Low-Noise Directional Studio Microphone Reference Design

Low-Noise Directional Studio Microphone Reference Design
AN-0284
Low-Noise Directional Studio Microphone Reference Design
CIRCUIT FUNCTION AND BENEFITS
The circuit shown in Figure 2 implements a professional-grade studio or live performance microphone using up to 32 analog MEMS
microphones connected to op amps and a difference amplifier. The circuit is designed to be very low-noise and its output is linear
for acoustic inputs up to 131 dB SPL (sound pressure level). The ±9 V and +1.8 V power rails are generated from two voltage
regulators powered from a single +9 V battery.
The INMP411 consists of a MEMS microphone element and an impedance-matching amplifier. This microphone supports acoustic
inputs up to 131 dB SPL and has a low-frequency response that is flat to 28 Hz. These features make this microphone ideal for fullbandwidth, wide dynamic range audio capture applications, such as in a recording studio or on stage.
The ADA4075-2 op amps are used to perform several different functions in this circuit, including a summing amplifier and all-pass
filter. This op amp is low-noise, low-power, and low-distortion, making it a good choice for a battery-powered high-performance
audio application.
The AD8273 converts the single-ended microphone signal into a differential signal that can be output on a standard microphone XLR
connector. The gain setting resistors are internal to the difference amplifier, so it can create a high-performance differential signal
with no external components. The difference amplifier has very low distortion, low noise and good output drive capability, making it
a good choice for driving a differential microphone output.
The power supplies for this circuit are generated from an ADP1613 dc-to-dc switching converter and an ADP1720 linear regulator
(see Figure 9 for schematic). The ADP1613 in a SEPIC-Ćuk configuration generates the ±9 V rails for the amplifiers and the ADP1720
generates the MEMS microphones' 1.8 V supply. These regulators efficiently generate the necessary voltage supplies with very low
ripple.
Figure 1. EVAL-CN0284EB1Z Evaluation Kit
InvenSense reserves the right to change the detail
specifications as may be required to permit improvements
in the design of its products.
InvenSense Inc.
1745 Technology Drive, San Jose, CA 94089 U.S.A
+1(408) 988–7339
www.invensense.com
Document Number: AN-0284
Revision: 1.0
Release Date: Preliminary 2/22/2014
AN-0284
100pF
+1.8V
VDD
INMP411
GND
2.49k Ω
OUTPUT
47µF
750Ω
−
+9V
9V
BATT
20kΩ
+
1/2
×16
+9V
ADP1613,
ADP7120
POWER
SUPPLY
CIRCUIT
(SEE FIGURE 8)
+
−9V
ADA4075-2
−9V
+1.8V
+1.8V
GND
2.49k Ω
OUTPUT
100pF
+1.8V
VDD
INMP411
GND
2.49k Ω
OUTPUT
750Ω
47µF
−
20k Ω
×16
+1.8V
VDD
INMP411
GND
1/2
LOW-PASS
SHELVING FILTER
442 Ω
2.49k Ω
−9V
ADA4075-2
1.27k Ω
10nF
+9V
−
2.49k Ω
+
1/2
−9V
ADA4075-2
10nF
1.27kΩ
+9V
−
+
−9V
1/2
ADA4075-2
0.1µF
AD8273
1.27kΩ
1.27kΩ
10k Ω
−
+
6kΩ
−
+9V
−9V
1/2
+9V
12kΩ
3.92k Ω
2.49k Ω
+9V
2.49k Ω
+
INVERTING
ALLPASS FILTER
NON-INVERTING
ALLPASS FILTER
+
12kΩ
6kΩ
12kΩ
6kΩ
ADA4075-2
−
2.49k Ω
49.9Ω
47µF
49.9Ω
47µF
XLR MALE
VDD
INMP411
+
OUTPUT
12kΩ
6kΩ
−9V
Figure 2. Microphone Circuit Diagram
(Simplified Schematic: All Connections and Decoupling Not Shown)
CIRCUIT DESCRIPTION
An array of many INMP411 MEMS microphones are closely spaced in this circuit to improve the overall SNR of the system to a point
that it can be used for very low noise recording studio applications. The circuit can be used with either one or two clusters of
microphones, depending on the desired directionality. Using two microphone arrays and some simple filtering enables some basic
beamforming to achieve a supercardioid directional response; while the directional response of a single microphone array is
basically omnidirectional, like the response of a single MEMS microphone.
Following the summing, filtering, and beamforming, the signal is still single-ended. The AD8273 difference amplifier converts this
single-ended signal into a differential signal that is output on the XLR jack.
This circuit can be powered from a single 9 V battery, and two regulators generate the ±9 V and +1.8 V supplies. The amplifiers use
the bipolar supply, while the MEMS microphones operate from the single low-voltage 1.8 V supply.
Summing Amplifiers
The ADA4075-2 is used as a summing amplifier in two places in this circuit. First, the outputs of each of the 16 INMP411 mics in the
array are summed together in an inverting summing amplifier with a gain of 0.31. The input summing resistors are all 2.49 kΩ,
therefore the contribution of each microphone to the output is equal. Every time the number of microphones in the array is
doubled, the overall SNR increases by about 3 dB. This is because the microphone signals sum coherently, increasing the amplitude
of the output by 6 dB for each doubling of the number of microphones, while the noise adds incoherently, adding 3 dB to the noise
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floor. The result is a net increase of 3 dB in the SNR. This circuit uses 16 INMP411 microphones, each with an individual SNR of 62
dBA, therefore after four doublings of the number of microphones, the SNR of the array improves by 12 dB to 74 dBA.
The INMP411 has an output impedance of 200 Ω, so the 2.49 kΩ summing resistor reduces the amplitude of each mic’s output signal
by 7.5%, or 0.7 dB.
