Application Note
SiSonic
Design
Guide
Rev 3.0
Application Note
Table of Contents
1.0 MEMS MICROPHONE TECHNOLOGY .......................................................................... 3
2.0 CHOOSING THE RIGHT SISONIC MICROPHONE ...................................................... 3
2.1 PACKAGE TYPE ................................................................................................................. 3
2.2 OUTPUT FORMAT ............................................................................................................. 7
2.3 RF PROTECTION LEVEL ................................................................................................ 10
2.4 SISONIC PRODUCT MATRIX ........................................................................................ 12
3.0 MECHANICAL DESIGN CONSIDERATIONS .............................................................. 14
3.1 CHOOSING THE MIC AND PORT HOLE LOCATIONS .............................................. 14
3.2 ACOUSTIC PATH DESIGN ............................................................................................. 14
3.3 WIDEBAND FREQUENCY RESPONSE ......................................................................... 18
3.4 ECHO AND NOISE PROBLEMS ..................................................................................... 19
3.5 PCB LAND PATTERN AND SOLDER STENCIL PATTERN ....................................... 20
4.0 ELECTRICAL DESIGN CONSIDERATIONS ................................................................ 21
4.1 POWER SUPPLY ............................................................................................................... 21
4.2 GROUND ........................................................................................................................... 21
4.3 GAIN CONTROL ............................................................................................................... 21
4.4 MICROPHONE TO CODEC INTERFACE CIRCUIT ..................................................... 22
4.5 SISONIC 2-WIRE CIRCUIT ............................................................................................. 23
4.6 MINIMIZING NOISE PICK-UP ........................................................................................ 24
5.0 MANUFACTURING INFORMATION ............................................................................. 26
5.1 PICK-AND-PLACE SETTINGS........................................................................................ 26
5.2 REWORK ........................................................................................................................... 28
5.3 HANDLING AND STORAGE .......................................................................................... 28
5.4 QUALIFICATION TESTING ............................................................................................ 28
5.5 SENSITIVITY MEASUREMENTS .................................................................................. 30
6.0 ADDITIONAL RESOURCES ............................................................................................. 30
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Application Note
1.0 MEMS MICROPHONE TECHNOLOGY
SiSonic MEMS microphones are on the cutting edge of acoustic technology and have gained
wide acceptance in many consumer electronics products including smart phones, feature phones,
entry phones, laptops, tablet PCs, netbooks, PDAs, and DSCs. The principle of operation for
SiSonic microphones is similar to that for traditional Electret Condenser Microphones (ECMs),
but since MEMS microphones are manufactured using silicon wafer processes they have smaller
form factors, improved performance in varied environmental conditions, and improved ease-ofuse in designs.
Purpose: This application note explains the package types, output formats, and RF protection
levels available in SiSonic microphones. It also provides information on mechanical design,
electrical design, and on using SiSonic microphones in a mass production environment.
2.0 CHOOSING THE RIGHT SISONIC MICROPHONE
SiSonic microphone models vary by package type, output format, and RF protection level.
The choice of package is driven by the mechanical requirements of the design, the output format
by the interface chipset and the application, and the RF protection level by the proximity to
antennas and other RF noise sources. The information in this section will help you choose the
right SiSonic microphone for your application.
2.1 PACKAGE TYPE
2.1.1 Top-port and Bottom-port SiSonic
Diagrams of the basic construction of SiSonic microphones and port-hole locations are shown in
the figures below.
Diaphragm & Backplate
Acoustic Port Hole
Wirebonds
Lid
Encapsulation
Wall
CMOS
Base
MEMS
Back Volume
Figure 1: The construction of a top-port SiSonic microphone
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Application Note
Back Volume
Diaphragm & Backplate
Wirebonds
Lid
Encapsulation
Wall
CMOS
Base
MEMS
Acoustic Port Hole
Figure 2: The construction of a bottom-port Zero-Height SiSonic microphone
Top-port SiSonic microphones allow for traditional microphone placement and gasket design,
while bottom-port Zero-Height SiSonic microphones are particularly suited for ‘thin’ product
designs. Having both package choices gives designers more options for microphone placement
within the design constraints. The diagrams below show typical acoustic path mechanical
designs when using top-port and bottom-port microphones, along with typical frequency
responses for each package.
