ssm2166p Analog Devices, Inc., ssm2166p Datasheet - Page 13

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

STEP 5. Adjusting the Rotation Point
Set the input level to 15 mV, and observe the output on the
oscilloscope. Adjust R3—ROTATION PT ADJ CW until the
output level just begins to drop, then reverse so that the output
is 500 mV. The limiting has now been set 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. The compression, which is the ratio
of output change to input change, in dB, has now been set to 2:1.
STEP 7. Setting the Noise Gate
With the input set at 100 µV, 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.” The noise gate has now been set to 100 µV. The range
of the noise gate is from 0.3 mV to over 0.5 mV relative to the
output of the buffer. To fit this range to the application, it may
be necessary to attenuate the input or apportion the buffer gain
and VCA gain differently.
STEP 8. Listening
At this time, it may be desirable to connect an electret micro-
phone 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 kΩ resistor). Experi-
ment 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. The compression ratio
REV. B
–13–
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. Recording Values
With the power removed from the test fixture, measure and
record the values of all potentiometers, including any fixed
resistance in series with them. If the averaging capacitor, C4,
has been changed, note its value, too.
SUMMARY
We have implemented the transfer condition of Figure 2. For
inputs below the 100 µV noise gate threshold, circuit and
background noise will be minimized. Above it, the output will
increase 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
increase. 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 ensure that the buffer is not being driven into
clipping 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. Customized
parts are available for large volume users. For further informa-
tion, contact an ADI sales representative.
SSM2166