The feedback resistor in the summing amplifier is 750 Ω, which gives the amplifier an overall gain of 0.31 (−10 dB) following the
summing node. The ADA4075-2 is unity gain stable, so this amplifier can be used for a gain <1. If another op amp were to be used in
this circuit, its closed-loop gain should be checked to ensure that it is stable at this low gain.
This reduction in level ensures the output at the XLR plug is similar to a typical recording microphone. A microphone with a
sensitivity above about −30 dBV may be considered too “hot” and therefore difficult to use in a typical recording setup. If a
microphone with higher sensitivity is desired, then the size of this feedback resistor can be increased thereby increasing the gain.
Other than maintaining typical microphone output levels, the output level is limited only by the supply voltages.
The input of the ADA4075-2 is also ac-coupled because the INMP411 output is biased at 0.8 V. The output of this stage and the rest
of the amplifier stages are biased at 0 V.
The second ADA4075-2 summing amplifier is used either to sum two in-phase arrays or to sum one microphone array with another
delayed and inverted array for beamforming processing.
Polarity
The amplifier summing the signals from the 16 INMP411 microphones is inverting. This is done to preserve the polarity of the input
acoustic signal. The INMP411 has an acoustically-inverted output, meaning that a positive pressure input results in a negative output
voltage. This signal is inverted in the first amplifier stage so that it is not inverted with respect to the acoustic input at the XLR output
jack. All other stages in the signal path are non-inverting, except for the inversion necessary in one of the two microphone paths for the
beamforming processing.
Beamforming
Beamforming involves processing the output of multiple microphones (or in this case, multiple mic arrays) to create a directional
pickup pattern. For recording and live sound applications it is important that the microphone only picks up sound from one
direction, such as from the singer or instrument, and attenuates the sound that is off the main axis. Beamforming is implemented in
this design using analog delays, an equalization filter, and a summing amplifier.
0°
+
d
–
DELAY
Figure 3. Endfire Beamforming Array
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As shown in Figure 3, a two-element array is set up by placing two microphone boards distance, d, apart. A cardioid pattern
(Figure 4) is achieved by delaying the signal from one array board by amount of time it takes sound to travel between the two
boards, and subtracting this delayed signal from the signal from the first microphone array board. With this type of spatial response,
the microphone rejects sounds from the sides and rear, while picking up sounds incident to the front of the microphone.
0°
0dB
–30°
30°
–10dB
–20dB
–60°
60°
–30dB
–90°
90°
–30dB
–30dB
–20dB
–120°
120°
–10dB
–150°
0dB
150°
180°
Figure 4. Cardioid Response
Allpass Filters
In this design, the two microphone array boards are separated by about 18 mm. To create a perfect cardioid pattern, the rear
microphone array’s output needs to be delayed by 52.4 μs. A series of two allpass filters are used to delay the signal as shown in
Figure 5.
INVERTING
ALLPASS FILTER
NON-INVERTING
ALLPASS FILTER
2.49k Ω
2.49k Ω
2.49k Ω
1.27k Ω
10nF
+9V
−
2.49k Ω
+
1/2
−9V
ADA4075-2
10nF
1.27kΩ
−
+
+9V
−9V
1/2
ADA4075-2
Figure 5. Non-Inverting and Inverting Allpass Filters
In the allpass filter design, there is a trade-off between the length of the delay and the frequency range over which the delay is
constant. As the group delay at lower frequencies is increased, the group delay is constant over a smaller frequency range. In this
circuit, each allpass filter contributes about 25 μs of delay up to about 4 kHz (with 10% tolerance). Above this frequency, the group
delay at the filter output starts to decrease rapidly (Figure 6), which quickly degrades the performance of the beamforming
algorithm. At these higher frequencies, the primary contribution to the design’s directionality is the physical size of each microphone
board (1.1” diameter).
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60
GROUP DELAY (µs)
50
40
30
20
10
0
10
100
1k
10k
FREQUENCY (Hz)
Figure 6. Two-Stage Allpass Filter Group Delay
The delayed signal from the rear array also needs to be inverted so that it can be subtracted from the front array’s signal in the
summing amplifier circuit. This is done by implementing the first allpass filter with a non-inverting output and the second allpass
filter with an inverting output.
The group delay (tG) of the filter is calculated by the equation:
tG =
2 RC
1 + ( ff0 ) 2
Where f0 = 1/(2πRC). In this design, R = 1.27 kΩ and C = 10 nF, so f0 is 12.5 kHz. The design and equations for these allpass filters
were taken from Analog Devices Tutorial MT-202 and the Linkwitz Lab website.
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Shelving Filter
The frequency response of the signal following the beamforming processing has a first-order roll off at low frequencies. If this is
uncorrected, the microphone’s output has poor low-frequency response, therefore a shelving filter is used to boost the response at
these frequencies. The first-order non-inverting shelving low-pass filter shown in Figure 7 is designed to have a gain of 20 dB at low
frequencies with a shelf rising from 4 kHz down to 400 Hz. The frequency response is shown in Figure 8. This shelving filter flattens
out at 20 dB of gain at about 100 Hz, so the microphone’s response still rolls off with a −6 dB/octave slope below this.
Summing the outputs from the front and rear microphone arrays boards also is implemented in this amplifier.
0.1µF
3.92kΩ
442Ω
MIC1+
+9V
−
1.27kΩ
MIC1–
+
1.27kΩ
10kΩ
−9V
1/2
ADA4075-2
Figure 7. Shelving Filter
25
GAIN (dB)
20
15
10
5
0
10
100
1k
FREQUENCY (Hz)
Figure 8. Shelving Filter Response
The gain (VOUT/VIN) of the filter is calculated by the equation:
1+
VOUT
= G1
VIN
1+
( )
()
f 2
f2
f 2
f1
Where G1 = f2/f1 = 1+R2/R1. The filter’s corner frequencies f1 and f2 are calculated by:
f1 =
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1
2πCR2
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AN-0284
f2 =
1
 R1R 2 
2πC 

 R1 + R 2 
In this design, R1 = 442 Ω, R2 = 3.92 kΩ and C = 0.1 μF, so f1 is 406 Hz and f2 = 4.1 kHz. The design and equations for this shelving
lowpass filter were also taken from the Linkwitz Lab website.
