SSM2120 Datasheet, PDF (5/12 Pages) Analog Devices – Dynamic Range Processors/Dual VCA

2V
1s
100
90
10
0%
Figure 4. Detector Output
2V
100
90
SSM2120/SSM2122
The decay rates are linear ramps that are dependent on the
current out of the LOG AV pin (set by R_REF) and the value of
C_AV. The integration or decay time of the circuit is derived from
the formula:
Decrementation Rate (in dB/s) =
I_REF × 333
C _AV
Table I. Settling Time (t_S) for C_AV = 10 ␮F. t_S = t_S (C_AV = 10 ␮F)
5 dB
3 dB
2 dB
1 dB
10 dB Step
20 dB Step
30 dB Step
40 dB Step
50 dB Step
60 dB Step
11.28 ms
16.65
18.15
18.61
21.46
26.83
28.33
27.79
30.19
35.56
37.06
37.52
(+144 µs)
(+46 µs)
46.09
51.46
52.96
53.42
10
0%
50ms
Figure 5. Overlayed Detector Output
DYNAMIC ATTACK AND DECAY RATES
Figure 5 shows the output levels overlayed using a storage
scope. The attack rate is determined by the step size and the
value of C_AV. The attack time to final value is a function of the
step size increase. Table I shows the values of total settling
times to within 5 dB, 3 dB, 2 dB and 1 dB of final value with
C_AV = 10 µF. When step sizes exceed 40 dB, the increase in
settling time for larger steps is negligible. To calculate the attack
time to final value for any value of C_AV, simply multiply the
value in the chart by C_AV/10 µF.
APPLICATIONS
The following applications for the SSM2120 use both the VCAs
and level detectors in conjunction to assimilate a variety of
functions.
The first section describes the arrangement of the threshold
control in each control circuit configuration. These control
circuits form the foundation for the applications to follow which
include the downward expander, compressor/limiter and
compandor.
THRESHOLD CONTROL
Figure 6a shows the control circuit for a typical downward
expander while Figure 6b shows a typical control curve. Here,
the threshold potentiometer adjusts V_T to provide a negative
unipolar control output. This is typically used in noise gate,
downward expander, and dynamic filter applications. This
potentiometer is used in all applications to control the signal
level versus control voltage characteristics.
R_IN
L
R_IN
MONO
OR R
REC_IN
R_LL
2V
MONO – R_IN = 10kΩ
STEREO – R_IN = 20kΩ
THRESHOLD
CONTROL
|I_IN|
V_T
V+
V–
R_T
39kΩ CON_OUT ^RCON
1kΩ
LOG AV
C_AV
1.5MΩ
V–
TO +V_C
200Ω
V–
THRESHOLD
+
–
*
*LOWER LIMIT CAN BE FIXED
BY CONNECTING A RESISTOR
R_LL FROM REC_IN TO GROUND
V_IN – dB
a. Control Circuit
b. Typical Downward Expander
Control Curve
Figure 6. Noise Gate/Downward Expander Control Circuit and Typical Response
REV. C
–5–