Product case
Standard Height
Top-port SiSonic
Gasket
PCB
Figure 3: Typical acoustic path design using a top-port SiSonic microphone
Product case
Minimized Height
Gasket
PCB
Zero-Height
SiSonic
Figure 4: Typical acoustic path design using a bottom-port Zero-Height SiSonic microphone
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Application Note
Frequency Response
-20
Top Port
SPQ2409HE5H (top port)
-25
SPA2410LR5H (bottom port)
-30
Ultrasonic
-35
dB
-40
-45
-50
-55
-60
100
1,000
10,000
100,000
Frequency (Hz)
Figure 5: Comparison of the frequency response of main SiSonic package types
2.1.2 Mesh Lid for Ultrasonic Applications
Ultrasonic SiSonic microphones have an open mesh lid design that uses an acoustically
transparent mesh as show in the picture below.
Figure 6: Mesh lid construction of ultrasonic mic
The open lid and mesh allow ultrasonic frequencies from 20 kHz to 80 kHz or more to be
captured by the MEMS. The frequency response of ultrasonic Sisonic is shown below.
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Application Note
SPM0404UD5 Typical Frequency Response
-40.00
dB V/0.1Pa
-50.00
-60.00
-70.00
-80.00
-90.00
1
10
100
1000
10000
100000
Frequency (Hz)
Figure 7: Ultrasonic SiSonic Frequency Response
The output of the ultrasonic SiSonic microphone must be processed by an amplifier, codec, or
A/D converter that can extract the ultrasonic frequencies needed by the application, usually by
using a higher sample rate and/or lower decimation rate. Ultrasonic SiSonic is ideally suited for
applications with natural ultrasonic emissions, or for those requiring a separate transmitter and
receiver or multiple transmitters at different frequencies. Potential applications include:
• Consumer Products
o Ultrasonic pen
o Gesture recognition
• Automotive Sensors:
o Parking assistant, curb detection
o Passenger detection (within cabin)
o Forward sensing of obstacles
• Industrial Applications
o Equipment monitoring (mechanical attrition of ball-bearing)
o Position sensors (for plant use)
o Gas tube or pipe monitoring (flow or leaks)
o Security systems (motion detection)
• Military
o Impulse detection (gunshot detection)
o Fire-arm positioning sensor
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Application Note
2.1.3 Package Size
The microphone footprint decides the minimum PCB area required by the mic, and the mic
height sets the minimum vertical space required to integrate the mic into the final product.
Dimensions of each SiSonic package are shown in the table and figure below.
Package
name
Mini
SPK
Ultra
Mini
SPQ
SPA
SPY
Port-hole location
Top
Bottom (Zero-Height)
Mesh Top (Ultrasonic)
Bottom (Zero-Height)
Top
Bottom (Zero-Height)
Top
Bottom (Zero-Height)
Bottom (Zero-Height)
Representative
Model numbers
SPM0410HR5H
SPM0410LR5H
SPM0404UD5
SPK0413LM4H
SPU0409HD5H
SPU0410HR5H
SPU0410LR5H
SPQ2410HR5H
SPA2410LR5H
SPY0824LR5H
Length
(mm)
Width Height Footprint
(mm) (mm) (mm2)
4.72
3.76
4.00
3.76
3.76
3.35
3.00
3.00
2.95
3.00
2.24
2.50
1.90
1.25
1.40
1.00
0.90
1.10
1.10
1.10
0.98
0.90
17.7
12.0
11.1
11.3
8.4
8.4
5.7
Table 1: SiSonic Packages, Port-Hole Locations, and Dimensions
SPM series
Mini
SPK Series
4.72 x 3.76
x 1.25mm
4.00 x
3.00 x
1.00mm
SPU series
Ultra Mini
3.76 x
2.95 x
1.10mm
SPQ
series
3.76 x
2.24 x
1.10
SPA
series
3.35 x
2.50 x
0.98
SPY
series
3.0 x
1.9 x
0.90
Figure 8: Comparison of SiSonic Package Sizes
2.2 OUTPUT FORMAT
2.2.1 Unity-gain Analog and Amplified Outputs
Unity-gain SiSonic has a simple buffered output as shown in the figure below. A coupling
capacitor on the output is required to pass acoustic frequencies to the chipset input while
isolating the DC voltages. The coupling capacitor forms a high pass filter with the input
resistance of the next stage, and is typically 0.1µF or larger to give a corner frequency below
100Hz.