If a single microphone array board is used in the design, then the allpass and shelving filter circuits are not needed, and the output
of the summing amplifier can be connected directly to the AD8273 for the single-ended-to-differential conversion.
Single-Ended-to-Differential Conversion
The AD8273 difference amplifier performs the single-ended-to-differential conversion for the output signal. This amplifier is
configured for a gain of 1 (0 dB) to keep the microphone’s output level from being too high. One of the internal amplifiers of the
AD8273 is set up in a non-inverting configuration, and the other is inverting. Individually, each of these amplifiers has a gain of 0.5
(−6 dB), but the single-ended-to-differential conversion results in a +6 dB gain, resulting in the overall 0 dB gain in this sub-circuit.
The AD8273 does not require any external components except for the power supply decoupling capacitors; all gain-setting resistors
are internal to the IC, so they are very well-matched, and the amplifier’s output has very low distortion.
The AD8273 has good output drive capability and can easily drive a highly capacitive load. This is necessary because the output
might be connected to a long (multiple meters) XLR cable.
The outputs of the AD8273 drive 49.9 Ω series resistors and a 47 μF ac-coupling capacitor. The series capacitor is necessary because
the microphone could be connected to an input that provides +48 V phantom power, which is often used as a bias and supply for an
electret microphone. Phantom power can typically only supply less than 10 mA, which is more than this circuit uses, so it is not used
as the supply. The capacitor’s rating is 63 V so the circuit is protected from the 48 V bias that could be present on both the + and −
outputs.
±9 V Power Supply
All of the amplifiers in this design are powered from ±9 V supplies. These voltages are generated from an ADP1613 in a SEPIC-Ćuk
configuration. This circuit generates the positive and negative supply rails from a single input voltage as shown in Figure 8.
The design for the bipolar supply design was created using Analog Devices’ ADIsimPower™ tools. The ADP161x SEPIC-Ćuk Design
Tool takes some basic design parameters as an input and generates a schematic, BOM, and performance specifications for the given
circuit. This design was created using the following specifications:
•
VIN minimum = 7.5 V
•
VIN maximum = 9.0 V
•
VOUT = 9.0 V
•
IOUT = 40 mA
•
Ambient temperature = 55°C
•
Design optimized for lowest cost
The total current drawn from the battery when the complete system is operational is 82 mA. Of this total current, about 17.5 mA is
used by each microphone board, and 47 mA is used by the power board. With this load, a typical 9V battery lasts about five hours.
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±9V SUPPLY
47µH
C 3
1.0µF
1
B
47µH
VIN
SW 5
3
EN
FB 2
SS
COMP 1
1.0µF
0Ω
FREQ
GND
7
4
10µF
VBATT
2
3
+9V
1.0µF
1.0µF
6
8
1.0µF
1
4
1.0µF
AVDD
ADP1720
ADP1613
E 2
5.1V
+
1.0µF
+
10µF
1.0µF
71.5kΩ
2
IN
OUT 3
4
EN
ADJ 1
5
GND
162Ω
+
GND
12.4kΩ
3
2
GND
4
1
GND
VBATT
+1.8V SUPPLY
–9V
6
7
8
24.3kΩ
1.0µF
49.9kΩ
11.3kΩ
10nF
100pF
5.36kΩ
Figure 9. Details of Power Supply Circuit
+1.8 V Power Supply
The +1.8 V supply is used to power the INMP411 MEMS microphones and is generated from the ADP1720 linear regulator. This
regulator drops the regulated +9 V supply to the necessary 1.8 V in a very small PCB footprint, requiring only one small (1 μF) bypass
capacitor on both the input and the output and two resistors to set the output voltage. The INMP411 draws a maximum of 220 μA
with a 1.8 V supply, so the highest current needed from this regulator output (with 32 INMP411s connected) is 7.04 mA. At this
maximum load, the regulator dissipates 50.7 mW:
P = (9 V – 1.8 V) × 7.04 mA = 50.7 mW
Additional Circuitry
This section describes the function of additional components that are used on the EV_CN0284-EB1Z but are not part of the core
circuit design. This primarily includes RF filtering and overvoltage protection circuitry
RF Filtering
Between the AD8273's differential output and the XLR plug, there are a few components intended to filter high-frequency noise that
may be picked up by the board or the microphone XLR cable. Each half of the differential signal has an LC filter, to remove EMI or RF
noise. There is also a common mode choke between the two differential legs to remove common mode currents, while allowing
differential currents to pass.
The connection of the microphone signals between the microphone boards and the power board also have an LC filter to reduce
high-frequency noise that may be picked up by the ribbon cable connection. A similar LC filter (Figure 10) is also used on the output
of all of the regulated voltage supplies.
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SUPPLY
VOLTAGE
REGULATOR
OUTPUT
1800Ω AT 100MHz
1.0nF
Figure 10. RF Noise Filter
Spike Suppression
Back-to-back 5.1 V Zener diodes are connected between each side of the differential output signal and ground. These diodes are
used to clamp voltage spikes greater than ±5.1 V that may be conducted on the output cable.
Low Battery Indicator
LED D11 is used to indicate when the battery voltage is low and should be replaced. The LED is placed in series between the battery
positive terminal and the regulated +9 V supply. When the battery voltage droops below 9 V by more than the LED forward voltage (2 V),
then the LED indicator turns on. The output voltage of many 9 V batteries starts to drop rapidly once it falls below 7 V.
