DM00103199
AN4426
Application note
Tutorial for MEMS microphones
Introduction
This application note serves as a tutorial for MEMS microphones, providing general
characteristics of these devices, both acoustic and mechanical, as well as summarizing the
portfolio available from ST.
MEMS microphones target all audio applications where small size, high sound quality,
reliability and affordability are key requirements. Based on MEMS (Micro-ElectricalMechanical Systems) sensor technology, our microphones meet price points set by
traditional electric condenser microphones (ECM), while featuring superior reliability and
robustness. MEMS microphones from ST are designed using common techniques but also
with industry-unique and innovative packaging that offers slimmer form factors and
outperforms traditional devices. Both analog- and digital-input, top- and bottom-port
solutions are available. Their best-in-class SNR makes ST's MEMS microphones suitable
for applications beyond typical consumer applications, such as phonometers and soundlevel meters that require a high dynamic range.
January 2014
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Contents
AN4426
Contents
1
Mechanical specifications, construction details . . . . . . . . . . . . . . . . . . 4
2
Acoustic parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.1
Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.2
Directionality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.3
SNR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.4
Dynamic range and acoustic overload point . . . . . . . . . . . . . . . . . . . . . . .11
2.5
Equivalent input noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
2.6
Frequency response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.7
Total harmonic distortion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.8
PSRR and PSR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3
MEMS microphone portfolio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
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List of figures
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
Figure 14.
Figure 15.
Figure 16.
Figure 17.
Figure 18.
MEMS microphone inside package . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
MEMS transducer mechanical specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Capacitance change principle. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4 x 5 package . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3 x 4 package - bottom port . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3 x 4 package - top port . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Faraday cage in ST’s MEMS microphones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
RF immunity simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
EMC test setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
RF test disturbance signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
RF immunity test results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Omnidirectional microphone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
A-weighted filter response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Acoustic and electrical relationship - analog. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Acoustic and electrical relationship - digital . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
MP45DT02 frequency response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
MEMS microphone portfolio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
MEMS microphone notation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
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AN4426
Mechanical specifications, construction details
A microphone is a dual-die device consisting of two components, the integrated circuit and
the sensor, which are housed in a package using techniques that are proprietary to ST.
Figure 1. MEMS microphone inside package
MEMS sensor
ASIC (Application specific
integrated circuit)
AM17596v1
The sensor uses MEMS technology (Micro-Electrical-Mechanical Systems) and it is
basically a silicon capacitor. The capacitor consists of two silicon plates/surfaces. One plate
is fixed while the other one is movable (respectively, the green plate and the grey one
shown in the following figure). The fixed surface is covered by an electrode to make it
conductive and is full of acoustic holes which allow sound to pass through. The movable
plate is able to move since it is bonded at only one side of its structure. A ventilation hole,
allows the air compressed in the back chamber to flow out and consequently allows the
membrane to move back. The chamber allows the membrane to move inside but also, in
combination with the chamber created by the package will affect the acoustic performance
of the microphones in terms of frequency response and SNR.
Figure 2. MEMS transducer mechanical specifications
So basically the microphone MEMS sensor is a variable capacitor where the transduction
principle is the coupled capacitance change between a fixed plate (back plate) and a
movable plate (membrane) caused by the incoming wave of the sound.
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Figure 3. Capacitance change principle
The integrated circuit converts the change of the polarized MEMS capacitance into a digital
(PDM modulated) or analog output according to the microphone type. Finally the MEMS
microphone is housed in a package with the sound inlet placed in the top or in the bottom
part of the package, hence the top-port or bottom-port nomenclature of the package. ST
manufactures microphones using industry-wide techniques, but also has developed
innovative packaging to achieve improved performance of the microphones. Packaging
techniques will be discussed in further detail.
The 4x5 package is widely used to house the digital microphone MP45DT02. It is a common
packaging technique in a top-port configuration where the ASIC is placed under the sound
inlet with glue on top (glob top) in order to protect the circuit from light and the MEMS sensor
is placed beside the integrated circuit(a). The two silicon components are fixed to the
substrate and the pads of the device are on the bottom side. The resonant chambers are
identified depending on the position of each chamber with respect to the membrane and the
incoming sound. In this case, considering the incoming direction of the sound, the front
chamber is created by the package and the chamber inside the MEMS, behind the MEMS
membrane, is the back chamber. This configuration allows protecting the MEMS from dust
and particles falling into the package but results in a low SNR and frequency response with
a peak in the audio band.
