTROL

CIRCUITRY

POWER

DOWN

POWER
DOWN

GAIN

COMPRESSION
RATIO SET

COMP

AVG

2.2 F

AVG
CAP

GAIN

ADJUST

OUTPUT

10 F

V+

GATE

ROT PT

GND

10 F

OPTIONAL

0.1 F

R1 = 10k

Figure 14. Functional Block Diagram and Typical Application

bandwidth is unaffected). The GBW plots are shown in Figure
10b. The lower 3 dB cutoff frequency of the SSM2166 is set by
the input impedance of the VCA (1 k

) 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 minimum when the gain is at a maximum, thereby optimiz-
ing the usable dynamic range of the part. A photograph of the
SSM2166's wideband peak-to-peak output noise is illustrated in
Figure 10b.

The 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

F. 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 aver-
aging 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 dy-
namics. Electrolytic capacitors are recommended here for low-
est cost and should be in the range of 2

F to 47

F. Capacitor

values from 18

F to 22

F have been found to be more appro-

priate in voiceband 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·C

AVG

milliseconds where C

AVG

is in

F. 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 con-
trolled 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 Fig-
ure 15 for a C

AVG

of 2.2

F (Figure 15a) and 22

F (Figure

15b). In each of these photographs, the input signal to the
SSM2166 (not shown) is a series of tone bursts in 6 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 photographs, the
attack time of the rms level detector is dependent only on C

AVG

but the release times are linear ramps whose decay times are
dependent on both C

AVG

and the input signal step size. The

rate of release is approximately 240 dB/s for a C

AVG

of 2.2

and 12 dB/s for a C

AVG

of 22

100

100mV

100ms

6dBV

66dBV

85dBV

Figure 15a. RMS Level Detector Performance with
C

AVG

= 2.2

SSM2166

REV. A

100

100mV

6dBV

66dBV

85dBV

Figure 15b. RMS Level Detector Performance with
C

AVG

= 22

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 VCA's gain control is logarithmic--a linear change in
control signal causes a dB 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 13.

Compression Ratio. Changing the scaling of the control sig-
nal fed to the VCA causes a change in the circuit's compression
ratio, "r." This effect is shown in Figure 16. The compression
ratio can be set by connecting a resistor between the COMP
RATIO pin (Pin 10) and GND. Lowering R

COMP

gives smaller

compression ratios as indicated in Figure 5, with values of about
17 k

or less resulting in a compression ratio of 1:1. AGC per-

formance is achieved with compression ratios between 2:1 and
15:1, and is dependent on the application. A 100 k

potentiom-

eter may be used to allow this parameter to be adjusted. On the
evaluation board (Figure 22), an optional resistor can be used to
set the compression equal to 1:1 when the wiper of the potenti-
ometer is at its full CCW position.

INPUT dB

OUTPUT dB

15:1

5:1

2:1

1:1

VCA GAIN

Figure 16. 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 il-
lustrated in Figure 9. The effect on rotation point is shown in
Figure 17. By varying a resistor, R

ROT PT

, connected between the

positive supply and the ROTATION POINT SET pin (Pin 11),

the rotation point may be varied from approximately 20 mV rms
to 1 V rms. From the figure, the rotation point is inversely propor-
tional to R

ROT PT

. For example, a 1 k

resistor would typically

set the rotation point at 1 V rms, whereas a 55 k

resistor would

typically set the rotation point at approximately 30 mV rms.

Since limiting occurs for signals larger than the rotation point
(V

> V

), the rotation point effectively sets the maximum

output 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 will cover the entire desired in-
put signal range. Occasional larger signal transients will then be
attenuated by the action of the limiter.

RP1

INPUT dB

OUTPUT dB

RP2

RP3

VCA GAIN

r:1

Figure 17. Effect of Varying the Rotation Point

VCA Gain Setting and Muting. The maximum gain of the
SSM2166 is set by the GAIN ADJUST pin (Pin 2) via R

GAIN

This resistor, with a range between 1 k

and 20 k

, will cause

the nominal VCA gain to vary from 0 dB to approximately
20 dB, respectively. To set the VCA gain to its maximum can
also be achieved by leaving the GAIN ADJUST pin in an
OPEN condition (no connect). Figure 18 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 k

resistor.