PERFORMANCE AND MEASUREMENTS
Sensitivity Performance
The sensitivity of the microphone array is higher than that of each individual microphone because their outputs are summed. The
INMP411 has a sensitivity of −46 dBV. With the outputs of sixteen microphones summed together, and the other gains applied to the
signal path, the sensitivity of the circuit with a single array is −33 dBV. If the beamforming part of the circuit is enabled when two array
boards are used, the sensitivity is −27 dBV.
Frequency Response Performance
15
NORMALIZED AMPLITUDE (dB)
10
5
0
–5
–10
100
1k
FREQUENCY (Hz)
Figure 11. On-Axis (0°) Frequency Response
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AN-0284
The frequency response of the microphone design is dependent on the orientation of the microphone. Figure 11 shows the
frequency response for the sound incident on-axis. Off-axis, there is less of a low-frequency roll-off caused by the beamforming
processing, so the fixed shelving filter results in a higher low frequency boost.
The dip in the response at 8 kHz is a result of the beamforming processing; this is the point at which the inverted signal from the rear
array board is most nearly in-phase with the non-inverted front microphone signal, resulting in significant attenuation of the overall
subtracted output signal.
The summing amplifers and single-ended-to-differential circuit all have very flat frequency response across the audio bandwidth, so
they do not contribute significantly to the overall frequency response of the system design. The AD8273 has very good output drive
capability, so it doesn’t contribute a roll-off at high frequencies, even with long, highly-capacitive XLR cables connected.
Noise Performance
Summing the outputs of 16 INMP411 microphones theoretically improves the SNR by 12 dB from that of a single microphone. For each
doubling of the number of microphones in the array, the SNR should increase by about 3 dB. This is because the signals add
coherently and increase the amplitude by 6 dB, while the noise adds incoherently for a 3 dB increase in noise level.
The INMP411 has an A-weighted 62 dB SNR, so the array of 16 microphones has a theoretical SNR of about 74 dBA. For a −33 dBV
sensitivity, this means that the noise floor across a 20 kHz bandwidth is at −107 dBV (4.47 μV rms). This is equal to an acoustic noise
floor of 20 dB SPL. In practice, the measured noise floor is typically 1-2 dB higher than this, possibly because the spacing of the 16
microphones in the summing array is large enough that the individual microphone signals are not perfectly coherent.
The op amp and difference amplifier circuits following the microphones are significantly lower-noise than the microphones
themselves, so the microphones are the limiting factor in the noise of the overall design.
When two of the 16× microphone array boards are used for beamforming, the overall system SNR is degraded by about 3 dB. The
EV_CN0284-EB1Z board’s SNR when configured in a beamforming array is about 71 dB, which is an equivalent acoustic noise floor
(or equivalent input noise) of 21 dB SPL.
THD & Linearity Performance
The primary contribution to the distortion in the circuit comes from the INMP411 microphones. The other analog amplifier
components operate in a linear region, with the signals not close to the supply rails. The INMP411 has an acoustic overload point of
131 dB; this is the point at which THD is 10% for an individual microphone and is commonly referred to as the clipping point in audio
applications. All microphones in the array are simultaneously exposed to similar SPLs, so the circuit’s total distortion curve looks very
similar to that of an individual microphone. The distortion curve for the INMP411 is shown in Figure 12.
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THD + N (%)
10
1
0.1
90
100
110
120
130
140
INPUT LEVEL (dB SPL)
Figure 12. INMP411 dB SPL vs %THD+N
With a −27 dBV sensitivity, the microphone’s output with 131 dB SPL input is 2.27 VRMS (6.40 VP-P). This is well within the linear region
for the amplifiers operating with a ±9 V supply, so the linearity and distortion is controlled by the INMP411 performance. Figure 13
shows the linearity (input in dB SPL vs. output in dBV) for this circuit design.
15
10
OUTPUT AMPLITUDE (dBV)
5
0
–5
–10
–15
–20
–25
–30
–35
90
100
110
120
INPUT LEVEL (dB SPL)
130
140
Figure 13. Measured Linearity of Circuit
Directionality Performance
The INMP411 microphones are omnidirectional, when used individually. When put in a large array like this, the design has some
directionality. That is, the level of the output depends on the orientation of the array with regard to the position of the sound
source.
A single array board exhibits little directionality at low frequencies (<4 kHz). In the mid frequency range, the array boards attenuate
the sounds from the rear and sides by as much as 5 dB to 6 dB because of the acoustic shadowing from the PCB itself. At some
higher frequencies where the wavelength of sound is on the order of the size of the array PCB, such as around 8 kHz, the array has
significant directionality. Directionality measurements with a single microphone board are shown in Figure 14.
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345
0
0
15
–2
330
30
–4
315
45
–6
–8
300
–10
60
–12
–14
285
75
–16
–18
–20
270
90
105
255
240
120
225
135
150
210
195
180
165
250Hz
500Hz
1kHz
2kHz
4kHz
8kHz
Figure 14. Directionality of a Single Array Board at Different Frequencies
The primary purpose of having two array boards spaced a fixed distance apart from each other is to create a directional response. The
combination of the distance between the two boards and the beamforming circuit results in an array that has considerable rejection of
sounds from the sides and rear of the assembly. Figure 15 shows the response of two boards whose faces are spaced 18 mm apart
from each other. The measurements of this setup show a supercardioid directional response, with significant attenuation of sounds
from the rear and sides, but with a small rear lobe. The nulls in the directional response of this design are at about 135° and 225°. At
most frequencies in the voice range (250 Hz to 4 kHz), there is at least 15 dB of off-axis attenuation.