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Application Note
VDD
+
Output
+
_
Ground
Figure 9: Unity-gain SiSonic Block Diagram
In far-field applications like teleconferencing and video recording, the desired acoustic signal is
in the mic far-field and may require additional amplification. Amplified SiSonic microphones
add up to 20dB of gain to the analog output signal before transmitting it to the chipset or audio
codec. Amplifying the signal at the mic versus at the codec improves the overall system Signalto-Noise Ratio (SNR) by increasing the transmitted signal size relative to noise in the traces.
The amplification in the mic and codec must be chosen appropriately so that the acoustic signal
does not saturate either mic or codec during operation.
VDD
+
+
Output
_
R1 = 22kΩ
R2 = 2.44kΩ
Ground
Gain Control
R3 (set by designer)
C1 (set by designer)
Figure 10: Amplified SiSonic Block Diagram
2.2.2 Differential SiSonic
SiSonic is also available with a differential output driver that offers stronger noise immunity than
a single-ended output due to the common mode rejection of noise picked up in traces.
Differential SiSonic includes an integrated amplifier that provides up to 14dB of additional
amplification. The figure below illustrates a typical differential circuit configuration.
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Application Note
Vdd
Output-
+
-
Output+
R1 = 9.8KΩ
R2 = 2.44kΩ
Ground
Gain Control
R3 (set by customer)
C1 (set by customer)
Figure 11: Block Diagram of Differential SiSonic with Amplified Output
2.2.3 Low Frequency SiSonic
Low frequency SiSonic is for applications such as music recording and automotive sensing
where low frequency audio signals are being captured. Low frequency SiSonic comes in an
Ultra Mini Zero-Height package, and has a low-frequency roll-off at about 5 Hz as shown below.
20.00
Relative Sensitivity (dB V/Pa)
10.00
0.00
-10.00
-20.00
1
10
100
1000
10000
Frequency (Hz)
Figure 12: Low Frequency SiSonic Frequency Response Curve
2.2.4 PDM Digital SiSonic
Pulse Density Modulated (PDM) Digital SiSonic microphones have a Sigma-Delta Analog to
Digital Converter (ADC) integrated into the microphone that accepts a 1.0 MHz to 3.25 MHz
clock, and returns over-sampled PDM data at the supplied clock frequency. Decimation and
filtering performed by the receiving chipset convert the PDM data stream into the PCM data for
use by application software. The primary advantage of the digital interface is noise immunity,
with secondary benefits of reduced overall system power consumption. Because the mic output
is a relatively large digital signal, only extreme noise can cause a bit change, and the half-cycle
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Application Note
PDM format used makes the audio data relatively immune to even multi-bit errors. Digital
SiSonic microphones are ideal for designs requiring relatively long mic signal traces, especially
designs using multiple microphones. Mic traces can be routed to virtually any part of the design
with few constraints on routing and a higher probability of success with the first layout, at the
same time requiring fewer protective components.
The L/R select signal configures the microphone to drive the data line after either the rising edge
(Data_H) or falling edge (Data_L) of the clock. A Data_H and Data_L microphone can
multiplex data over the same output trace for multi-mic applications, reducing the interface pin
count. A basic block diagram of a Digital SiSonic design is shown in Figure 7.
Vdd
L/R to Vdd for Data_H Mic
L
Gnd
Digital Sisonic
Clock
(1.0 - 3.25 MHz)
Digital Sisonic
Data
(½ Cycle
PDM format)
Vdd
R
Two mic outputs can
L/R to Gnd for Data_L
be multiplexed on
Gnd
same data line
Figure 13: PDM Digital SiSonic Interface
DSP
Digital SiSonic microphones require a chipset with a PDM audio interface. Contact Knowles for
more information on Digital SiSonic microphones and validated chipset vendors.