INPUT dB

OUTPUT dB

VCA GAIN

r:1

Figure 18. Effect of Varying the VCA Gain Setting

SSM2166

REV. A

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

GAIN

smaller than 1 k

. Switching Pin 2 through 330

or less

to ground will mute the output. Either a switch connected to
ground or a transistor may be used, as shown in Figure 19. To
avoid audible "clicks" when using this MUTE feature, a capaci-
tor (C5 in figure) can be connected from pin 2 to GND. The
value of the capacitor is arbitrary and should be determined em-
pirically, but a 0.01

F capacitor is a good starting value.

GAIN

ADJUST

GAIN

330

MUTE

(CLOSED SWITCH)

NOTE: ADDITIONAL CIRCUIT DETAILS
OMITTED FOR CLARITY.

SSM2166

Figure 19. Details of SSM2166 Mute Option

Downward Expansion Threshold. The downward expan-
sion, or noise gate, threshold is determined via a second refer-
ence voltage internal to the control circuitry. This second
reference can be varied in the SSM2166 using a resistor, R

GATE

connected between the positive supply and the NOISE GATE
SET pin (Pin 9) of the SSM2166. The effect of varying this
threshold is shown in Figure 20. The downward expansion
threshold may be set between 300

V rms and 20 mV rms by

varying the resistance value between Pin 9 and the supply volt-
age. Like the ROTATION PT ADJUST, the downward expan-
sion threshold is inversely proportional to the value of this
resistance: setting this resistance to 1 M

sets the threshold at

approximately 250

V rms, whereas a 10 k

resistance sets the

threshold at approximately 20 mV rms. This relationship is
illustrated in Figure 4. A potentiometer network is provided on
the evaluation board for this adjustment. In general, the down-
ward expansion threshold should be set at the lower extreme of
the desired range of the input signals, so that signals below this
level will be attenuated.

VCA GAIN

INPUT dB

OUTPUT dB

DE1

DE3

DE2

r:1

Figure 20. Effect of Varying the Downward Expansion
(Noise Gate) Threshold

Power-Down Feature

The supply current of the SSM2166 can reduced to under
100

A by applying an active HIGH, 5 V CMOS compatible

input to the SSM2166's POWER DOWN pin (Pin 12). In this
state, the input and output circuitry of the SSM2166 will assume a
high impedance state; as such, the potentials at the input pin
and the output pin will be determined by the external circuitry
connected to the SSM2166. The SSM2166 takes approximately
200 ms to settle from a POWER-DOWN to POWER-ON com-
mand. For POWER-ON to POWER-DOWN, the SSM2166
requires more time, typically less than 1 s. Cycling the power
supply to the SSM2166 can result in quicker settling times: the
off-to-on settling time of the SSM2166 is less than 200 ms,
while the on-to-off settling time is less than 1 ms. In either
implementation, transients may appear at the output of the de-
vice. In order to avoid these output transients, MUTE control
of the VCA's gain as previously mentioned should be used.

PC Board Layout Considerations

Since the SSM2166 is capable of wide bandwidth operation and
can be configured for as much as 80 dB of gain, special care
must be exercised in the layout of the PC board which contains
the IC and its associated components. The following applica-
tions hints should be considered and/or followed:

(1) In some high system gain applications, the shielding of in-
put wires to minimize possible feedback from the output of the
SSM2166 back to the input circuit may be necessary.

(2) A single-point ("star") ground implementation is recom-
mended in addition to maintaining short lead lengths and PC
board runs. The evaluation board layout shown in Figure 23 for
the SSM2166 demonstrates the single-point grounding scheme.
In applications where an analog ground and a digital ground are
available, the SSM2166 and its surrounding circuitry should be
connected to the system's analog ground. As a result of these
recommendations, wire-wrap board connections and grounding
implementations are to be explicitly avoided.

(3) The internal buffer of the SSM2166 was designed to drive
only the input of the internal VCA and its own feedback net-
work. Stray capacitive loading to ground from the BUF

OUT

in excess of 5 pF to 10 pF can cause excessive phase shift and
can lead to circuit instability.

