ssm2167 Analog Devices, Inc., ssm2167 Datasheet - Page 10

• Manufacturer Part Number: ssm2167
• Description: Low Voltage Microphone Preamplifier With Variable Compression And Noise Gating
• Manufacturer: Analog Devices, Inc.

SSM2167
SETTING THE NOISE GATE THRESHOLD
(DOWNWARD EXPANSION)
The noise gate threshold is a programmable point using an external
resistor (RGATE) that is connected between Pin 7 (GATE THRS)
and V
−40 dBV and −55 dBV, as shown in Table 5. The downward
expansion threshold is inversely proportional to the value of this
resistance: setting this resistance to 0 Ω sets the threshold at
approximately 10 mV rms (−40 dBV), whereas a 5 kΩ resistance
sets the threshold at approximately 1 mV rms (−55 dBV). This
relationship is illustrated in Figure 19. It is not recommended to
use more than 5 kΩ for the R
of the SSM2167 prevents the noise gate from being lowered
further without causing problems.
Table 5. Setting Noise Gate Threshold
Noise Gate (dBV)
−40
−48
−54
−55
DD
V
. The downward expansion threshold may be set between
DE1
V
DE2
V
Figure 19. Effects of Varying the Downward
DE3
Expansion (Noise Gate) Threshold
r:1
1
INPUT (dB)
1
GATE
resistor because the noise floor
Value of R
0 Ω (short to V+)
1 kΩ
2 kΩ
5 kΩ
V
RP
VCA GAIN
GATE
Rev. C | Page 10 of 12
ROTATION POINT (LIMITING)
Input signals above a particular level, the rotation point, are
attenuated (limited) by internal circuitry. This feature allows the
SSM2167 to limit the maximum output, preventing clipping of
the following stage, such as a codec or ADC. The rotation point
for SSM2167 is set internally to −24 dBV (63 mV rms).
SHUTDOWN FEATURE
The supply current of the SSM2167 can be reduced to under
10 μA by applying an active low, 0 V CMOS-compatible input
to the SHUTDOWN pin (Pin 3) of the SSM2167. In this state,
the input and output circuitry of the SSM2167 assumes a high
impedance state; as such, the potentials at the input pin and the
output pin are determined by the external circuitry connected
to the SSM2167. The SSM2167 takes approximately 200 ms to
settle from a shutdown to power-on command. For power-on to
shutdown, the SSM2167 requires more time, typically less than
1 sec. Cycling the power supply to the SSM2167 can result in
quicker settling times: the off-to-on settling time of the SSM2167 is
less than 200 ms, whereas the on-to-off settling time is less than
1 ms. The SSM2167 shutdown current is related to both temper-
ature and voltage.
PCB LAYOUT CONSIDERATIONS
Because the SSM2167 is capable of wide bandwidth operation
and can be configured for as much as 60 dB of gain, special care
must be exercised in the layout of the PCB that contains the IC
and its associated components. The following applications hints
should be considered for the PCB.
The layout should minimize possible capacitive feedback from
the output of the SSM2167 back to its input. Do not run input
and output traces adjacent to each other.
A single-point (star) ground implementation is recommended
in addition to maintaining short lead lengths and PCB runs. In
applications where an analog ground and a digital ground are
available, the SSM2167 and its surrounding circuitry should be
connected to the analog ground of the system. As a result of
these recommendations, wire-wrap board connections and
grounding implementations are to be explicitly avoided.