Figure 4. 4 x 5 package
The 3x4 package is used in ST to produce both the bottom- and the top-port digital
microphones, MP34DB01 and MP34DT01. Considering the bottom configuration first, this
structure is depicted in the following figure. The ASIC and the MEMS sensor are fixed to the
substrate and the pads of the device are on bottom side as well. The sound inlet is obtained
by drilling the substrate according to the position of the MEMS sensor. The package
a. 4 x 5 microphone with and without glob top are both available.
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encloses all the components. In this configuration the front chamber is the cavity of the
MEMS sensor and the package creates the back chamber. This design optimizes the
acoustic performance of the microphone in terms of SNR and also allows obtaining a flat
response across the entire audio band. The drawback of this solution is represented by the
assembly of this microphone. Usually the bottom-port microphones are soldered on the
PCB. The thickness of the board modifies the volume of the front chamber, degrading the
flat response of this type of microphone (refer to AN4427, “Gasket design for optimal
acoustic performance in MEMS microphones” for details). In order to minimize the artifacts
caused by this environment, a flex cable is recommended to be used. Additionally, the
bottom-port microphones have a ringed metal pad around the hole. A very careful soldering
process is required to avoid dust or soldering paste from entering in the sound port,
damaging the MEMS membrane.
Figure 5. 3 x 4 package - bottom port
The 3x4 top-port configuration is basically a mirrored bottom-port microphone. The ASIC
and the sensor are placed close to each other, the sensor is still under the sound inlet but
these two components are attached to the top of the structure, in other words, the ASIC and
MEMS are fixed to the package lid, not to the substrate. The pads are on the substrate and
thus on the bottom side of the microphone. This configuration, covered by ST patents,
allows optimizing all the benefits of the bottom-port microphone in terms of acoustic
performance (i.e. maximized SNR and flat band) and all the benefits related to the top-port
configuration during the assembly process.
Figure 6. 3 x 4 package - top port
Another package used in ST is a 3.76 mm by 2.95 mm, approximately called 3 by 3. This
package is a bottom-port configuration and is used for the analog microphone MP33AB01
which is fully aligned with the digital bottom-port device. Summarizing, it has the same
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package construction and the same performance. This is the best microphone offered by ST
in terms of SNR.
MEMS microphones housed in a plastic package are protected from radiated disturbances
by embedding in the plastic package a metal shield which serves as a Faraday cage. The
model in the following figure shows how the Faraday cage is implemented in ST’s plastic
packages.
Figure 7. Faraday cage in ST’s MEMS microphones
The next figure shows the simulation of an electric field in open space. By applying an
electric field source outside the microphone package, the Faraday cage is able to
considerably attenuate the field inside the microphone structure. The temperature grade of
the E field is an easy way to plot the results.
Figure 8. RF immunity simulation
In addition to the simulation, ST has a dedicated test to evaluate immunity, “Microphone
durability to EMC disturbances (Digital Microphone EMC Immunity Test Rev0.3)”.
Microphones are subjected to RF disturbances using a proper jig with the following setup.
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Figure 9. EMC test setup
Attenuator+50Ω
Termination
EMC jig
RF out
IN
Signal generator
OUT
+33dBm @450,900° 1800MHz
Power Meter
RF in
Coupler
RF Amplifier
RF antenna
Amplified RF out
Dig Mic
Quality Board
Labview
Routine
VDD
gnd
Data
CLK
DECIMATOR BOARD
Only level shifters are used to
bring out 3.3V level signals
and send them to Labview
routine
PC
Basically the test consists of placing the microphone under an antenna radiating a
disturbance signal of 1 kHz AM modulated in the range [0.8, 3] GHz. The RF amplitude
differs depending on the frequency range according to the following criteria:

+33dBm in the range [0.8, 2.4]

+17dBm in the range [2.4, 3.0]
Figure 10. RF test disturbance signal
The carrier of the disturbance is 1 kHz since it is an audio signal. Hence, the RF immunity of
the microphone is evaluated by measuring the residual of the carrier at the output of the
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microphone. The next figure shows the result of the peak at 1 kHz measured when applying
the RF disturbance on top of an MP34DT01.