(4) When using high impedance sources (

5 k

), system gains

in excess of 60 dB are not recommended. This configuration is
rarely appropriate, as virtually all high impedance inputs provide
larger amplitude signals that do not require as much amplifica-
tion. When using high impedance sources, however, it can be
advantageous to shunt the source with a capacitor to ground at
the input pin of the IC (Pin 7) to lower the source impedance at
high frequencies, as shown in Figure 21. A capacitor with a value
of 1000 pF is a good starting value and sets a low pass corner at
31 kHz for 5 k

sources. In those applications where the source

ground is not as "clean" as would be desirable, a capacitor (illus-
trated as C7 on the evaluation board) from the VCA

input to

the source ground might prove beneficial. This capacitor is
used in addition to the grounded capacitor (illustrated as C2 on
the evaluation board) used in the feedback around the buffer,
assuming that the buffer is configured for gain.

SSM2166

REV. A

1000pF

0.1 F

AUDIO IN

> 5k )

+IN

SSM2166

NOTE: ADDITIONAL CIRCUIT DETAILS
OMITTED FOR CLARITY.

Figure 21. Circuit Configuration for Use with High
Impedance Signal Sources

The value of the C7 should be the same as C6, the capacitor
value used between BUF

OUT

and VCA

. This connection makes

the source ground noise appear as a common-mode signal to the
VCA, allowing the common-mode noise to be rejected by the
VCA's differential input circuitry. C7 can also be useful in
reducing ground loop problems and in reducing noise coupling
from the power supply by balancing the impedances connected
to the inputs of the internal VCA.

SSM2166 Evaluation Board

A schematic diagram of the SSM2166 evaluation board, avail-
able upon request from Analog Devices, is illustrated in Figure
22. As a design aid, the layouts for the topside silkscreen,
topside and backside metallization layers are shown in Figures
23a, b, and c. Although not shown to scale, the finished dimen-
sion of the evaluation board is 3.5 inches by 3.5 inches, and
comes complete with pin sockets and a sample of the SSM2166.

Figure 22. Evaluation Board

ROT PT.

ADJ

NOISE

GATE

ADJ

0.1 F

R12
100k

NOISE

GATE

POWER

V+

BUF

OUT

VCA

10 F

VCA

+INPUT

MIC

PWR

INPUT

JACK

1/8"

PHONE

C1
0.1 F

C7
10 F

GAIN

ADJUST

MUTE
SWITCH

R11

330

R10
20k

C5
0.01 F

GAIN

ADJ

AVG
CAP

C4
22 F

INPUT

SSM2166

10k

1 F

COMP

RATIO

R6
100k

COMP

RATIO

OP113

OUTPUT
JACK
RCA
PHONO

OUTPUT

50k

ROTATION

PT ADJ

10k

GND

SSM2166

REV. A

Figure 23c. Evaluation Board Backside Metallization
(Not to Scale)

Signal sources are connected to the SSM2166 through a 1/8"
phone jack where a 0.1

F capacitor couples the input signal to

the SSM2166's +IN pin (Pin 7). As shown in Figure 22 and in
microphone applications, the phone jack shield can be optionally
connected to the board's ground plane (Jumper J1 inserted into
board socket pins labeled "1" and "2") or to the SSM2166's
VCA

input at Pin 4 (Jumper J1 inserted into board socket pins

labeled "1" and "3"). If the signal source is a waveform or
function generator, the phone jack shield is to be connected
to ground.

For ease in making adjustments for all of the SSM2166's con-
figuration parameters, single-turn potentiometers are used
throughout. Optional Jumper J2 connects the COMP RATIO
pin to ground and sets the SSM2166 for no compression (that
is, compression ratio = 1:1). Optional Jumper J3 connects the
SSM2166's POWER DOWN input to ground for normal opera-
tion. Jumper J3 can be replaced by an open-drain logic buffer
for a digitally-controlled shutdown function. An output signal
MUTE function can be implemented on the SSM2166 by con-
necting the GAIN ADJUST pin (Pin 2) through a 330

resis-

tance to ground. This is provided on the evaluation board via
R11 and S1. A capacitor C5, connected between Pin 2 and
ground and provided on the evaluation board, can be used to
avoid audible "clicks" when using the MUTE function.