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340
350
330
0
0
10
20
30
–5
320
40
–10
310
50
–15
–20
300
60
–25
290
70
–30
280
80
–35
270
90
–40
100
260
250
110
240
120
230
130
220
140
150
210
200
190
180
170
160
150Hz
250Hz
500Hz
1kHz
2kHz
4kHz
8kHz
Figure 15. Directionality of a 2-Board Endfire Array at Different Frequencies
COMMON VARIATIONS
Other op amps or difference amps can be used, depending on the need of the specific design. Many op amps with lower noise, lower
power, or lower operating voltage rails than the ADA4075-2, some of which may be a better choice if the design parameters are
different than what’s presented here. Application Note AN-1165 lists many op amps that would be suitable alternatives.
The ADMP510 is another analog microphone in an even smaller package and has a lower noise floor than the INMP411. The ADMP510
has an SNR of 65 dBA, while the mic in this design has a 62 dBA SNR. A summed array of 16 ADMP510 microphones could have an
SNR as high as 77 dBA. However, the ADMP510’s acoustic overload point is 124 dB SPL, compared with 131 dB SPL for the INMP411.
This makes the ADMP510 a better choice for a microphone designs that may not be used in loud environments and require a lower
noise floor.
A fixed-output version of the ADP1720 could also be used if the microphones operate from a 3.3 V supply. This version would
eliminate the need for the two external output voltage setting resistors.
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CIRCUIT EVALUATION AND TEST
A hardware evaluation kit for this reference circuit is available as part number EV_CN0284-EB1Z.
Equipment Needed
The EV_CN0284-EB1Z kit includes:
•
•
•
•
Two microphone array boards with sixteen INMP411 microphones.
One output and power board that includes the voltage regulators and output amplifiers.
Two ribbon cables to connect the two boards.
One +9 V battery.
The evaluation kit’s output is on a standard XLR plug that can be connected to an audio preamp, mixer, sound card, or any other
microphone interface.
Getting Started
The evaluation kit can be configured with either one or two microphone array boards. Below are the steps to follow for evaluating
the circuit.
1.
2.
3.
4.
5.
Using the provided 10-pin ribbon cables, connect the microphone array boards to the power board. If just one microphone
board is used, connect it to header J1 on the power board. If using two microphone boards, the front microphone board
should be connected to J1 and the rear microphone board to J2.
Set switch S2 on the power board either to add (+ position, switch down) or take the difference of (− position, switch up)
the signals from the two microphone boards. If you are configuring a beamforming array with two microphone boards, set
the switch for the difference of the two signals; this signal path also enables the low-pass shelving filter. If using just one
microphone board, set the switch to +.
Insert a 9 V battery in the battery pack BT1 on the back side of the power board. If a battery is not available, power from an
external source can be connected to header J4, with the positive voltage on the bottom pin and ground on the top.
Connect an XLR cable between the output plug (J3) on the power board and the input of a microphone preamp or mixer.
Turn on the circuit by moving switch S1 to the right. This connects the battery to the power circuit and the evaluation kit is
now active.
Document Number: AN-0284
Revision: 1.0.
Page 14 of 24
AN-0284
Figure 16. Two Microphone Boards with 18 mm Spacer
Document Number: AN-0284
Revision: 1.0.
Page 15 of 24
AN-0284
SCHEMATICS, LAYOUTS, AND BILLS OF MATERIALS
AVDD
C1
0.10uF
5
VDD
R1
1
GND
GND
GND
OUTPUT
4
2
3
INMP411_MEMS_MIC
2k43
AVDD
U1
C2
0.10uF
5
VDD
R2
1
GND
GND
GND
OUTPUT
4
2
3
INMP411_MEMS_MIC
AVDD
U2
C21
2k43
U17-C
ADA4075-2ARZ
+9V
C5
V+
V-
0.10uF
0.10uF
C4
4
2
3
INMP411_MEMS_MIC
GND
GND
3
GND
GND
2
GND
GND
4
-9V
4
2
3
0.10uF
GND
GND
4
2
3
AVDD
GND
GND
GND
3
GND
GND
2k43
+
47uF
4
2
3
2
3
+
O
-9V
1
R24
20k0
R19
R18
6
2k43
5
R20
0.10uF
GND
GND
GND
4
2
3
INMP411_MEMS_MIC
AVDD
U10
10nF
2k43
AVDD
C13
C11
0.10uF
0.10uF
R11
1
C12
4
2
3
INMP411_MEMS_MIC
AVDD
U13
GND
GND
GND
4
2
3
R12
2k43
INMP411_MEMS_MIC
U12
0.10uF
5
VDD
R14
1
OUTPUT
GND
1
OUTPUT
2k43
C14
0.10uF
5
VDD
GND
3
R13
1
OUTPUT
2k43
GND
2
AVDD
GND
GND
GND
4
INMP411_MEMS_MIC
U11
GND
GND
OUTPUT
5
VDD
GND
5
VDD
4
2
3
INMP411_MEMS_MIC
AVDD
U14
C15
2k43
0.10uF
5
VDD
R15
1
GND
GND
GND
OUTPUT
4
2
3
INMP411_MEMS_MIC
AVDD
U15
2k43
C16
0.10uF
5
VDD
1
GND
GND
GND
OUTPUT
4
2
3
R16
2k43
INMP411_MEMS_MIC
U16
Figure 17. Microphone Board Schematic
Document Number: AN-0284
Revision: 1.0.