2.3 RF PROTECTION LEVEL
SiSonic microphones have integrated RF protection to help prevent RF noise from getting into
the acoustic signal. Standard SiSonic microphones have a grounded Faraday cage integrated into
the mic package, while Enhanced RF SiSonic microphones also have RC filters built into the
base PCB of the microphone. MaxRF SiSonic have the best RF performance, from a soldered
metal can package and RF filtering built into both the CMOS and the package of the
microphone. The figures below show conceptually how both radiated RF noise and conducted
RF noise are shorted to ground in RF-protected SiSonic mics.
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Application Note
2.3.1 Standard and Enhanced RF SiSonic
Antenna
Radiated Noise
Signal
Ground
Ground Plane
PCB
Signal Trace
Conducted Noise
Figure 14: Enhanced RF SiSonic with FR4 package and filtering in the mic base
2.3.1 MaxRF SiSonic
Antenna
Radiated Noise
RC
RC
Signal
Ground
Ground Plane
PCB
Signal Trace
Conducted Noise
Figure 15: MaxRF SiSonic with metal can package and additional filtering in the CMOS chip
Figure 16: Cross-section a metal can package
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Application Note
2.4 SISONIC PRODUCT MATRIX
The table below summarizes the design needs, features, packages, and applications for the
various SiSonic models.
Design
Requirement
SMD reflow
Mini
Mini
ZeroHeight
SPK
ZeroHeight
Ultra
Mini
Ultra
Mini
ZeroHeight
SPQ
SPA,
SPY
ZeroHeight
X
X
X
X
X
X
X
X
X
Thin design
Small
footprint
Amplified
output
Differential
output
Max RF
Protection
Wind Noise
Filtering
Digital PDM
interface
X
X
X
X
X
X
X
X
X
X
Mounted in standard lead-free
solder reflow processes
Bottom-mount design allows
“zero” height requirements on
top side of PCB
X
Minimal use of board space
Far-field applications, improved
system SNR
Better noise immunity from
balanced design
X
X
Benefit/application
X
X
X
X
X
X
Best RF noise immunity
Higher low-frequency roll-off
reduces wind noise.
Best noise immunity, no analog
circuits in chipset, ideal for
multi-mic designs
Table 2: Summary of Design Needs met by SiSonic
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Application Note
Package
name
Port-hole
location
Output Format
Non-amplified
(analog)
Amplified (analog)
Ultrasonic (analog)
Standard
Enhanced
Maximum
Standard
Enhanced
Enhanced
Standard
Enhanced
Enhanced
Maximum
Enhanced
Standard
Representative
Model numbers
SPM0404HD5H
SPM0404HE5H
SPM0410HR5H
SPM0408HD5H
SPM0408HE5H
SPM0406HE3H
SPM0423HD4H
SPM0423HM4H
SPM0404LE5H
SPM0410LR5H
SPM0408LE5H
SPM0404UD5
Digital PDM
Enhanced
SPK0813LM4H
Standard
Enhanced
Maximum
Maximum
Enhanced
Maximum
Maximum
SPU0409HD5H
SPUL409HE5H
SPU0410HR5H
SPU0414HR5H
SPU0409LE5H
SPU0410LR5H
SPU1410LR5H
Maximum
SPQ2410HR5H
Maximum
SPA2410LR5H
Maximum
SPY0824LR5H
Non-amplified
(analog)
Top
Amplified (analog)
Differential (analog)
Mini
Digital PDM
Bottom
(Zero-Height)
SPK
Mesh Top
Bottom
(Zero-Height)
Non-amplified
(analog)
Top
Ultra Mini
Bottom
(Zero-Height)
SPQ
SPA
SPY
Top
Bottom
(Zero-Height)
Bottom
(Zero-Height)
Amplified (analog)
Non-amplified
(analog)
Low Frequency
Non-amplified
(analog)
Non-amplified
(analog)
Non-amplified
(analog)
RF Protection
Table 3: A Summary of Available SiSonic Models. (Please contact Knowles for the latest
information.)