To configure the SSM2166's input buffer for gain, provisions for
R1, R2, and C2 have been included. To configure the input
buffer for unity-gain operation, R1 and R2 are removed, and a
direct connection is made between the IN pin (Pin 6) and the
BUF

OUT

pin (Pin 5) of the SSM2166.

The output stage of the SSM2166 is capable of driving > 1 V
rms (3 V p-p) into > 5 k

loads, and is externally available

through an RCA phono jack provided on the board. If the out-
put of the SSM2166 is required to drive a lower load resistance

Figure 23a. Evaluation Board Topside Silkscreen
(Not to Scale)

Figure 23b. Evaluation Board Topside Metallization
(Not to Scale)

SSM2166

REV. A

or an audio cable, then the onboard OP113 can be used. To
use the OP113 buffer, insert Jumper J4 into board socket pins
labeled "4" and "5" and insert Jumper J5 into board socket pins
labeled "6" and "7." If the output buffer is not required, re-
move Jumper J5 and insert Jumper J4 into board socket pins "5"
and "7." There are no blocking capacitors either on the input
nor at the output of the buffer. As a result, the output dc level
of the buffer will match the output dc level of the SSM2166,
which is approximately 2.3 V. A dc blocking capacitor may be
inserted on Pins 6 and 7. An evaluation board and setup proce-
dure is available from your Analog Devices representative.

Setup Procedure with Evaluation Board

To illustrate how easy it is to program the SSM2166, we will
take a practical example. The SSM2166 will be used interface
an electret-type microphone to a post-amplifier. You can use
the evaluation board or the circuit configuration shown in Figure
22. The signal from the microphone was measured under actual
conditions to vary from 1 mV to 15 mV. The post-amplifier
requires no more than 500 mV at its input. The required gain
from the SSM2166 is, therefore:

TOT

= 20

log (500/15) = 30 dB

We will set the input buffer gain to 20 dB and adjust the VCA
gain to 10 dB. The limiting or "rotation" point will be set at
500 mV output. From prior experience, we will start with a 2:1
compression ratio, and a noise gate threshold that operates be-
low 100

V. These objectives are summarized in Figure 24, and

we will fine-tune them later on. The transfer characteristic we
will implement is illustrated in Figure 25.

INPUT RANGE
OUTPUT RANGE
LIMITING LEVEL
COMPRESSION
BUFFER GAIN
VCA GAIN
NOISE GATE

1-15 mV

TO 500 mV

500 mV

2:1

20 dB
10 dB

100 V

Figure 24. Objective Specifications

Note: the SSM2166 processes the output of the buffer, which in
our example is 20 dB or ten times the input level. Use the oscil-
loscope to verify that you are not driving the buffer into clipping
with excessive input signals. In your application, you should
take the minimum gain in the buffer consistent with the average
source level as well as the crest factor (ratio of peak to rms).

INPUT mV

500

0.1

1.0

OUTPUT mV

ROTATION POINT

COMPRESSION

REGION

LIMITING REGION

GATE THRESHOLD

Figure 25. Transfer Characteristic

Evaluation Board

If you build your own breadboard, keep the leads to Pins 3, 4,
and 5 short. A convenient evaluation board is available from
your sales representative. The R and C designations refer to the
demonstration board schematic of Figure 22 and parts list,
Figure 28.

Test Equipment Setup

The recommended equipment and configuration is shown in
Figure 26. A low noise audio generator with a smooth output
adjustment range of 50

V to 50 mV is a suitable signal source.

A 40 dB pad would be useful to reduce the level of most genera-
tors by 100

to simulate the microphone levels. The input volt-

meter could be connected before the pad, and need only go
down to 10 mV. The output voltmeter should go up to 2 volts.
The oscilloscope is used to verify that the output is sinusoidal,
that no clipping is occurring in the buffer, and to set the limiting
and noise gating "knees."

VOLTMETER

SIGNAL

GENERATOR

SSM2166

EVALUATION

BOARD

VOLTMETER

OSCILLOSCOPE

Figure 26. Test Equipment Setup

STEP 1. Configure the Buffer

The SSM2166 has an input buffer that may be used when the
overall gain required exceeds 20 dB, the maximum user-
selectable gain of the VCA. In our example, the desired output
is 500 mV for an input around 15 mV, requiring a total gain of
30 dB. We will set the buffer gain at 20 dB, and adjust VCA
for 10 dB. In the socket pins provided on the evaluation board,
Insert R1 = 100 k

, and R2 = 11 k

. You have set the buffer

gain to 20 dB (

10).

