SSM2167-2RM-REEL AD [Analog Devices], SSM2167-2RM-REEL Datasheet - Page 7

• Manufacturer Part Number: SSM2167-2RM-REEL
• Description: Low Voltage Microphone Preamplifier with Variable Compression and Noise Gating
• Manufacturer: AD [Analog Devices]

The bandwidth of the SSM2167 is quite wide at all gain settings.
The upper 3 dB point is over 1 MHz at gains as high as 30 dB.
The GBW plots are shown in TPC 3. The lower 3 dB cutoff
frequency of the SSM2167 is set by the input impedance of
the VCA (1 kΩ) and C2. While the noise of the input buffer is
fixed, the input-referred noise of the VCA is a function of gain.
The VCA input noise is designed to be a minimum when the
gain is at a maximum, thereby maximizing the usable dynamic
range of the part.
Level Detector
The SSM2167 incorporates a full-wave rectifier and a patent-
pending, true rms level detector circuit whose averaging time
constant is set by an external capacitor (C
the AVG CAP pin (Pin 8). For optimal low-frequency operation
of the level detector down to 10 Hz, the value of the capacitor
should be 2.2 µF. Some experimentation with larger values
for C
low-frequency ambient background noise. The value of the aver-
aging capacitor affects sound quality: too small a value for this
capacitor may cause a “pumping effect” for some signals, while
too large a value can result in slow response times to signal
dynamics. Electrolytic capacitors are recommended here for
lowest cost and should be in the range of 2 µF to 22 µF.
The rms detector filter time constant is approximately given by
10
controls both the steady state averaging in the rms detector as
well as the release time for compression; that is, the time it takes
for the system gain to increase due to a decrease in input signal.
The attack time, the time it takes for the gain to be reduced
because of a sudden increase in input level, is controlled mainly by
internal circuitry that speeds up the attack for large level changes.
This limits overload time to less than 1 ms in most cases.
The performance of the rms level detector is illustrated in TPC 12
for a C
these photographs, the input signal to the SSM2167 (not shown) is
a series of tone bursts in six successive 10 dB steps. The tone bursts
range from –66 dBV (0.5 mV rms) to –6 dBV (0.5 V rms). As
illustrated in the photographs, the attack time of the rms level
detector is dependent only on C
ramps whose decay times are dependent on both C
input signal step size. The rate of release is approximately 240 dB/s
for a C
Control Circuitry
The output of the rms level detector is a signal proportional to
the log of the true rms value of the buffer output with an added
dc offset. The control circuitry subtracts a dc voltage from this
signal, scales it, and sends the result to the VCA to control the
gain. The VCA’s gain control is logarithmic—a linear change in
control signal causes a dB change in gain. It is this control law
that allows linear processing of the log rms signal to provide the
flat compression characteristic on the input/output characteristic
shown in Figure 1.
AVG
C
AVG
AVG
AVG
may be necessary to reduce the effects of excessive
of 2.2 µF and TPC 11 for a C
of 2.2 µF, and 12 dB/s for a C
milliseconds where C
AVG
AVG
, but the release times are linear
is in µF. This time constant
AVG
AVG
of 22 µF. In each of
of 22 µF.
AVG
) connected to
AVG
and the
Setting the Compression Ratio
Changing the scaling of the control signal fed to the VCA causes a
change in the circuit’s compression ratio, “r.” This effect is shown
in Figure 4. Connecting a resistor (R
sets the compression ratio. Lowering R
sion ratios as indicated in Table I. AGC performance is achieved
with compression ratios between 2:1 and 10:1, and is dependent
on the application. Shorting R
setting the compression equal to 1:1. If using a compression resis-
tor, using a value greater than 5 kΩ is recommend. If lower than
5 kΩ is used, the device may interpret this as a short, 0 Ω.
Compression Ratio
1:1
2:1
3:1
5:1
10:1
Table I. Setting Compression Ratio
V
DE1
V
DE2
V
DE
V
DE3
15:1
5:1
2:1
r:1
1:1
1
1
COMP
1
INPUT – dB
INPUT – dB
1
will disable the AGC function,
COMP
Value of R
0 Ω (short to V+)
15 kΩ
35 kΩ
75 kΩ
175 kΩ
COMP
V
) between Pin 8 and V
V
RP
RP
gives smaller compres-
SSM2167
VCA GAIN
COMP
VCA GAIN
DD