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Application Note
3.0 MECHANICAL DESIGN CONSIDERATIONS
The purpose of this section is to provide mechanical design information relating to the
microphone including:
• Choosing locations for the mic(s) and acoustic port hole(s) in the case
• Designing the acoustic path, including gasket design and assembly considerations
• Designing for a wideband frequency response
• Echo prevention and troubleshooting
• Optimizing the land pattern, solder stencil design, and soldering process
3.1 CHOOSING THE MIC AND PORT HOLE LOCATIONS
Choosing a location for the microphone in a design can be challenging. For analog mics in
particular, the traces from mic to chipset should be kept as short as possible and as far as possible
from potential noise sources. However, the layout of many mobile product designs require that
the mic and traces be near antennas, power amplifiers, motors, hard disk drives, switching power
supplies, etc. The design engineer must also consider the available board space, component
height restrictions, port-hole location(s), acoustic path dimensions, and gasket size, location, and
ease-of-assembly in mass production when choosing a mic location.
The external acoustic port hole in the product housing should be located near the mic to simplify
the gasket and associated mechanical design. The port-hole must also be far enough from
speakers and other acoustic noise sources to minimize the strength of these unwanted signals at
the microphone input. In near-field use modes like talk mode in a mobile phone, the port-hole
location is more critical than in far-field modes since small changes in distance can change the
strength of the acoustic signal arriving at the microphone. In both types of applications, the port
hole should be located where it won’t be blocked during normal use.
If there are multiple mics in a design, then the mic and port-hole locations are further constrained
by the related product use-modes and any audio algorithm requirements. Picking good locations
for the microphones and port holes early in the design process can prevent costly PCB layout or
plastics changes late in the product design cycle.
3.2 ACOUSTIC PATH DESIGN
The acoustic path guides external sound into the microphone. The overall frequency response of
the microphone in the product design is determined by the standalone microphone frequency
response and the physical dimensions of each part of the acoustic path, including the case porthole, gasket(s), and PCB port-hole. The acoustic path must not have leaks that can cause echo or
noise problems, and needs to be designed for manufacturability.
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Application Note
3.2.1 Acoustic Path Dimensions
A short, wide acoustic path has minimal effects on the mic response while a long, narrow path
can create peaks in the audio band, potentially causing a “tinny” sound as higher frequencies are
amplified. A good acoustic path design gives a flat sensitivity frequency response across the
target acoustic frequency range. The designer must measure the frequency response of the
microphone with its acoustic path and make adjustments if the performance doesn’t meet design
goals. Possible changes include:
1.
2.
3.
4.
5.
6.
A larger case port hole
A thinner case at the case port hole
A wider gasket cavity
A shorter acoustic path from changing the mic or case port hole location
A larger and/or thinner PCB hole (for bottom-port mic designs)
Adding a screen or mesh as an acoustic resistance to extend the flat frequency response
range (see section 3.3)
The codec or baseband chipset that processes the audio signal from the mic generally includes a
low-pass filter with a cutoff frequency just above the desired acoustic range to remove the
unwanted higher frequency components. The frequency response curves below compare the
sensitivity of a standalone microphone, a microphone with a short, wide acoustic path design
(gasket A), a microphone with a long, narrow acoustic path design (gasket B), and both gaskets
with a 6 kHz filter representative of a typical low-pass digital filter.
0
Simulated Top-port Mic Response
-10
Mic with gasket A
Mic with gasket B
Gasket A with 6kHz LPF
-20
Sensitivity (dB)
Gasket B with 6kHz LPF
-30
-40
-50
-60
-70
100
1000
10000
Frequency (Hz)
Figure 17: The Effect of Acoustic Path Design on Microphone Frequency Response
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Application Note
The gasket should provide a short, wide acoustic path from the port hole in the case to the
microphone. For designs using top-port SiSonic microphones, the gasket port-hole should have
at least a 0.5mm larger diameter than the microphone port hole to allow for x-y tolerances in the
gasket port-hole, mic port-hole, and gasket placement. At the same time, the gasket port-hole
must be small enough to ensure a complete seal of the gasket to the mic since any leaks could
cause echo, noise, or frequency response problems. A simple acoustic path design for a top-port
SiSonic microphone is shown below.
Gasket Cavity
Case Port-Hole
Product case
Noise
Standard Height
Optional Screen
Microphone Port-Hole
Top-port SiSonic
PCB
Figure 18: A Simple, Effective Acoustic Path Design for a Top-Port Microphone
For designs using Zero-Height SiSonic microphones, the acoustic path also includes the solder
ring between the microphone and PCB, and the through-hole in the PCB. The PCB acoustic hole
must be large enough to give a good frequency response, but small enough for PCB design rules
governing the distance from solder pads to drilled holes. The inside of the PCB acoustic hole
must be un-plated so that solder will not wick into the hole and block the hole. A simple
acoustic path design for a bottom-port SiSonic microphone is shown below.