Figure 11. RF immunity test results
RF immunity results of MP34TD01
-60
1kHz Peak value [dBFS]
-65
-70
-75
-80
-85
-90
-95
-100
-105
-110
0
500
1000
Carrier Frequency [MHz]
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2000
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2
Acoustic parameters
2.1
Sensitivity
The sensitivity is the electrical signal at the microphone output to a given acoustic pressure
as input. The reference of acoustic pressure is 1 Pa or 94 [email protected] The sound
pressure level, expressed in decibel, dBSPL=20*Log(P/Po) where Po = 20µPa is the
threshold of hearing. 20*Log(1Pa/20µPa) = 94 dBSPL

For analog microphones the sensitivity is expressed in mVRMS/Pa or dBV/Pa

For digital microphones the sensitivity is expressed in dBFS
dBV dBFS. It is not correct to compare different units. As given in the above equations,
dBV is in reference to 1VRMS instead of dBFS where the reference is the digital full scale.
2.2
Directionality
The directionality indicates the variation of the sensitivity response with respect to the
direction of the arrival of the sound. MEMS microphones from ST are omnidirectional which
means that there is no sensitivity change to every different position of the source of the
sound in space. The directionality can be indicated in a Cartesian axis as sensitivity drift vs.
angle or in a polar diagram showing the sensitivity pattern response in space.
The following figure depicts the directionality in these two reference systems.
Figure 12. Omnidirectional microphone
2.3
SNR
The signal-to-noise ratio specifies the ratio between a given reference signal to the amount
of residual noise at the microphone output. The reference signal is the standard signal at the
microphone output when the sound pressure is 1Pa @ 1 kHz (microphone sensitivity). The
noise signal (residual noise) is the microphone electrical output at silence.
This parameter includes both the noise of the MEMS element and the ASIC. Concerning
this sum, the main contribution to noise is given by the MEMS sensor, the integrated circuit
contribution can be considered negligible. Typically, the noise level is measured in an
anechoic environment and A-weighting the acquisition. The A-weighted filter corresponds to
the human ear frequency response.
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Acoustic parameters
Figure 13. A-weighted filter response
2.4
Dynamic range and acoustic overload point
The dynamic range is the difference between the minimum and maximum signal that the
microphone is able to generate as output.
2.5

The minimum signal is the smallest audio signal that the microphone can generate
distinctly from noise. In other words, the minimum signal is equivalent to the residual
noise.

The maximum audio signal is that which the microphone can generate without
distortion. It is also called acoustic overload point (AOP). Actually, the specification
allows up to 10% in terms of distortion at the acoustic overload point.
Equivalent input noise
A microphone is a sound-to-electricity transducer which means that any output signal
corresponds to a specific sound as input. The equivalent input noise (EIN) is the acoustic
level, expressed in dBSPL, corresponding to the residual noise as output.
For example, a digital microphone with a sensitivity of -26 dBFS and a 63 dB as SNR:
Residual noise = -26-63 = -89 dBFS this sum transposed in the acoustic domain is:
EIN= 94-63= 31 dBSPL
The following figures summarize the relationship between the acoustic and electric domains
related to each of the parameters listed above. Figure 14 and Figure 15 illustrate this for
analog and digital microphones, respectively.
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Figure 14. Acoustic and electrical relationship - analog
Figure 15. Acoustic and electrical relationship - digital
DIGITAL MICROPHONE EXAMPLE
Acoustic
Domain (dBSPL)
120
Digital
Domain (dBFS)
AOP
-10
Sensitivity
Line
90
80
70
60
50
Noise
Line
EIN
31 dBSPL
12/18
-20
-26dBFS
-30
SNR=63dB
94dBSPL
Dynamic Range=89dB
110
100
0
-40
-50
-60
-70
40
-80
30
-90
20
-100
10
-110
0
-120
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-89 dBFS
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2.6
Acoustic parameters
Frequency response
The frequency response of a microphone in terms of magnitude indicates the sensitivity
variation across the audio band. This parameter also describes the deviation of the output
signal from the reference 0 dB. Typically, the reference for this measurement is exactly the
sensitivity of the microphone @ 0 dB = 94 dBSPL @ 1 kHz. The frequency response of a
microphone can vary across the audio frequency band depending on three parameters: the
ventilation hole, the front chamber geometry, and back chamber geometry. The ventilation
hole and the back chamber geometry have an impact on the behavior at low frequencies
while the behavior at high frequencies depends on the geometry of the front chamber only.
Behavior at high frequencies can be a resonance peak caused by the Helmholtz effect. This
resonance is the phenomenon of air resonance in a cavity. As a matter of fact, it depends on
the dimension of the front chamber of the microphone, representing the sound cavity in
which the air resonates. A microphone with a flat frequency response is suitable when
natural sound and high intelligibility of the system is required. The following figure shows the
response of the MP45DT02. It shows a roll-off at low frequencies and a peak around 18 kHz
caused by the large front chamber (b) of this microphone.
