SSM2165-2P AD [Analog Devices], SSM2165-2P Datasheet - Page 5

• Manufacturer Part Number: SSM2165-2P
• Description: Microphone Preamplifier with Variable Compression and Noise Gating
• Manufacturer: AD [Analog Devices]

APPLICATIONS INFORMATION
The SSM2165 is a complete microphone signal conditioning
system in a single integrated circuit. Designed primarily for
voiceband applications, this integrated circuit provides amplifi-
cation, rms detection, limiting, variable compression, and down-
ward expansion. The internal rms detector has a time constant
set by an external capacitor. An integral voltage-controlled
amplifier (VCA) provides up to 40 dB of gain in the signal path
with approximately 30 kHz bandwidth. The device operates on
a single +5 V supply, accepts input signals up to 1 V
duces output signal levels at limiting of 320 mV and 250 mV for
the SSM2165-1 and SSM2165-2 respectively, into loads > 5 k .
The SSM2165 contains an input buffer and automatic gain
control (AGC) circuit for audio and voice band signals. Circuit
operation is optimized by providing user-adjustable compression
ratio and time constant. A downward expansion (noise gating)
feature reduces background and circuit noise below 500 V.
The rotation point determines the output signal levels before
limiting (referred to the input), and is 40 mV for the SSM2165-1
and 100 mV for the SSM2165-2.
1
REV. A
All signals are in rms volts or dBu (0 dBu = 0.775 V rms).
Figure 10. General Input/Output Characteristics of the
SSM2165
Figure 8. Small Signal Transient Response
INPUT – dB
DOWNWARD
EXPANSION
REGION
DOWNWARD
EXPANSION
THRESHOLD
(NOISE GATE)
100
0%
90
10
V
DE
COMPRESSION
REGION
LIMITING
THRESHOLD
(ROTATION POINT)
T
C
R
COMP RATIO = 1:1
V
V
20mV
A
IN
IN
AVG
L
= +25 C
= 10k
= 12.5mV (–1)
= 40mV (–2)
1
= 2.2 F
r
1
1
V
rp
10 s
LIMITING
REGION
VCA GAIN
1
, and pro-
–5–
THEORY OF OPERATION
Figure 10 illustrates the general transfer characteristic for the
SSM2165 where the output level in dBu is plotted as a function
of the input level in dBu (0 dBu = 0.775 V rms). For input
signals in the range of 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
compression ratio of approximately 10:1. 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 “rotation point,” and is different for the
SSM2165-1 and SSM2165-2, see Table I.
SSM2165
–1
–2
*At limiting.
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.
When the compression is set to 2:1, a –2 dB change of the
input signal level in the compression region causes –1 dB
change of the output level. Likewise, at 10:1 compression, a
–10 dB change of the input signal level in the compression
region causes a –1 dB change in the output level. The gain of
the system with an input signal level of V
the compression ratio, and is different for the SSM2165-1 and
SSM2165-2 (see Figures 1a and 1b). The “nominal gain” of
the system is 18 dB for the SSM2165-1, and 8 dB for the
SSM2165-2. System gain is measured at V
in dB.
Input signals below V
approximately 1:3. As a result, the gain of the system is small
for very small input signal levels below V
be quite large for input signals above V
expansion threshold, V
dash versions.
COMP
. Input signals above V
Figure 9. Large Signal Transient Response
Table I. Characteristics vs. Dash Number
Rotation Point
40 mV (–25.7 dBu)
100 mV (–17.7 dBu)
100
90
0%
10
DE
DE
DE
are downward expanded at a ratio of
, is fixed at 500 V (–64 dBu) for both
T
C
R
COMP RATIO = 1:1
V
V
200mV
A
AVG
L
IN
IN
(Downward Expansion) to V
= +25 C
= 10k
= 400mV (–2)
= 125mV (–1)
RP
= 2.2 F
are compressed with a fixed
Gain
18 dB
8 dB
DE
DE
RP
RP
. The downward
, even though it may
is fixed regardless of
10 s
and is (V
SSM2165
Output*
320 mV (–6 dBu)
250 mV (–8 dBu)
OUT
RP
– V
IN
)