Page 16 of 24
+
2k43
U18-B
O
R21
7
ADA4075-2ARZ
2k43
2
-
3
+
C20
Inverting Allpass filter
C19
R10
1
R22
2k43
1k27
OUTPUT
1
3
5
7
9
HEADER5X2_0.05IN_SM
U17-A
2k43
INMP411_MEMS_MIC
U8
C10
5
VDD
J1
ADA4075-2ARZ
R8
1
GND
INMP411_MEMS_MIC
AVDD
C17
C8
OUTPUT
U9
2k43
0.10uF
5
VDD
0.10uF
OUTPUT
750R
R7
INMP411_MEMS_MIC
U7
R9
2
4
6
8
10
R17
1
TP6 TP7
+9V
100pF
C9
2
-9V
C18
C7
GND
AVDD
4
U17-B
7
O
+ADA4075-2ARZ
AVDD
2k43
AVDD
U6
OUTPUT
1
-
TP1 TP2 TP3 TP4 TP5
INMP411_MEMS_MIC
5
VDD
5
VDD
5
R6
1
OUTPUT
2k43
INMP411_MEMS_MIC
U4
6
0.10uF
5
VDD
R4
1
OUTPUT
8
4
C24
C6
0.10uF
5
VDD
V+
V-
4
2k43
AVDD
U5
8
C23
R5
1
GND
3
AVDD
U3
OUTPUT
2k43
GND
GND
GND
2
INMP411_MEMS_MIC
GND
GND
4
5
VDD
R3
1
OUTPUT
U18-C
ADA4075-2ARZ
+9V
AVDD
C3
5
VDD
C22
10nF
U18-A
O
1
ADA4075-2ARZ
Non-Inverting Allpass filter
R23
1k27
AN-0284
+9V
C7
TP5 TP4 TP3 TP2 TP1
+9V
+9V
C9
L1
HEADER5X2_0.05IN_SM
-9V
0.10uF
-9V
MIC1-
1800 Ohm @ 100 MHz
C1
100pF
MIC1+
R7
R8
OPEN
100pF
6
R1
MIC1+
1
+INA
6
REFA
SENSEB
-INB
SPDT_SLIDE_CAS-120TA_SMD
OUTB
5
1 +INB
7 NC
NC
REFB
12
5.1V
C11
R11
13
49R9
47uF
14
C13
+
100pF
C31
10
100pF
C12
R12
9
49R9
8
47uF
D3
C14
+
1
4
.
.
J3
2
2
3
3
1
L7
COM_MODE_CHOKE_SMD_2020
100pF
+
G
L6
5.1V
1800 Ohm @ 100 MHz
4
C10
R9
2
R4
MIC2-
L3
R5
1800 Ohm @ 100 MHz
L4
-9V
3
2
3k92
R3
C4
Microphone 2 Input
-9V
U2-A
+
O
1
R6
10k0
C5
MIC2+
1800 Ohm @ 100 MHz
D4
5.1V
0.10uF
ADA4075-2ARZ
MIC2-
0R00
-
3
1k27
HEADER5X2_0.05IN_SM
OUTA
S2
3
0.10uF
1k27
2
4
6
8
10
SENSEA
TP11TP10
1800 Ohm @ 100 MHz
D2
-INA
Bottom: Sum out of phase
442R
+9V AVDD
1
3
5
7
9
+
7
C6
R10
TP9 TP8
J2
O
L5
U1
Top: Sum in phase
U2-B
ADA4075-2ARZ
R2
1k27
MIC1+
TP14TP13
-
5
1k27
MIC2+
2
0R00
C2
HEADERS 1 & 2
1, 3, 5, 7, 9: GND
2: -9V
4: +9V
6: AVDD (MEMS mic power)
8: Inverted mic signal
10: Non-inverted mic signal
D1
5.1V
AD8273
11
1800 Ohm @ 100 MHz
L2
+VS
2
4
6
8
10
0.10uF
-VS
J1
4
C8
AVDD
1
3
5
7
9
8
V+
V-
XLR-MALE
Microphone 1 Input
TP7 TP6
U2-C
ADA4075-2ARZ
0.10uF
OPEN
C3
100pF
100pF
-9V
L10
1800 Ohm @ 100 MHz
D7
1.0uF
C23
1.0uF
.
.
+
10uF
C24
C25
1.0nF
C21
3
2
C3
Q1
1
B
.
.
COUPLED_INDUCTOR_47UH
1
2
C19
1.0uF
4
L8
E 2
C28
L11
C22
1.0uF
1.0uF
1.0uF
+
C26
10uF
C27
6
8
U3
ADP1613ARMZ
VIN
SW
EN
FB
SS
COMP
R18
K
1
71K5
FREQ 7 4 GND
0R00
+/-9V Supply
D8
Green Diffused
R17
11k3
R16
C16
1.0uF
C17
C18
10nF
100pF
+9V
R15
5k36
Low Battery LED
VBATT
R22
249R
A
K
D11
Red Diffused
Figure 18. Power Board Schematic
Document Number: AN-0284
Revision: 1.0.
Page 17 of 24
EN
ADJ
1
R19
VBATT
1800 Ohm @ 100 MHz
24k3
U4
R20
49k9
C29
1.0uF
SPDT_SLIDE_EG1218
S1
3
2
1
D10
C30
D9
1.0nF
9V_BATTERY_HOLDER
A
5
2
4
9V Battery Connection
AVDD
L12
BT1
R21
499R
1.0nF
3
C15
D6
1800 Ohm @ 100 MHz
D5
5.1V
1.0uF
ADP1720ARMZ-R7
2
3
IN
OUT
+9V
C20
3
+1.8V Supply
VBATT
COUPLED_INDUCTOR_47UH
R14
162R
R13
12k4
1
J4
4
1
L9
5 GND
6 GND
7 GND
8 GND
VBATT
2
AN-0284
TOP VIEW
BOTTOM VIEW
Figure 19. Microphone Board Layout
Document Number: AN-0284
Revision: 1.0.
Page 18 of 24
AN-0284
TOP VIEW
BOTTOM VIEW
Figure 20. Power Board Layout
Document Number: AN-0284
Revision: 1.0.