Case Port-Hole
Product case
Gasket Cavity
Optional
Screen
Noise
Minimized Height
PCB
Zero-Height
SiSonic
Noise
PCB Port-Hole
Microphone Port-Hole
Figure 19: A Simple, Effective Acoustic Path Design for a Bottom-Port Microphone
Knowles provides free simulation services for acoustic path designs. These simulations show the
approximate frequency response of SiSonic microphones with the gasket, case, and PCB to show
if the frequency response is appropriate for the application. A summary of some of the
recommended minimum dimensions for SiSonic acoustic path design is shown in the table
below. Case holes and gasket ports can be non-circular, and will generally give similar
performance as a circular hole with the same cross-sectional area.
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Application Note
Microphone Package
Mini Top-port
Mini Zero-Height
SPK Zero-Height
SPA Zero-Height
SPY Zero-Height
Ultra Mini Top-port
SPQ Top-port
Ultra Mini Zero-Height
Microphone
Port-hole
diameter
0.84 mm
Recommended
PCB Hole
Diameter
N/A
Recommended Recommended
Gasket Cavity
Case Hole
Diameter
Diameter
D > 1.5 mm
1.5 > D ≥ 1.0 mm
0.25 mm
0.9> D >0.4 mm
D > 1.5 mm
1.5 > D ≥ 1.0 mm
0.50 mm
N/A
D > 1.5 mm
1.5 > D ≥ 1.0 mm
0.25 mm
0.6> D >0.3 mm
D > 1.5 mm
1.5 > D ≥ 1.0 mm
Table 4: Recommended Acoustic Path Dimensions
3.2.2 Gasket Material and Assembly
A gasket must be made of acoustically opaque material that prevents sound from passing through
it. The material must seal completely to the case and to the microphone or PCB. In a stack-up
tolerance analysis, the gasket must form a compression fit in worst case (large gap) conditions,
while compressing enough in small gap conditions to avoid bulges in the product case or the
walls of the acoustic path. Good acoustic sealing prevents echo, noise, and frequency response
problems that can result from resonant air volumes inside the product housing and from alternate
paths to the mic port-hole.
The manufacturability of the mic-gasket-case assembly must also be considered. The assembly
process must be designed to reliably align the holes in the gasket to the holes in the case and mic
or PCB in volume production. Side-port or end-port gasket designs are more difficult to
assemble, since the required gasket compression force is often parallel to the surface of the
microphone and perpendicular to the usual case compression force as shown in figure (a) below.
These types of gaskets can have problems with leaks during assembly, but a well-designed
assembly process or a gasket design such as that shown in (b) can form good seals.
Product case
Compression Force
Optional Screen
End-port Gasket
Top-port SiSonic
PCB
Compression
Force
Potential gasket leak
(a)
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Application Note
Product case
Compression Force
Optional Screen
End-port Gasket
Top-port SiSonic
PCB
(b)
Figure 20: Example of an End-port Gasket Designs
Common gasket materials include various kinds of rubber and compressible, closed-cell foams.
Knowles can design and source gaskets for SiSonic microphones, so if interested please contact
Knowles for more information.
3.3 WIDEBAND FREQUENCY RESPONSE
Wideband and super wideband audio applications such as video and music recording use a
frequency range beyond the traditional communications voiceband of 3.4 kHz, and are
increasingly being used in new mobile product designs. A flat response is required from each
part of the design from microphone through the entire signal processing path. Bottom-port
SiSonic mics are well suited for wideband audio, but for top-port microphones the acoustic path
can cause a peak in the desired frequency range. In this case, the flat response can be extended
by adding acoustically resistive material such as a screen or mesh across the acoustic path of the
microphone to dampen the peak. An optimized gasket design together with an acoustic
resistance can extend the flat frequency response range of the top-port microphones to up to 15
kHz as shown in the figure below.