STEP 2. Initialize Potentiometers

With power off, preset the potentiometers per the table of Fig-
ure 27 below.

GAIN ADJUST
(VCA)

FUNCTION

ROTATION
POINT

COMPRESSION
RATIO

NOISE GATE

ZERO

1 M

INITIAL

RESISTANCE

INITIAL

POSITION

CCW

RANGE

020 k

050 k

0100

01 M

POT

R10

0 dB; CW TO INCREASE

VCA GAIN

1 V; CW TO REDUCE

ROTATION POINT

1:1; CW TO INCREASE

COMPRESSION

300 V; CCW TO

INCREASE THRESHOLD

EFFECT OF CHANGE

Figure 27. Initial Potentiometer Settings

STEP 3. Test Setup

With power on, adjust the generator for an input level of 15 mV,
1 kHz. The output meter should indicate approximately 100 mV.
If not, check your setup.

STEP 4. Adjusting the VCA Gain

Set the input level to 15 mV. Adjust R10--GAIN ADJ CW for
an output level of 500 mV. You have now set the VCA gain to
10 dB.

SSM2166

REV. A

STEP 5. Adjusting the Rotation Point

Set the input level to 15 mV, and observe the output on the os-
cilloscope. Adjust R3--ROTATION PT ADJ CW until the
output level just begins to drop, then reverse so that the output is
500 mV. You have now set the limiting to 500 mV.

STEP 6. Adjusting the Compression Ratio

Set the input signal for an output of 500 mV but not in limiting.
Note the value (around 15 mV). Next, reduce the input to 1/10
the value noted, (around 1.5 mV), for a change of 20 dB. Next,
adjust R6--COMP RATIO CW until the output is 160 mV, for
an output change of 10 dB. You have now set the compression,
which is the ratio of output change to input change, in dB, to 2:1.

STEP 7. Setting the Noise Gate

With the input set at 100

V, observe the output on the oscillo-

scope, and adjust R7--ROT PT SET CCW until the output
drops rapidly. "Rock" the control back and forth to find the
"knee." You have set the noise gate to 100

V. The range of

the noise gate is from 0.3 mV to over 0.5 mV relative to the out-
put of the buffer. To fit this range to your application, you may
have to attenuate the input or apportion the buffer gain and VCA
gain differently.

STEP 8. Listening

At this time, you may want to connect an electret microphone to
the SSM2166, and listen to the results. Be sure to include the
proper power for the microphone's internal FET (usually +2 V
to +5 V dc through a 2.2 k

resistor). Experiment with the

settings to hear how the results change. Varying the averaging
capacitor, C4, changes the attack and decay times, which are
best determined empirically. Compression ratio will keep the
output steady over a range of microphone to speaker distance,
and the noise gate will keep the background sounds subdued.

STEP 9. Record Values

With the power removed from the test fixture, measure and
record the values of all potentiometers, including any fixed resis-
tance in series with them. If you have changed the averaging
capacitor, C4, note its value too.

SUMMARY

We have implemented the transfer condition of Figure 2. For
inputs below the 100

V noise gate threshold, circuit and back-

ground noise will be minimized. Above it, the output will in-
crease at a rate of 1 dB for each 2 dB input increase, until the
500 mV rotation point is reached at an input of approximately
15 mV. For higher inputs that would drive the output beyond
500 mV, limiting will occur, and there will be little further in-
crease. The SSM2166 processes the output of the buffer, which
in our example is 20 dB or ten times the input level. Use the os-
cilloscope to ensure that you are not driving the buffer into clip-
ping with the highest expected input peaks. Always take the
minimum gain in the buffer consistent with the average source
level and crest factor (ratio of peak to rms). The wide program
range of the SSM2166 makes it useful in many applications
other than microphone signal conditioning.

Other Versions

The SSM2165 is an 8-lead version of this microphone preamp
with unity buffer gain and preset noise gate threshold. Custom-
ized parts are available for large volume users. For further in-
formation, contact your sales representative.

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