ANALOG DEVICES CN-0262 Service Manual

Circuit Note

Microphone Preamp with 18 dB
Fixed Gain

Rev.0

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+

+

+

GND

INPUT AVG CAP

BUFOUT

SHUTDOWN

VCA

IN

V

DD

V

DD

R

GATE

R

COMP

OUTPUT

COMPRESSION

RATIO

GATE THRS

OUTPUT

1

2

4

3

5 6

7

9

8

V

DD

V

DD

VDD

VDD

ADMP504

SSM2167

GND

OUTPUT

SHUTDOWN

GND

10µF

0.1µF
GND

10µF

10µF

10µF

0.1µF

500kΩ

100kΩ

10

10475-001

Circuits from the Lab™ reference circuits are engineered and
tested for quick and easy system integration to help solve today’s
analog, mixed-signal, and RF design challenges. For more
information and/or support, visit www.analog.com/CN0262.

Low Noise Analog MEMS Microphone and Preamp

with Compression and Noise Gating

EVALUATION AND DESIGN SUPPORT

Circuit Evaluation Boards

SSM2167 Evaluation Board (SSM2167Z-EVAL)
ADMP504 Evaluation Board (EVAL-ADMP504Z-FLEX)

Design and Integration Files
Schematics, Layout Files, Bill of Materials

CIRCUIT FUNCTION AND BENEFITS

This circuit, shown in Figure 1, interfaces an analog MEMS
microphone to a microphone preamp. The ADMP504 consists
of a MEMS microphone element and an output amplifier.
Analog Devices’ MEMS microphones have a high signal-to­noise ratio (SNR) and a flat wideband frequency response,
making them an excellent choice for high performance,
low power applications.

The SSM2167 is a low voltage, low noise mono microphone
preamp that is a good choice for use in low power audio signal

CN-0262

Devices Connected/Referenced

ADMP504 Ultralow Noise Analog MEMS Microphone

SSM2167

chains. This preamp includes built-in compression and noise
gating, which gives it an advantage for this function over using
just an op amp in the preamp circuit. Compressing the dynamic
range of the microphone signal can reduce the peak signal levels
and add additional gain to low level signals. Noise gating
attenuates the level of signals below a certain threshold, so that
only desired signals, such as speech, are amplified, and noise in
the output signal is reduced. These features help to improve the
intelligibility of the voice signal picked up by the microphone.

CIRCUIT DESCRIPTION

The ADMP504 analog MEMS microphone is connected to the

SSM2167 INPUT pin through a 0.1 μF capacitor. This coupling

capacitor is necessary because the microphone’s output is biased
at 0.8 V, and the preamp’s input is biased at 0.4 V. The preamp’s
input and the ac coupling capacitor between the microphone
and preamp input form a high-pass filter.

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Figure 1. Analog MEMS Microphone Connection to Preamp

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CN-0262 Circuit Note

LIMITING

REGION

ROTATION POINT

1

r

1

1

DOWNWARD

EXPANSION

THRESHOLD

DOWNWARD

EXPANSION

REGION

V

DE

V

RP

OUTPUT (dB)

INPUT (dB)

COMPRESSION

REGION

10475-002

GAT E

COMP

The −3 dB corner frequency of this filter is 1/(2πRC), where C
is the capacitor size, and R is the preamp’s input impedance
(100 kΩ). For a 0.1 μF coupling capacitor, the high-pass filter’s
corner is 16 Hz. Increasing the capacitor size will lower the filter
corner frequency.

The time constant of the SSM2167’s true rms level detector is
controlled by the size of the capacitor connected to AVG CAP
(pin 6). A 10 μF capacitor results in a time constant of about
100 ms, which is a reasonable setting for speech signals. This
time constant controls the rms detector’s averaging, as well as
the compressor’s release time. A smaller capacitor used here will
give a shorter time constant, and a larger capacitor will result in
a longer time constant. The time constant in milliseconds is
calculated by 10 × C

, where C

AVG

is in μF.

AVG

Both the microphone and the preamp can be powered from a
single 2.5 V to 3.3 V supply.

The SSM2167 preamp requires some additional external passive
components for its operation:

• 10 μF capacitor between V

• 10 μF capacitor between pins VCA

and ground

DD

IN

and BUF

OUT

• AC coupling capacitor on OUTPUT pin

• 500 kΩ pull-up resistor on SHUTDOWN pin

• R

• R

: sets the threshold of the noise gate

GATE

: sets the compression ratio

COMP

The ADMP504 has a −38 dBV sensitivity, which means that an
input signal of 94 dB SPL (1 Pascal) will be output from the
microphone at −38 dBV. This microphone’s maximum input
level is 120 dB SPL, at which level its output will be −12 dBV. Its
dynamic range is 91 dB, so the microphone’s noise floor is at

−103 d B V.

Gain is applied to the SSM2167 input signal in three different
regions—downward expansion, compression, and limiting, as
shown in Figure 2. The SSM2167 applies a fixed 18 dB of gain to
input signals and can also apply an additional variable gain to
signals between the downward expansion point and rotation
point.

The downward expansion threshold is the boundary between
the downward expansion and compression regions. This point
is set by the selection of resistor R

(Table 1). The threshold

GATE

can be set at a point for input signals between 77 dB and 92 dB
SPL, or −55 dBV to −40 dBV input to the preamp.

Table 1. Noise Gate Threshold Settings

Preamp Noise Gate
Threshold (dBV)

Microphone
(dB SPL)

R

(kΩ)

−40 92 0 (short to VDD)

−48 84 1

−54 78 2

−55 77 5

The compression region lies between the downward expansion
and limiting regions. In this region, the dynamic range of the
input signal can be reduced, or compressed, so that the output
signal level is more smooth and constant. The SSM2167 can
achieve compression ratios of up to 10:1. This downward
compressor will increase the level of the signal below the
rotation point threshold. The level of compression is controlled
by resistor R

, as detailed in Table 2.

COMP

Table 2. Compression Ratio Settings

Compression Ratio R

Value (kΩ)

1:1 0 (short to VDD)
2:1 15
3:1 35
5:1 75
10:1 175

Figure 2. Gain Regions

The boundary between the compression and limiting regions is
fixed at a −24 dBV preamp input signal level, which
corresponds to a 108 dB SPL acoustic input to the microphone.
Above this point, after the 18

dB of fixed gain, the preamp

output will be limited to a −6 dBV level.

Figure 3 shows the voltage output levels vs. acoustic input levels
of the circuit for a selection of different compression ratios.

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