ANALOG DEVICES AN-583 Service Manual

AN-583

a

One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106 • Tel: [phone] • Fax: [phone] • www.analog.com

APPLICATION NOTE

Using the SSM2167 Evaluation Board

By Shawn Scarlett

SSM2167 EVALUATION BOARD

The evaluation board is a convenient means to evaluate
and understand the operation of the SSM2167. This
documentation provides a basic explanation of how the
board is set up and how to use it to evaluate the
SSM2167. This document is only a supplement and
makes several references to figures contained in the
product data sheet. The current data sheet is available
on-line at www.analog.com.

This document also contains the schematics and layout
for the evaluation board to allow easy modification and
experimentation. Applications support is available from
Analog Devices, Inc. on-line or in the United States at 1(800)
ANALOG D or 1(800) 262-5643.

Basic Connections and Setup

The board features simple, easy-to-use connections that
allow for a broad range of experimentation. The evalua­tion board is available only with the SSM2167-1.

Power and Ground

The SSM2167 is optimized for operation at 3 V;
however, it can operate from a wide range of supplies.
Please refer to the product data sheet for the maximum
limits. The SSM2167 board can be connected to a bench
power supply or two AA batteries. Power leads will need
to be soldered into place at JP3. Alternatively, a header
jack can be soldered to the board to allow easy discon­nection. The power and ground connections are critical
to the performance of the board, so care should be taken
to ensure proper connection.

Shutdown Connection

Located at JP3, along with the power and ground, is the
low current shutdown connection. The board has a pull-up
resistor installed so that if no connection is made at that
point, the part will operate in active mode. To enable the
shutdown feature, simply connect the SHUTDOWN pin
on JP3 to ground. If an in-depth analysis is to be done on
the shutdown current, the pull-up resistor R5 may be
removed. The shutdown pin should not be left floating. If
R5 is removed, be sure to connect the SHUTDOWN pin
directly to the supply or ground.

Signal Connections, Input, and Output

Signal sources can be connected to the evaluation board
with standard 3.5 mm jacks or leads which can be
soldered directly to the board at JP2 and JP3. The board
uses 3.5 mm stereo sockets with the signal on the tip and
the sleeve connected to the board ground; the ring is left
floating. At both jumper connections, a ground point is
available to prevent noise.

The input jack is configured for maximum versatility. By
default, it is designed for use with an electret micro­phone or input source. It can be changed easily to
accommodate dynamic microphones.

Electret Microphones

The evaluation board is configured with a 2.2 kΩ resistor
(R4) to the supply for use as a biasing resistor. It is con­nected directly to the signal path so that both the jack
and the jumper (JP2) can be used as input connections.
Standard computer electret microphones can be used with­out modification because the ring and tip are connected
internally in the microphone. The input jack and JP2 are
decoupled from the input pin via a 0.1 µF capacitor (C1).

Dynamic Microphones

If the board is to be used with a dynamic or self-powered
microphone, R4 should be removed.

Output Signal

The output jack is decoupled from the circuit via a 10 µF
capacitor (C2). The connection can be made at either JP1
or the standard 3.5 mm jack, with the signal connection
to the tip of the plug. For listening tests, the output sig­nal can be connected directly from the 3.5 mm jack to an
RCA input on a normal audio amplifier using a standard
stereo adapter cable. In this case, use the left (white or
black) RCA connection because it corresponds with the
tip of the 3.5 mm jack. The output of the SSM2167 is not
sufficient to drive headphones or other output transduc­ers without external amplification.

SIGNAL

GENERATOR

AC VOLTMETER

SSM2167

EVALUATION

BOARD

OSCILLOSCOPE

AC VOLTMETER

REV. A

Figure 1. Test Equipment Setup

© Analog Devices, Inc., 2003

AN-583

Test Equipment Setup

The recommended equipment and configuration is shown
in Figure 1. 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 is useful to reduce the level of
most generators by 100´ to simulate microphone levels.
The input voltmeter can be connected before the pad
and need only go down to 10 mV. The output voltmeter
should go up to 2 V. The oscilloscope is used to verify
that the output is sinusoidal, that no clipping is occur­ring in the buffer, and that the noise-gating threshold
is set.

STEP 1. Connections

Connect power, ground, input, and output as discussed above.

STEP 2. Test Setup

To confirm the operation of the board and test setup,
first put JP4 in the 1:1 position and JP5 in the –55 dB
position. With power on, adjust the generator for an input
level of 15 mV, 1 kHz. The output meter should indicate
approximately 100 mV. If it does not, check the setup.
Substituting the SSM2167-2 will give an output of approxi­mately 40 mV.

STEP 3. Listening

Connect a microphone to the SSM2167 and listen to the
results. Be sure to include the proper power for the micro­phone as discussed above. Experiment with the settings to
hear how the results change. Compression ratio will keep
the output steady over a range of source-to-microphone
distances, and the noise gate will keep the background
sounds subdued.

STEP 5. Adjusting the Compression Ratio

The evaluation board provides three different settings
for the compression ratio in the same manner as the
noise-gate threshold. Experiment with different com­pression ratios to determine what will sound best in a
given system; starting with a 2:1 ratio is recommended.
High compression ratios will exaggerate the effect of the
noise gate because compression ratio determines the
gain at the noise gate, as shown in Figure 4 of the data
sheet. Compression of 10:1 should be used only in sys­tems where the noise floor is well below the noise gate.
Most systems require between 2:1 and 5:1 compression
for best results.

Listening Test

The final step in evaluating the SSM2167 should be a
listening test. The improvement in vocal clarity can be
heard by recording the SSM2167 output or listening to it
live. Ideally, the evaluation board should be connected
to an existing system. The impact of the compression is
demonstrated by shorting out RCOMP (R9 through R12).
When the RCOMP resistor is shorted, the VCA will revert
to the audible 1:1 compression setting. This will not
the noise-gate or limiting settings. The effect of turning
off the compression will be most noticeable when the
input signal is between –50 dBV and –40 dBV. Evaluating
the SSM2167 within the end application will also give
the best indication of how high the noise-gate threshold
should be set. The noise level of the system will be
greatly influenced by the design of the system, including
cooling fans, hard drives, handling, and other sources of
acoustic noise.

affect

STEP 4. Setting the Noise-Gate Threshold

The evaluation board provides three different preset values
of noise-gate threshold. Experiment with these values
by varying the gate. The board also provides landing
pads for a custom value that can be extrapolated from
the table given in the data sheet, or TPC 1 in the Typical
Performance Characteristics section of the data sheet.
Using above 5 kΩ is not recommended because
extremely low noise-gate thresholds may approach the
noise floor of the system.

The highest setting (–48 dB) is recommended to start an
evaluation. If the input signal is not sufficient to surpass
the threshold, lower the setting. In most applications, the
input signal will easily overcome this setting. If the gate
is set too low, the background noise will be amplified well
into the audible range. By examining the function
(Figure 1) in the data sheet, the maximum gain of the part
can be determined when the input signal is at the
noise-gate threshold. The dashed line on the transfer
function represents unity gain; the distance between the
dashed line and the solid line represents the VCA gain.

Listening tests are the most critical part of an evaluation.
Since test equipment and signal generators do not rep­resent audio signals well, listening is the best way to
evaluate the benefits of the SSM2167. The evaluation
board makes it easy to implement the SSM2167 and to
pick appropriate application settings. The end result is a
noticeable improvement in signal clarity and a system
that is easy for customers to use.

Figure 2. SSM2167 Evaluation Board; Top Layer
Including Component Identification and Placement

–2–

REV. A