Application Note No. 110
A pp li c at i on N ot e , R ev . 1 . 1, S e p. 2 00 7
A p p li c a t i o n N o t e N o . 1 1 0
U s i n g I n fi n e o n ’ s S i l i c o n M i c r o p ho n e S M M 3 10
t o g e t he r w i t h I n f i n e on ’ s M i c r o p h o n e F i l t er
BGF200
R F & P r o t e c ti o n D e v i c e s
Edition 2007-09-03
Published by
Infineon Technologies AG
81726 München, Germany
© Infineon Technologies AG 2009.
All Rights Reserved.
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LIMITATION WARRANTIES OF NON-INFRINGEMENT OF INTELLECTUAL PROPERTY RIGHTS OF ANY
THIRD PARTY) WITH RESPECT TO ANY AND ALL INFORMATION GIVEN IN THIS APPLICATION NOTE.
Information
For further information on technology, delivery terms and conditions and prices please contact your nearest
Infineon Technologies Office (www.infineon.com).
Warnings
Due to technical requirements components may contain dangerous substances. For information on the types in
question please contact your nearest Infineon Technologies Office.
Infineon Technologies Components may only be used in life-support devices or systems with the express written
approval of Infineon Technologies, if a failure of such components can reasonably be expected to cause the failure
of that life-support device or system, or to affect the safety or effectiveness of that device or system. Life support
devices or systems are intended to be implanted in the human body, or to support and/or maintain and sustain
and/or protect human life. If they fail, it is reasonable to assume that the health of the user or other persons may
be endangered.
Application Note No. 110
Application Note No. 110
Revision History: 2007-09-03, Rev. 1.1
Previous Version:
Page
Subjects (major changes since last revision)
Application Note
3
Rev. 1.1, 2007-09-03
Application Note No. 110
Overview
1
Overview
Infineon’s microphone filter and ESD protection device BGF200 can not only be used together with electret
condenser microphones, but also with Infineon’s silicon microphone SMM310. Please note that Infineon’s silicon
microphone SMM310 does not relay on BGF200, because the SMM310 provides also sufficient insensitivity
against electromagnetic interference from mobile phones. However, when there is a long unshielded cable, wire
or trace (longer than approximately 1 inch) between the microphone and the microphone preamplifier, as it can be
in mobile phones for instance, Infineon’s BGF200 is the ideal device to suppress electromagnetic interference
from GSM or UMTS transmitters as well as to protect the microphone preamplifier from ESD events that are
injected into the unshielded cable.
Unlike electret condenser microphones Infineon’s silicon microphone SMM310 does not need any external bias
circuit and therefore pin A3 of BGF200 must be left open. On pin A2 of BGF200 an AF (audio frequency) bypass
capacitor of at least 1 µF is required to provide sufficient AF grounding for the filter as well as to block DC (see
Figure 3). On the other hand, as this capacitor acts as a high-pass filter, it also helps to suppress low-frequency
wind noise. Therefore, do not connect pin A2 directly to ground. Figure 4 shows the frequency response of
Infineon’s microphone filter BGF200 with two different bypass capacitors (680nF and 1 µF). When a 680nF
bypass capacitor is used, the lower 3 dB cut-off frequency is already 160Hz, while for a 1 µF bypass capacitor the
lower cut-off frequency is 130 Hz. Thus, one can calculate the lower 3dB cut-off frequency f3dB for the bypass
capacitance C as follows:
1 µF
f3dB = 130Hz × ----------C
(1)
Since most microphone amplifiers have differential inputs, BGF200 offers also a differential output that can be
directly connected—through AC coupling capacitors—to the microphone preamplifier. Figure 1 shows the
schematic of BGF200 where one can see the integrated on-chip ESD protection diodes connected to pin C3 to
protect the microphone preamplifier from ESD events of up to ±15 kV as per the IEC 61000-4-2 specification [1].
B3, GND1
A3
C3
1n0
C3
R3
50R
R1
2k2
C2
1n0
C1
1n0
B2, GND2
A2
C2, GND3
R2
2k2
R5
50R
B1
C5
150p
C4
1n0
C1
R4
2k2
BGF200_Schematic.vsd
Figure 1
Schematic of BGF200 (microphone filter and ESD protection device)
Figure 2 shows a typical application example for Infineon’s silicon microphone SMM310 when the microphone is
placed close to the preamplifier. The microphone’s internal capacitance CEMC between the microphone’s output
and ground is approximately 30 pF and therefore one should also place a 30 pF capacitance between the
Application Note
4
Rev. 1.1, 2007-09-03
Application Note No. 110
Overview
preamplifier’s IN- pin and ground for proper suppression of RF common mode signals. The AC coupling capacitors
CCO are typically 100 nF.
Figure 3 shows an application example when there is a long cable between the silicon microphone and the
preamplifier. For this purpose Infineon’s microphone filter BGF200 is used to suppress electromagnetic
interference injected into the cable from GSM or UTMS transmitters in mobile phones for instance. Furthermore,
the microphone filter BGF200 also prevents the preamplifier from being damaged due to ESD events injected into
the cable. Therefore, BGF200 should be placed as close as possible to the preamplifier. Infineon’s silicon
microphone SMM310 can withstand ESD events of ±4 kV—as defined by the Human Body Model [2]—between
any two pins. Furthermore, Infineon’s SMM310 has a metal cover which is connected to ground and is therefore
much more robust against electrostatic air discharges on the sound inlet hole than electret condenser
microphones. Typically, the ESD robustness is 10kV when electrostatic air discharges as per the IEC 61000-4-2
specification occur directly at the sound inlet hole of the microphone.
Microphone Preamp
SMM310
V+
VDD (4)
CEMC
OUT (1)
C CO
IN+
GND (2,3)
C CO
INV-
30 pF
GND
X
Figure 2
AN110_Application_Example1.vsd
Typical application example for Infineon’s silicon microphone SMM310
Microphone Preamp
V+
SMM310
VDD (4)
ESD event
A3
B1
C3
OUT (1)
CEMC
BGF200
C1
GND (2,3)
long cable
B2,B3,C2
A2
IN+
CCO
INCCO
VGND
1 µF
AN110_Application_Example2.vsd
Figure 3
Typical application example for Infineon’s silicon microphone SMM310 together with
Infineon’s microphone filter BGF200
Application Note
5
Rev. 1.1, 2007-09-03
Application Note No. 110
Overview
0
3 dB
Voltage Gain (dB)
-5
-10
-15
-20
BGF200 with 1 µF
-25
BGF200 with 680 nF
-30
10
100
1000
Frequency (Hz)
10000
100000
AN110_Frequency_Response.vsd
Figure 4
Frequency response of BGF200 with different bypass capacitors at pin A2
Please, feel free to contact Infineon’s application experts for further recommendations and technical support.
References
[1]
IEC 61000-4-2, Electromagnetic Compatibility (EMC) Part 4: Testing and measurement techniques –
Section 2: “Electrostatic discharge immunity test,” International Electrotechnical Commission, 1995.
[2]
JESD22-A114D, “Electrostatic Discharge (ESD) sensitivity testing Human Body Model (HBM),” JEDEC Solid
State Technology Association, March 2006.
Application Note
6
Rev. 1.1, 2007-09-03
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