0
Mic and Gasket 1
-10
Gasket 1, Screen 1
-20
Sensitivity (dB)
Gasket 1, Screen 2
Mic and Gasket 2
-30
Gasket 2, Screen 3
-40
-50
-60
-70
100
1000
10000
Frequency (Hz)
Figure 21: Using an Acoustic Resistance Screen to Extend the Flat Frequency Response
An acoustic resistance can be inserted between gasket and microphone (top-port mics), gasket
and case, or PCB and gasket (bottom-port mics), and can also protect from dust and liquids.
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Application Note
Examples of possible acoustic resistance screen locations are shown in the figures in the previous
2 sections of this app note. The acoustic resistance of a material is inversely proportional to its
effective area, so a larger effective area has less resistance. A larger area also reduces the effect
of variations in the material properties. The thickness of the material should be taken into
account when calculating height stack-up tolerances and the compression of gasket materials.
3.4 ECHO AND NOISE PROBLEMS
Echo problems are most likely caused by a poor gasket seal. A leak in the gasket seal allows the
speaker output or other noise to propagate inside the product case into the mic port, with little
loss in strength. An easy way to test for a gasket leak is to block the acoustic port hole in the
case. If the echo problem persists, then the echo is likely caused by a gasket leak and can be
fixed by a gasket design change. A gasket leak may also cause the microphone to pick up audio
noise from other sources such as a camera zoom motor or a chirping capacitor. The figure below
shows a design with a gasket leak.
Product case
Noise from speaker,
motor, etc.
Gasket Leak
Top-port SiSonic
PCB
Figure 22: Echo or noise from a gasket leak.
In product use modes such as conference call mode the speaker output must be strong, so extra
care must be taken to prevent echo. Assuming a good gasket design between the microphone
and case, the strength of the speaker output at the microphone input is determined by the shortest
path from the speaker to the microphone for sound traveling outside of the product case. The
SPL output level of the speaker in open air decreases proportional to 1/R, and the sound intensity
with 1/R2. Once again, blocking the case port hole of the product can help determine if this is
the source of echo. If the echo disappears when the case port hole is blocked, then the speaker
output signal is too strong for the mic location. An external echo path such as this can be
addressed with the following changes:
1.
2.
3.
Reduce or limit the speaker output level.
Increase the path length from speaker to microphone by changing the location of the
microphone and/or speaker in the design until the echo is reduced to an acceptable level.
Use echo cancelation software to remove the speaker signal from the mic input.
The IntelliSonic software package from Knowles includes echo cancellation, noise cancellation,
and beam-forming functions for 2 microphone arrays for laptops using Windows 7/Vista.
SiSonic Design Guide - AN16
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Application Note
IntelliSonic can reduce echo and remove background noise signals to significantly improve SNR.
Contact Knowles for further information about IntelliSonic.
3.5 PCB LAND PATTERN AND SOLDER STENCIL PATTERN
The PCB land pattern, the solder stencil pattern, the solder paste, and the reflow profile should
be designed to yield reliable solder joints. The solder joints serve as the electrical connection,
mechanical connection, and (for bottom-port mics) the acoustic seal between the mic and PCB.
The recommended PCB land pattern for each SiSonic model matches the microphone solder
pads dimensions. The solder stencil pattern must be optimized for production, and for bottomport SiSonic models must use a broken solder ring such as that shown in the figure below.
(a)
(b)
Figure 23: Comparison of SPY Zero-Height SiSonic (a) microphone solder pads and (b) a nonoptimized reference solder stencil pattern with a broken solder ring.
The solder stencil and land patterns should be designed while considering PCB design rules,
solder type, reflow profile, solder stencil thickness, etc. Design optimizations could include:
1. Increasing the land pattern size symmetrically to extend beyond the edge of the mic to
allow for visual inspection of the solder joint.
2. Splitting round pads in the land pattern into two semicircles to allow for better outgassing during reflow and reduce the occurrence of bubbles.
3. For bottom-port mics, reducing the PCB hole diameter or increasing the solder ring
diameter to meet PCB design rule requirements.
4. Optimizing the solder reflow profile for each unique board design to ensure good solder
joints between the mic and PCB.
5. For bottom-port mics, reducing the solder flux content of solder paste to prevent
excessive flux from entering the mic port hole during reflow.
6. Increasing the solder stencil thickness to ensure adequate solder volume for good solder
joints.
