AN4184
Application note
Microphone coupon boards STEVAL-MKI129Vx /MKI155Vx
based on digital microphones
Introduction
This application note briefly describes the microphone coupon boards STEVAL-MKI129Vx /
MKI155Vx from STMicroelectronics based on digital microphones.
The boards are offered in three different versions, depending on the hosted microphone:
STEVAL-MKI129V1 for the MP45DT02
STEVAL-MKI129V2 for the MP34DB01
STEVAL-MKI129V3 for the MP34DT01
STEVAL-MKI155V1 for the MP45DT02-M
STEVAL-MKI155V2 for the MP34DB02
STEVAL-MKI155V3 for the MP34DT01-M
Figure 1. STEVAL-MKI129V1/V2/V3
July 2014
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Contents
AN4184
Contents
1
Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2
Microphone measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3
Schematic hints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4
Evaluation boards details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
5
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4.1
STEVAL-MKI129V1 board details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
4.2
STEVAL-MKI129V2 board details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.3
STEVAL-MKI129V3 board details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.4
STEVAL-MKI155V1 board details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.5
STEVAL-MKI155V2 board details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.6
STEVAL-MKI155V3 board details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
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1
Description
Description
The STEVAL-MKI129Vx and MKI155Vx boards are the daughterboards used in the
STEVAL-MKI126 kit. For additional details, please refer to AN4146 “ STSmartVoice
demonstration board STEVAL-MKI126Vx” available on www.st.com.
The STEVAL-MKI126 board hosts four groups of three female headers as depicted in the
figure below. The J12, J25 and J26 group can host one of the three different types of
microphone audio adapters. The single PCB, hosting one microphone, can also be
detached and used alone.
Figure 2. Connections of microphone audio adapters
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Microphone measurements
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AN4184
Microphone measurements
One of the benefits of using the combination of these two kits STEVAL-MKI126 and
STEVAL-MKI129 or STEVA-MKI155 is the measurement of the different models of the
microphone. In this section a brief description of the microphone evaluation will be given.
The equipment used to measure the main parameters of a microphone is listed as follows:
Interface boards APWlink or ST audio hub (STEVAL-CCA035V1 or STEVAL-MKI138Vx
respectively)
STSmartVoice board (STEVAL-MKI126Vx)
Coupon board of the MP34DT01 (STEVAL-MKI129V3)
Reference microphone (G.R.A.S. Type 46AE)
Generic loudspeaker
Audio precision (Model 2722)
Figure 3. Test setup
The test is done by performing a synchronous acquisition of a couple of microphones, the
first is the reference calibrated microphone, the second is the device under test (DUT). The
two microphones are placed face-to-face to minimize the sound pressure differences
between the membranes of the two devices.
The reference microphone is used, exploiting the known parameters, to find the reference in
the acoustic domain. Once the sound pressure level near the membranes of the
microphones has been set to 1 PA, then by performing a reading of the DUT amplitude, the
sensitivity is found. The reference microphone is also used to measure the DUT frequency
response through a comparison between their sensitivities across the audio band. Since the
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Microphone measurements
reference microphone is an analog device, the analog interface of the STSmartVoice has to
be used in order to ensure synchronous measurements (refer to AN4146 for details on
using the board).
Table 1 summarizes the main features and corresponding values of the three MEMS
microphones, followed by figures showing the frequency response of each device.
Table 1. Main parameters of ST’s digital MEMS microphones
Features
ST MEMS MP45DT02
ST MEMS MP34DB01
ST MEMS MP34DT01
Sensitivity
-26 dBFS
-26 dBFS
-26 dBFS
S/N
61 dB
62 dB
63 dB
Directivity
OMNI directional
OMNI directional
OMNI directional
Lead free reflow issue
no
no
no
Current consumption
20 µA (sleep)
650 µA
20 µA (sleep)
600 µA
20 µA (sleep)
600 µA
Package dimensions
4.72 x 3.76 x 1.25 mm
3 x 4 x 1 mm
3 x 4 x 1 mm
Operating temperature
-40°C < T < +85°C
-30°C < T < +85°C
-30°C < T < +70°C
Figure 4. MP45DT02 frequency response (STEVAL-MKI129V1)
+30
Helmotz frequency (Peak is
just beyond the audio band)
+25
+20
+15
+10
+5
d
B
V
+0
-5
-10
-15
-20
-25
-30
20
50
100
200
500
1k
2k
5k
10k
20k
Hz
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AN4184
Figure 5. MP34DB01 frequency response (STEVAL-MKI129V2)
+30
Helmotz frequency (Peak appears in
audio band due to PCB via length)
+25
+20
+15
+10
+5
d
B
V
+0
-5
-10
-15
-20
-25
-30
20
50
100
200
500
1k
2k
5k
10k
20k
Hz
Figure 6. MP34DT01 frequency response (STEVAL-MKI129V3)
+30
+25
+20
+15
+10
+5
d
B
V
+0
-5
-10
-15
-20
-25
-30
20
50
100
200
500
1k
Hz
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2k
5k
10k
20k
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3
Schematic hints
Schematic hints
Digital microphones frequently operate in an environment where a codec provides the
suitable clock and consequently accepts the digital data of the microphone. Figure 7 below
depicts the two typical connections, part (a) shows the usage of the microphone in mono
mode, and part (b) shows the stereo mode.
