ssm2166p Analog Devices, Inc., ssm2166p Datasheet - Page 12

• Manufacturer Part Number: ssm2166p
• Description: Microphone Preamplifier With Variable Compression And Noise Gating
• Manufacturer: Analog Devices, Inc.

SSM2166
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 procedure 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 to interface
an electret-type microphone to a postamplifier. The evaluation
board or the circuit configuration shown in Figure 12 can be
used. The signal from the microphone was measured under
actual conditions to vary from 1 mV to 15 mV. The postamplifier
requires no more than 500 mV at its input. The required gain
from the SSM2166 is, therefore:
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
below 100 µV. These objectives are summarized in Figure 14,
and we will fine-tune them later on. The transfer characteristic
we will implement is illustrated in Figure 15.
Note: The SSM2166 processes the output of the buffer, which
in our example is 20 dB or 10 times the input level. Use the
oscilloscope to verify that the buffer is not being driven into
clipping with excessive input signals. In the application, take the
minimum gain in the buffer consistent with the average source
level as well as the crest factor (ratio of peak to rms).
500
40
INPUT RANGE
OUTPUT RANGE
LIMITING LEVEL
COMPRESSION
BUFFER GAIN
VCA GAIN
NOISE GATE
Figure 14. Objective Specifications
Figure 15. Transfer Characteristic
G
0.1
TOT
GATE THRESHOLD
COMPRESSION
= 20 × log (500/15) = 30 dB
REGION
1.0
INPUT – mV
2
ROTATION POINT
10
1
1 mV–15 mV
TO 500 mV
15
500 mV
100 V
LIMITING REGION
20 dB
10 dB
2:1
–12–
Evaluation Board
When building a breadboard, keep the leads to Pins 3, 4, and 5
short. A convenient evaluation board is available from an ADI
sales representative. The R and C designations refer to the
demonstration board schematic of Figure 12 and parts list,
Table I.
Test Equipment Setup
The recommended equipment and configuration is shown in
Figure 16. 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 generators
by 100× to simulate the microphone levels. The input voltmeter
could be connected before the pad, and need only go down to
10 mV. The output voltmeter should go up to 2 V. The oscillo-
scope is used to verify that the output is sinusoidal and that no
clipping is occurring in the buffer, and to set the limiting and
noise gating “knees.”
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 the VCA for 10 dB.
In the socket pins provided on the evaluation board, insert
R1 = 100 kΩ, and R2 = 11 kΩ. The buffer gain has been set to
20 dB (×10).
STEP 2. Initializing Potentiometers
With power off, preset the potentiometers per the table of
Figure 17 below.
STEP 3. Testing 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 the 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. The VCA gain has been set to 10 dB.
NOISE GATE
ROTATION
POINT
COMPRESSION
RATIO
GAIN ADJUST
(VCA)
FUNCTION
VOLTMETER
GENERATOR
SIGNAL
Figure 17. Initial Potentiometer Settings
AC
Figure 16. Test Equipment Setup
POT
R10
R3
R7
R6
0–20 k
0–50 k
RANGE
0–1 M
0–100
k
POSITION
INITIAL
CCW
CCW
CCW
EVALUATION
CW
SSM2166
BOARD
RESISTANCE
INITIAL
ZERO
ZERO
ZERO
1 M
0 dB; CW TO INCREASE
INCREASE THRESHOLD
1:1; CW TO INCREASE
EFFECT OF CHANGE
1 V; CW TO REDUCE
OSCILLOSCOPE
ROTATION POINT
300 V; CCW TO
VOLTMETER
COMPRESSION
VCA GAIN
AC
REV. B