7. Reducing the solder stencil thickness to reduce solder volume to minimize the occurrence
of solder balls.
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Application Note
4.0 ELECTRICAL DESIGN CONSIDERATIONS
The interface signals for analog SiSonic microphones include power, output, ground, and
sometimes additional signals. Since analog microphones have small amplitude outputs that are
susceptible to noise, care should be taken with the trace routing to avoid potential noise
problems. This section outlines recommendations for interfacing to each SiSonic signal, with an
emphasis on the interface between the microphone output and the codec or chipset.
4.1 POWER SUPPLY
SiSonic microphones have no change in sensitivity with supply voltage, so the system designer
only needs ensure that the supply voltage stays in the specified range, typically 1.5 to 3.6V.
Because SiSonic has a separate power supply line and an internal voltage regulator, it is less
susceptible to power supply noise than traditional ECMs. The Power Supply Rejection Ratio
(PSRR) for SiSonic microphones is typically 50dB before any amplification, but even with this
level of PSRR strong power supply noise can cause significant noise in the microphone output.
For example, a 10mV RMS noise signal attenuated by 50dB still results in a 30µV RMS ripple in
the microphone output. This can be significant compared to the 11µV RMS typical noise floor
of the microphone, and is equivalent to an acoustic input of about 46dB SPL. If there is strong
noise in the microphone power supply, then shunt capacitors may be added to stabilize the
supply as shown in Figure 28.
4.2 GROUND
All microphone ground pads should be connected to an analog ground plane through a short,
wide trace that is not daisy-chained from device to device. If there is strong noise in the ground
plane, some designs may benefit from a series ferrite bead in the ground path to isolate the
microphone from the noise. Amplified SiSonic models are designed to be drop-in replacements
for non-amplified models of the same package size, with one ground pad changed to be the gain
control pad. If it is anticipated that an amplified mic output may be needed in a design, the gain
control pad can be connected to ground through appropriate components to set the desired gain
(see the next section.) If non-amplified SiSonic is used in the final design, then the gain pad
components can be left unpopulated and the pad will be grounded internally by the nonamplified mic.
4.3 GAIN CONTROL
The gain of amplified SiSonic microphones is set using a resistor and capacitor connected to the
Gain Control terminal of the microphone, as shown in the circuit below.
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Application Note
V+
Term 4 =
Vout
Term 1 = Output
R1 = 22kΩ
R2 = 2.4kΩ
Term 3 = Ground
Term 2 = Gain Control
R3 (set by customer)
C1 (set by customer)
Figure 24: Gain control circuitry for amplified SiSonic
The value of R3 is chosen to give the desired gain value, with a maximum gain of 20dB when R3
is 0Ω. C1 allows proper DC biasing of the amp input, and should be chosen so that the corner
frequency (C.F.) of the high-pass filter formed by C1, R2, and R3 is well below the acoustic
range. If no additional gain is required, the gain terminal can be tied directly to the output
terminal for the same sensitivity as a non-amplified SiSonic. The gain terminal cannot be left
floating since this will add noise to the mic output. R3 and C1 are calculated using the following
formulas:
Setting Gain Formulas:
Gain of non-inverting Op-Amp is determined as:
G=1+ {R1 / (R2 + R3)}
Gain (dB) = 20 * log(G)
High-pass-filter Corner Frequency:
C.F. = 1 / {2π*(R2 + R3) * C1}
Figure 25: Formulas for calculating gain control component values
The R and C components should be located as near to the microphone as possible, since any
noise picked up in the Gain Control terminal could feed into the output of the microphone.
4.4 MICROPHONE TO CODEC INTERFACE CIRCUIT
The interface circuit between an analog SiSonic microphone output and the codec or baseband
chipset can be very simple depending on the design needs. If the codec input is self-biasing, then
the only interface component required is a coupling capacitor. This capacitor forms a high pass
filter with the input impedance of the codec or chipset, so is typically in the µF range for a 3dB
point 30dB (50dB typical)
< 5dB (single-ended)
Mechanical Shock
Operating Temperature
Vibration Sensitivity
Change in Sensitivity with Vdd
>10,000 G
-40 C to +100 C
~ -74dBv/G
No change, 1.5V to 3.6V
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