Figure 16. MP45DT02 frequency response
The frequency response of a microphone in terms of phase indicates the phase distortion
introduced by the microphone. In other words, the delay between the sound wave moving
the microphone membrane and the electrical signal at the microphone output results in that
this parameter includes both the distortion due to the membrane and the ASIC.
b.
A detailed explanation of the Helmholtz resonance principle and its effect on the chambers of the microphone
is given in Section 2: Acoustic parameters.
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Total harmonic distortion
THD is the measurement of the distortion affecting the electrical output signal of the
microphone given an undistorted acoustic signal as input. THD+N is expressed as a ratio of
the integer in a specified band of the power of the harmonics plus the power of noise and
the power of the undistorted signal (fundamental).
Equation 1
Typically ST indicates the THD+N measured in the (50 Hz - 4 kHz) band for a given
undistorted signal 1 kHz @ 100 dBSPL.
2.8
PSRR and PSR
PSRR indicates the capability of the ASIC to reject noise added to the supply voltage. To
evaluate this parameter, a tone of VIN = 100 mVpk-pk @ 217 Hz (GSM switching frequency
in phone applications) is added to the power supply and then the amplitude of the output is
measured. The added noise can be either a square wave or sinusoidal wave. Typically the
square wave is preferred since it is the worst case.
PSRR is the ratio of the residual noise amplitude at the microphone output (VOUT @ 217
Hz) to the added spurious signal on the supply voltage. It is typically expressed in dB as
given in the equation below:
Equation 2
PSRR = 20 x Log [(VOUT @ 217 Hz) / (VIN @ 217 Hz)]
The capability of the integrated circuit to reject noise added to the supply voltage can also
be expressed with another parameter that is the PSR. Basically it is simply a measurement
of the output when noise of 100 mVpk-pk @ 217 Hz is superposed to the supply voltage.
Consequently expressed in dB as given in the equation below:
Equation 3
PSR = 20 x Log (Vout @ 217 Hz)
To evaluate either the PSRR or PSR, proper sealing of the sound inlet or measurements
performed in an anechoic chamber are recommended to avoid mixing the superimposed
noise with that of the noise floor of the output. Finally, in the microphone datasheets PSR is
commonly given instead of PSRR.
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3
MEMS microphone portfolio
MEMS microphone portfolio
Figure 17. MEMS microphone portfolio
ST’s portfolio includes digital and analog microphones. The commercial products are
named using the notation depicted in the following figure.
Figure 18. MEMS microphone notation
MP34DB01
MP:
microphone
34: package
size
D: digital
(otherwise A
for analog)
B: bottom
port
(otherwise T
for top)
01: device
revision
EXAMPLE
MP45DT02 : MEMS microphone, 4x5 wide, digital, top-port, revision 02
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The following table provides a complete overview of the microphones offered by ST.
Additionally it serves as a summary for selecting the appropriate microphone among the ST
portfolio as the features of both digital and analog microphones are given.
Table 1. Features of MEMS microphones
Parameter
MP45DT02
MP34DB01
MP34DT01
MP33AB01
MP33AB01H
Sensitivity
-26 dBFS
-26 dBFS
-26 dBFS
-38 dBV
-38 dBV
Directivity
Omnidirectional
Omnidirectional
Omnidirectional
Omnidirectional
Omnidirectional
SNR
61 dB
62.5 dB
63 dB
63 dB
66 dB
AOP
120 dB
120 dB
120 dB
125 dB
125 dB
EIN
33 dB
31.5 dB
31 dB
31 dB
28 dB
THD+N
<5% @ 115 dB
<5% @ 115dB
<5% @ 115dB
<5% @ 120dB
<5% @ 120dB
PSR
-70 dBFS
-70 dBFS
-70 dBFS
-75 dBV
-75 dBV
Max. current
consumption
650 µA
600 µA
600 µA
250 µA
250 µA
Package
dimensions
4.72x3.76x1.25 mm
3x4x1 mm
3x4x1 mm
3.76x2.95x1 mm
3.76x2.95x1 mm
Port location
Top port
Bottom port
Top port
Bottom port
Bottom port
Operating
temperature
-40°C<T<+85°C
30°C<T<+85°C
30°C<T<+70°C
-30°C<T<+100°C
-30°C<T<+100°C
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Revision history
Revision history
Table 2. Document revision history
Date
Revision
09-Jan-2014
1
Changes
Initial release
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