Figure 7. Typical microphone application
Concerning the stereo configuration, the LR pins of the involved digital microphones must
be respectively set to Vdd and gnd. This setting will produce the output signals depicted in
Figure 8 below. The succession of data valid and Hi-Z allows shortening the digital outputs
between each other to share the same data line.
Figure 8. Output data timing in stereo configuration
Concerning the schematic, the project must follow the recommended connections of
Figure 7 according to the desired configuration. Additionally, two capacitors (C1 and C2)
can be considered to ensure suitable supply decoupling. Generally, ST recommends two
different values, C2 = 10 µF working at low frequency and C1 = 100 nF working in the higher
frequency domain. These capacitors are the recommended values when the application
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Schematic hints
AN4184
board supply voltage contains noise, they are not mandatory if the supply voltage is
relatively clean. If the designer can use only one capacitor, C1 =100 nF is the recommended
choice.
Impedance matching is another important topic that merits consideration in the design of the
board. Nowadays the use of microphones in applications such as smartphones, laptops or
tablets host these devices pretty far from the codec or even on a separate PCB connected
by flexible cables. In any case, every involved component (the codec, the digital microphone
and the trace) shows its own impedance. The mismatch of the impedance will generate
multiple reflections on the transmission line, causing a degradation of the performance and
probable EMI issues. The way to avoid such reflections consists of adding components,
which allows matching the impedance. Recommended clock trace termination methods are
given below and on the following pages.
Method 1
Series resistor: Rs = Z0 - R0
Figure 9. Matching the clock driver output impedance - driver side
The series resistor Rs added at the output of the driver is used to match the impedance
between the driver output and that shown by the transmission line. The advantage of this
solution is that its implementation is very easy. This solution will impact the speed of the
rising and falling edges. Reducing the speed of the edges will help, considering that the EMI
issue conversely causes the increase of the delay introduced by the transmission line.
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Schematic hints
Method 2
Shunt resistor: Rs = Z0
Figure 10. Matching the PCB line - microphone side
This solution is the same as that presented in Method 1, but concerns the microphone side.
It is a very easy solution to implement as well but can introduce non-negligible extra power
consumption. It also causes asymmetrical rising and falling clock edges.
Method 3
Two resistors:
R1 R2
-------------------- = Z 0
R1 + R2
Figure 11. Matching the PCB line - microphone side (using two resistors)
The solution in Method 3 is very similar to Method 2. This is impedance matching on the
microphone side done with two resistors, allowing less power consumption and symmetrical
rising and falling edges of the clock, however the number of components represents a
drawback.
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Method 4
Shunt resistor: Rs = Z0
Series capacitor to obtain = Rs * Cs > 2TPD (TPD line propagation delay)
Figure 12. Matching the PCB line near the microphone in a frequency range
Basically this network works in the same way as that shown in Method 2, but only above a
fixed frequency. This frequency depends on the time constant fixed by the resistor and
capacitor and it must be chosen greater than twice the propagation time delay. This is a very
useful solution, but it impacts the rising and falling edge of the clock and the transmission
line delay.
Method 5
Shunt capacitor where R0 = Z0 (j) II Cs
Figure 13. Matching the clock driver output impedance - driver side (with capacitor)
The capacitor Cs added at the output of the driver is used to match the impedance between
the driver output and that shown by the transmission line. The advantage of this solution is
that its implementation is very easy. It introduces negligible extra current consumption and,
as in Method 4, impacts the rising and falling edge of the clock.
Any of these solutions can be used to avoid the reflections on the transmission line, the
designer must select the method which is best suited to his application. The STEVALMKI129Vx coupon boards do not implement these techniques since their use is for
promotional and debugging proposes only.
The Cd capacitor in the previous figures represents the impedance shown by the digital
input of the MEMS microphones offered by STMicroelectronics. It is not included in the
previous equations because the methods listed are provided in order to properly terminate
the codec or the microphone with the transmission line; this capacitor simply impacts the
propagation delay.