Page 19 of 24
AN-0284
TABLE 1. MICROPHONE BOARD BILL OF MATERIALS
Qty
Reference
Value
Manufacturer
Part Number
20
C1-16 C21-24
0.10uF
Murata ENA
GRM155R71C104KA88D
1
R24
20k0
Yageo
RC0402FR-0720KL
1
100pF
Murata ENA
GRM1555C1H101JZ01D
20
C18
R1-R16 R18-19
R21-22
2k49
Yageo
2
C19-20
10nF
TDK Corp
C1608C0G1E103J
1
C17
47uF
Kemet
T520B476M010ATE035
2
R20 R23
1k27
Vishay/Dale
CRCW04021K27FKED
2
U17-18
ADA4075-2ARZ
Analog Devices Inc.
ADA4075-2ARZ
16
U1-16
INMP411
InvenSense, Inc
INMP411
1
J1
2x5 0.05" SM
SAMTEC
SHF-105-01-L-D-SM
7
TP1-7
5002
Keystone Electronics
5002
1
R17
750r
Mini Test Point White .1" OD
Chip Resistor 1% 63mW Thick
Film 0402
SAMTEC
FFSD-05-D-03.00-01-N-R
2x5 0.05" spacing, 3" cable
1
Description
Multilayer Ceramic 16V X7R
(0402)
Chip Resistor 1% 63mW Thick
Film 0402
Multilayer Ceramic 50V NP0
(0402)
Chip Resistor 1% 63mW Thick
Film 0402
Multilayer Ceramic 25V NP0
(0603)
Tantalum Capacitor 105deg
SMD
Chip Resistor 1% 63mW Thick
Film 0402
Ultralow Noise Amplifier at
Lower Power
Analog Microphone
5 x 2 Shroud Polarized Header
0.05inch Surface Mount
TABLE 2. POWER BOARD BILL OF MATERIALS
Qty
Reference
Value
1 BT1
Manufacturer
Part Number
MPD
BH9VPC
2 C11-12
C1-4 C13-14
8 C18 C31
C15-16 C19-20
8 C22-23 C28-29
47uF
Panasonic
EEE-1JA470UP
100pF
Murata ENA
GRM1555C1H101JZ01D
1.0uF
Murata ENA
GRM188R71C105KA12D
1 C17
10nF
TDK Corp
C1608C0G1E103J
1 C21
1.0uF
Panasonic EC
ECJ-3YB1E105K
2 C24 C26
10uF
Rohm
TCA1C106M8R
3 C25 C27 C30
1 C5
1.0nF
OPEN
Murata ENA
Do Not Stuff
GRM1555C1H102JA01D
Do Not Stuff
Document Number: AN-0284
Revision: 1.0.
Page 20 of 24
Description
9V Battery Holder, PC
Mount
Electrolytic Capacitor
85deg SMD
Multilayer Ceramic 50V
NP0 (0402)
Multilayer Ceramic 16V
X7R (0603)
Multilayer Ceramic 25V
NP0 (0603)
Multilayer Ceramic 25V
X7R 1206
SMD Tantalum
Capacitor 1206 16V
Multilayer Ceramic 50V
NP0 (0402)
Do Not Stuff
AN-0284
5 C6-10
0.10uF
Murata ENA
GRM155R71C104KA88D
1 D11
Red Diffused
Lumex Opto
SML-LX1206IW-TR
5 D1-5
5.1V
Diodes, Inc
DDZ9689-7
4 D6-7 D9-10
BAT54T1G
On Semiconductor
BAT54T1G
1 D8
Green Diffused
Lumex Opto
SML-LX1206GW-TR
2 J1-2
2x5 0.05" SM
SAMTEC
SHF-105-01-L-D-SM
1 J3
NC3MAH
Neutrik
NC3MAH
1 J4
Sullins Electronics
Corp
PBC02SAAN; or cut
PBC36SAAN
9 L1-6 L10-12
2-Jumper
1800 Ohm @ 100
MHz
muRata
BLM15HD182SN1
1 L7
DLW5BTN101SQ2L Murata Electronics
DLW5BTN101SQ2L
2 L8-9
47uH
Coilcraft
LPD4012-473
1 Q1
MMBT3904LT1G
ON Semiconductor
MMBT3904LT1G
1 R10
442R
Stackpole
RMCF0402FT442R
2 R11-12
49R9
Yageo
RC0402FR-0749R9L
1 R13
12k4
Panasonic EC
ERJ-2RKF1242X
1 R14
162R
Panasonic ECG
ERJ-2RKF1620X
4 R1-4
1k27
Vishay/Dale
CRCW04021K27FKED
1 R15
5k36
Yageo
RC0402FR-075K36L
1 R17
11k3
Yageo
RC0402FR-0711K3L
1 R18
1 R19
71K5
24k3
Yageo
Stackpole
RT0402FRE0771K5L
RMCF0402FT24K3
Document Number: AN-0284
Revision: 1.0.
