SSM2166

REV. A

–12–

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:

× 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

µV. 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

40

0.1

10

1.0

15

ROTATION POINT

COMPRESSION

REGION

1

2

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

µV 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.”

AC

VOLTMETER

SIGNAL

GENERATOR

SSM2166

EVALUATION

BOARD

AC

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

Ω, and R2 = 11 kΩ. 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

ZERO

ZERO

1 M

INITIAL

RESISTANCE

INITIAL

POSITION

CCW

CCW

CCW

CW

RANGE

0–20 k

0–50 k

0–100

k

0–1 M

POT

R10

R3

R6

R7

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.