CN-0207: High Performance iMems Analog Microphone's Simple Interface to

CN-0207: High Performance iMems Analog Microphone's Simple Interface to
Circuit Note
CN-0207
Devices Connected/Referenced
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/CN0207.
ADAU1761
SigmaDSP Stereo, Low Power, 96 kHz,
24-Bit Audio Codec with Integrated PLL
ADMP404
Analog MEMS Microphone
High Performance Analog MEMS Microphone’s Simple Interface
to SigmaDSP Audio Codec
Circuit Evaluation Boards
ADAU1761 Evaluation Board (EVAL-ADAU1761Z)
ADMP404 Evaluation Board (EVAL-ADMP404Z-FLEX)
Design and Integration Files
Schematics, Layout Files, Bill of Materials
CIRCUIT FUNCTION AND BENEFITS
The circuit shown in Figure 1 allows up to two analog MEMS
microphones to be interfaced to an audio codec. The ADMP404
consists of a MEMS microphone element and an output amplifier.
Analog Devices, Inc., MEMS microphones have a high signalto-noise ratio (SNR) and a flat wideband frequency response,
making them an excellent choice for high-performance, lowpower applications.
Up to two ADMP404 MEMS microphones can be input to an
ADAU1761 low power codec’s two ADCs.
The ADMP404 has a sensitivity of −38 dBV. In most applications, the MEMS microphone outputs must have some
additional gain, which is provided by the ADAU1761 internal
PGAs. The input PGAs can be set for up to 35.25 dB of gain in
0.75 dB steps, with an additional fixed 20 dB boost available
following the PGAs.
The PGA input and the ac-coupling capacitor between the
MEMS microphone and codec form a high-pass filter. The
−3 dB corner frequency of this filter is 1/(2πRC), where C is the
capacitor size, and R is the codec’s input impedance for a given
PGA gain setting. For a +20 dB gain setting (9.1 kΩ input
impedance) and a 2.2 μF capacitor, the high-pass filter’s corner
is 8 Hz. Increasing the PGA gain settings raises the filter corner
frequency, whereas increasing the capacitor size lowers the
corner frequency. See the ADAU1761 data sheet for details on
the input impedance at different settings.
CIRCUIT DESCRIPTION
The ADMP404 analog MEMS microphones are connected to
the ADAU1761 LINN and RINN input pins. These pins are
connected to the inverting inputs of the internal PGAs. The
only necessary passive components in this circuit are a single
0.1 μF bypass capacitor for each ADMP404 and a 2.2 μF
capacitor in series with each MEMS microphone output. The
bypass capacitors should be placed as close to the ADMP404
VDD pin (Pin 3) as possible. The ADAU1761 LINP and RINP
should be connected directly to the codec CM pin.
The ADMP404 power supply is provided from the ADAU1761
MICBIAS pin. MICBIAS can be set to either 0.9 × AVDD or
0.65 × AVDD, where allowable values of AVDD for the
ADAU1761 are between 1.8 V and 3.3 V. The ADMP404 VDD
supply should be between 1.5 V and 3.6 V.
0.1µF
ADAU1761
VDD
LEFT ADMP404 OUTPUT
MICROPHONE
2.2µF
GND
MICBIAS
VDD
ADMP404 OUTPUT
RIGHT
2.2µF
GND
LEFT
PGA
LDBOOST[1:0]
LINP
CM
0.1µF
MICROPHONE
LINN
RINP
–12dB
TO
+35.25dB
RIGHT
PGA
MUTE/
0dB/
20dB
RDBOOST[1:0]
RINN
–12dB
TO
+35.25dB
MUTE/
0dB/
20dB
09777-001
EVALUATION AND DESIGN SUPPORT
Figure 1. Analog MEMS Microphone Connection to the Audio Codec
(Simplified Schematic: Power Supply Decoupling and All Connections
Not Shown)
Rev. A
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CN-0207
Circuit Note
Register Settings
The following register bit fields must be set in the ADAU1761
to enable its PGAs and ADCs for microphone input on the left
channel. Setting 1 to Setting 4 should be repeated for the register
bit fields controlling the right channel if two MEMS
microphones are used. These settings are
1.
2.
3.
4.
5.
6.
7.
MX1EN in Register R4 to 0b1— enables the left channel
input mixer.
LDBOOST[1:0] in Register R5 to 0b01 for 0 dB additional
boost and to 0b10 for 20 dB additional boost.
LDVOL[5:0] in Register R8—sets the PGA input gain
between −12 dB and 35.35 dB.
LDEN in Register R8 to 0b1—enables the left differential
input path.
MBI in Register R10 to 0b0 for 0.90 × AVDD or to 0b1 for
0.65 × AVDD—sets the voltage level of the MICBIAS
output.
MBIEN in Register R10 to 0b1—enables the MICBIAS
output.
LDMUTE in Register R8 to 0b1—unmutes the left
differential input channel. To avoid pops and clicks, this
should be the last bit set.
A screen shot of the SigmaStudio™ register controls for the
analog input is shown in Figure 2.