Page 21 of 24
Multilayer Ceramic 16V
X7R (0402)
Red Diffused
6.0millicandela 635nm
1206
Zener 5.1V 500mW
SOD-123
Schottky 30V 0.2A
SOD123 Diode
Green Diffused
10millicandela 565nm
1206
5 x 2 Shroud Polarized
Header 0.05inch Surface
Mount
PC-Mount Male XLR-3
Receptacle
2-pin Header
Unshrouded Jumper
0.10"
Chip Ferrite Bead 1800
Ohm @ 100 MHz
Common Mode Choke
100 Ohm 100MHz
Miniature Coupled
Inductor
NPN Gen Purp
Transistor
Chip Resistor 1% 63mW
Thick Film 0402
Chip Resistor 1% 63mW
Thick Film 0402
Chip Resistor 1%
100mW Thick Film 0402
Chip Resistor 1% 63mW
Thick Film 0402
Chip Resistor 1% 63mW
Thick Film 0402
Chip Resistor 1% 63mW
Thick Film 0402
Chip Resistor 1% 63mW
Thick Film 0402
Chip Resistor 1% 63mW
Thick Film 0402
Chip Resistor 1% 63mW
AN-0284
1 R20
49k9
Yageo
RC0402FR-0749K9L
1 R21
604R
Panasonic EC
ERJ-6ENF6040V
1 R22
280R
Panasonic EC
ERJ-6ENF2800V
3 R5 R8 R16
0R00
Panasonic ECG
ERJ-2GE0R00X
1 R6
1 R7
10k0
OPEN
Yageo
Do Not Stuff
RC0402FR-0710KL
OPEN
1 R9
3k92
Stackpole
RMCF0402FT3K92
1 S1
SPDT
E-Switch
EG1218
SPDT
Copal Electronics
Keystone
Electronics
CAS-120TA
Analog Devices Inc.
AD8273ARZ
Analog Devices Inc.
ADA4075-2ARZ
Analog Devices Inc.
ADP1613ARMZ-R7
Analog Devices Inc.
ADP1720ARMZ-R7
1 S2
TP1-11 TP1313 14
5002
1 U1
1 U2
ADA4075-2ARZ
1 U3
1 U4
Document Number: AN-0284
Revision: 1.0.
ADP1720ARMZ-R7
Page 22 of 24
5002
Thick Film 0402
Chip Resistor 1% 63mW
Thick Film 0402
Chip Resistor 1%
125mW Thick Film 0805
Chip Resistor 1%
125mW Thick Film 0805
Chip Resistor 5%
100mW Thick Film 0402
Chip Resistor 1% 63mW
Thick Film 0402
Do Not Stuff
Chip Resistor 1% 63mW
Thick Film 0402
SPDT Slide Switch PC
Mount
SPDT Slide Switch SMD J
Hook
Mini Test Point
White .1" OD
Very Low Distortion,
Dual-Channel, High
Precision Difference
Amplifier
Ultralow Noise Amplifier
at Lower Power
650 KHZ /1.3 MHZ STEPUP PWM DC-TO-DC
SWITCHING CONVERTER
WITH 2.0 A CURRENT
LIMIT
Adjustable, High
Voltage, Micropower
Linear Regulator
AN-0284
LEARN MORE
Elko, Gary W. and Kieran P. Harney, "A History of Consumer Microphones: The Electret Condenser Microphone Meets Micro-ElectroMechanical-Systems," Acoustics Today, April 2009.
Jung, Walt. Op Amp Applications, Analog Devices, 2002.
Lewis, Jerad, “Understanding Microphone Sensitivity," InvenSense, 2014.
Lewis, Jerad, "Microphone Specs Explained," Application Note AN-1112, InvenSense, 2014.
Lewis, Jerad, "Microphone Array Beamforming," Application Note AN-1140, InvenSense, 2014.
Lewis, Jerad, "Op Amps for MEMS Microphone Preamp Circuits," Application Note AN-1165, InvenSense, 2014.
Tutorial MT-202, “Allpass Filters,” Analog Devices
Data Sheets and Evaluation Boards
INMP411 Data Sheet
INMP411 Flex Evaluation Board (EV_INMP411-FX)
Document Number: AN-0284
Revision: 1.0.
Page 23 of 24
AN-0284
Compliance Declaration Disclaimer:
InvenSense believes this compliance information to be correct but cannot guarantee accuracy or completeness. Conformity
documents for the above component constitutes are on file. InvenSense subcontracts manufacturing and the information contained
herein is based on data received from vendors and suppliers, which has not been validated by InvenSense.
Environmental Declaration Disclaimer:
InvenSense believes this environmental information to be correct but cannot guarantee accuracy or completeness. Conformity
documents for the above component constitutes are on file. InvenSense subcontracts manufacturing and the information contained
herein is based on data received from vendors and suppliers, which has not been validated by InvenSense.
This information furnished by InvenSense is believed to be accurate and reliable. However, no responsibility is assumed by
InvenSense for its use, or for any infringements of patents or other rights of third parties that may result from its use. Specifications
are subject to change without notice. InvenSense reserves the right to make changes to this product, including its circuits and
software, in order to improve its design and/or performance, without prior notice. InvenSense makes no warranties, neither
expressed nor implied, regarding the information and specifications contained in this document. InvenSense assumes no
responsibility for any claims or damages arising from information contained in this document, or from the use of products and
services detailed therein. This includes, but is not limited to, claims or damages based on the infringement of patents, copyrights,
mask work and/or other intellectual property rights.
Certain intellectual property owned by InvenSense and described in this document is patent protected. No license is granted by
implication or otherwise under any patent or patent rights of InvenSense. This publication supersedes and replaces all information
previously supplied. Trademarks that are registered trademarks are the property of their respective companies. InvenSense sensors
should not be used or sold in the development, storage, production or utilization of any conventional or mass-destructive weapons
or for any other weapons or life threatening applications, as well as in any other life critical applications such as medical equipment,
transportation, aerospace and nuclear instruments, undersea equipment, power plant equipment, disaster prevention and crime
prevention equipment.
©2014 InvenSense, Inc. All rights reserved. InvenSense, MotionTracking, MotionProcessing, MotionProcessor, MotionFusion,
MotionApps, DMP, and the InvenSense logo are trademarks of InvenSense, Inc. Other company and product names may be
trademarks of the respective companies with which they are associated.
©2014 InvenSense, Inc. All rights reserved.
Document Number: AN-0284
Revision: 1.0.
Page 24 of 24
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