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Evaluation boards details
4
Evaluation boards details
4.1
STEVAL-MKI129V1 board details
Figure 14. STEVAL-MKI129V1 schematic
8
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Table 2. Bill of material - STEVAL-MKI129V1 (MP45DT02)
Designator
Type
Description
Footprint
Manufacturer
C1
Capacitor
100 nF, 16 V, 10%
CAP0603
Murata
C2
Capacitor
10 µF, 10 V, 10%
CAP0805
Murata
J1, J2
Pin array
Header 2.5 mm x 3 (male)
Header (male)
Any source
U1
Microphone
MP45DT02
MP45DT02
STMicroelectronics
Figure 15. STEVAL-MKI129V1 layout - top view
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Figure 16. STEVAL-MKI129V1 layout - bottom view
4.2
STEVAL-MKI129V2 board details
Figure 17. STEVAL-MKI129V2 schematic
U1
MP34DB01
1
2
3
DATA
GND
CLK
C1
C2
100nF 10uF
3
VDD
L/R
2
4
DATA
CLK
GND
5
1
VDD
LR
1
2
3
J2
J1
Table 3. Bill of material - STEVAL-MKI129V2 (MP34DB01)
Designator
Type
Description
Footprint
Manufacturer
C1
Capacitor
100 nF, 16 V, 10%
CAP0603
Murata
C2
Capacitor
10 µF, 10 V, 10%
CAP0805
Murata
J1, J2
Pin array
Header 2.5 mm x 3 (male)
Header (male)
Any source
U1
Microphone
MP34DB01
MP34DB01
STMicroelectronics
Figure 18. STEVAL-MKI129V2 layout - top view
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Figure 19. STEVAL-MKI129V2 layout - bottom view
4.3
STEVAL-MKI129V3 board details
Figure 20. STEVAL-MKI129V3 schematic
1
2
3
U1 MP34DT01
DATA
GND
CLK
C1
C2
100nF 10uF
3
VDD
L/R
2
4
DATA
CLK
GND
5
1
VDD
LR
1
2
3
J1
J2
Table 4. Bill of material - STEVAL-MKI129V3 (MP34DT01)
Designator
Type
Description
Footprint
Manufacturer
C1
Capacitor
100 nF, 16 V, 10%
CAP0603
Murata
C2
Capacitor
10 µF, 10 V, 10%
CAP0805
Murata
J1, J2
Pin array
Header 2.5 mm x 3 (male)
Header (male)
Any source
U1
Microphone
MP34DT01
MP34DT01
STMicroelectronics
Figure 21. STEVAL-MKI129V3 layout - top view
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Figure 22. STEVAL-MKI129V3 layout - bottom view
4.4
STEVAL-MKI155V1 board details
Figure 23. STEVAL-MKI155V1 schematic
Table 5. Bill of material - STEVAL-MKI155V1 (MP45DT02-M)
Designator
Type
Description
Footprint
Manufacturer
C1
Capacitor
100 nF, 16 V, 10%
CAP0603
Murata
C2
Capacitor
10 µF, 10 V, 10%
CAP0805
Murata
J1, J2
Pin array
Header 2.5 mm x 3 (male)
Header (male)
Any source
U1
Microphone
MP45DT02-M
MP45DT02-M
STMicroelectronics
Figure 24. STEVAL-MKI155V1 layout - top view
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Figure 25. STEVAL-MKI155V1 layout - bottom view
4.5
STEVAL-MKI155V2 board details
Figure 26. STEVAL-MKI155V2 schematic
Table 6. Bill of material - STEVAL-MKI155V2 (MP34DB02)
Designator
Type
Description
Footprint
Manufacturer
C1
Capacitor
100 nF, 16 V, 10%
CAP0603
Murata
C2
Capacitor
10 µF, 10 V, 10%
CAP0805
Murata
J1, J2
Pin array
Header 2.5 mm x 3 (male)
Header (male)
Any source
U1
Microphone
MP34DB02
MP34DB02
STMicroelectronics
Figure 27. STEVAL-MKI155V2 layout - top view
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Figure 28. STEVAL-MKI155V2 layout - bottom view
4.6
STEVAL-MKI155V3 board details
Figure 29. STEVAL-MKI155V3 schematic
Table 7. Bill of material - STEVAL-MKI155V3 (MP34DT01-M)
Designator
Type
Description
Footprint
Manufacturer
C1
Capacitor
100 nF, 16 V, 10%
CAP0603
Murata
C2
Capacitor
10 µF, 10 V, 10%
CAP0805
Murata
J1, J2
Pin array
Header 2.5 mm x 3 (male)
Header (male)
Any source
U1
Microphone
MP34DT01-M
MP34DT01-M
STMicroelectronics
Figure 30. STEVAL-MKI155V3 layout - top view
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Figure 31. STEVAL-MKI155V3 layout - bottom view
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Revision history
5
AN4184
Revision history
Table 8. Document revision history
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Date
Revision
Changes
12-Mar-2013
1
Initial release
21-Jul-2014
2
Added STEVAL-MKI155Vx family
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AN4184
IMPORTANT NOTICE – PLEASE READ CAREFULLY
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Information in this document supersedes and replaces information previously supplied in any prior versions of this document.
© 2014 STMicroelectronics – All rights reserved
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