[PIN 11]
LINN
IN 1
[PIN 10]
LINP
[PIN 6]
LAUX
LINPG[2:0]
MX1AUXG[2:0]
AUX LEFT
INPUT
LEFT
AUXILIARY BYPASS
09777-002
IN 3
The ADMP404 can also be replaced with an ADMP401,
ADMP405, or ADMP504, which are also analog MEMS
microphones. The ADMP401 has a −42 dBV sensitivity,
whereas the ADMP404 has a −38 dBV sensitivity. The
ADMP405 is identical to the ADMP404 except that the
ADMP405 has a low frequency cutoff at 200 Hz vs. the
ADMP404’s 100 Hz cutoff. This higher frequency cutoff makes
the ADMP405 attractive for reducing low frequency wind noise.
The ADMP504 has the same −38 dBV sensitivity as the
ADMP404, but its noise floor is 3 dB lower. The ADMP504 and
ADMP404 are pin and footprint-compatible.
The SigmaStudio GUI software requires a PC with the following:
Windows® 7, Windows Vista, or Windows XP Professional or
Home Edition with SP2, 128 MB of RAM (256 MB recommended), 50 MB of available hard disk space, 1024 × 768 screen
resolution, and USB 1.1/2.0 data port.
LDBOOST[1:0]
LDVOL[5:0]
A mono microphone circuit using a single ADMP404 can be
set up by simply removing one of the ADMP404 MEMS
microphones and its associated capacitors. The other
connections remain the same in this mono configuration.
Equipment Needed
MIXER
ENABLE
LINE P +
This circuit can also be set up with an ADAU1361 instead of an
ADAU1761. The primary difference between these two codecs
is that the ADAU1761 has a SigmaDSP® processor core and the
ADAU1361 does not. The ADAU1781 SigmaDSP codec can also
be used.
Evaluation boards for the ADMP404 and ADAU1761 are
available and can easily be connected as described below.
MIXER 1
–
COMMON VARIATIONS
CIRCUIT EVALUATION AND TEST
LINNG[2:0]
LINE N
A complete design support documentation package for this
circuit note can be found at http://www.analog.com/CN0207DesignSupport.
Figure 2. SigmaStudio PGA Input Configuration for the ADMP404
MEMS Microphone Input to the ADAU1761
In its default performance setting (MPERF = 0), the MICBIAS
output sources up to approximately 2 mA of current to supply
the microphone VDD supply. The ADMP404 draws a maximum
of 250 μA; therefore, this bit does not need to be set to high
performance (MPERF = 1), even to supply power to two
microphones. The codec’s high-performance bias mode is only
needed when biasing electret microphones, not for providing
supply to MEMS microphones.
In addition, the ADAU1761 Evaluation Board (EVALADAU1761Z) and the ADMP404 Evaluation Board (EVALADMP404Z-FLEX) are required.
Getting Started
The EVAL-ADMP404Z-FLEX has three output wires: VDD,
GND, and OUTPUT. The VDD wire should be connected to
J15 or J18 on the EVAL-ADAU1761Z board to supply power
from the ADAU1761 MICBIAS pin. The output wire of the
ADMP404 board can be connected to the tip of a mono 3.5 mm
audio plug, with ground connected to the ring. This plug
connects to the ADAU1761 evaluation board’s two analog input
jacks: J20 and J22. The ADAU1761 evaluation board has 10 μF
Rev. A | Page 2 of 3
Circuit Note
CN-0207
ac-coupling capacitors rather than the 2.2 μF capacitors shown
in Figure 1.
Data Sheets and Evaluation Boards
From this point, follow the documentation for the EVALADAU1761Z regarding software installation, setup, and
operation of the system.
ADAU1761 Evaluation Board
ADAU1761 Data Sheet
ADMP401 Data Sheet
The SigmaStudio software is used to program and tune the
registers and SigmaDSP core in the ADAU1761. SigmaStudio
can be downloaded from http://www.analog.com/sigmastudio.
ADMP404 Data Sheet
ADMP405 Data Sheet
ADAU1361 Data Sheet
Functional Block Diagram
ADAU1781 Data Sheet
The documentation for the ADAU1761 evaluation board
(EVAL-ADAU1761Z) describes the system setup and gives a
complete schematic of the board. The only external connections
required are the USB connection to the PC and to the audio
outputs of the ADAU1761 evaluation board.
ADAU1361 Evaluation Board
Setup and Test
REVISION HISTORY
See the EVAL-ADAU1761Z board documentation for additional details regarding circuit description, jumper settings,
setup, and testing.
12/11—Rev. 0 to Rev. A
ADAU1781 Evaluation Board
ADMP404 Evaluation Board
Changes to Circuit Note Title .......................................................... 1
Changes to Evaluation and Design Support .................................. 1
Changes to Circuit Function and Benefits..................................... 1
Changes to Circuit Description....................................................... 1
Changes to Common Variations ..................................................... 2
LEARN MORE
CN0207 Design Support Package:
http://www.analog.com/CN0207-DesignSupport.
Elko, Gary W., and Kieran P. Harney. "A History of Consumer
Microphones: The Electret Condenser Microphone Meets
Micro-Electro-Mechanical-Systems," Acoustics Today (April
2009).
4/11—Revision 0: Initial Version
Nielsen, Jannik, Hammel Nielsen, and Claus Fürst. “Toward
More Compact Digital Microphones,” Analog Dialogue
(September 2007).
Lewis, Jerad. AN-1112 Application Note, Microphone Specs
Explained. Analog Devices.
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©2011 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
CN09777-0-12/11(A)
Rev. A | Page 3 of 3
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