NSC LMV1091TM

LMV1091
Dual Input, Far Field Noise Suppression Microphone
Amplifier
General Description
Key Specifications
The LMV1091 is a fully analog dual differential input, differential output, microphone array amplifier designed to reduce
background acoustic noise, while delivering superb speech
clarity in voice communication applications.
The LMV1091 preserves near-field voice signals within 4cm
of the microphones while rejecting far-field acoustic noise
greater than 50cm from the microphones. Up to 20dB of farfield rejection is possible in a properly configured and using
±0.5dB matched micropohones.
Part of the Powerwise™ family of energy efficient solutions,
the LMV1091 consumes only 600μA of supply current providing superior performance over DSP solutions consuming
greater than ten times the power.
The dual microphone inputs and the processed signal output
are differential to provide excellent noise immunity. The microphones are biased with an internal low-noise bias supply.
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Far Field Noise Suppression Electrical *
SNRIE
Supply voltage
Supply current
Standby current
Signal-to-Noise Ratio (Voice band)
Total Harmonic Distortion + Noise
PSRR (217Hz)
34dB (typ)
26dB (typ)
2.7V to 5.5V
600μA (typ)
0.1μA (typ)
65dB (typ)
0.1% (typ)
99dB (typ)
*FFNSE at f = 1kHz
Features
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No loss of voice intelligibility
Low power consumption
Shutdown function
No added processing delay
Differential outputs
Adjustable 12 - 54dB gain
Excellent RF immunity
Available in a 25–bump micro SMD package
Applications
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Mobile headset
Mobile and handheld two-way radios
Bluetooth and other powered headsets
Hand-held voice microphones
System Diagram
30092240
© 2009 National Semiconductor Corporation
300922
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LMV1091 Dual Input, Far Field Noise Suppression Microphone Amplifier
October 28, 2009
LMV1091
Typical Application
30092215
FIGURE 1. Typical Dual Microphone Far Field noise Cancelling Application
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LMV1091
Connection Diagrams
25ump micro SMD package
30092214
Top View
Order Number LMV1091TM
See NS Package Number TMD25AAA
25–Bump micro SMD Marking
micro SMD Package View
30092216
Bottom View
30092231
Top View
X = Plant Code
YY = Date Code
TT = Die Traceability
ZA4 = LMV1091TM
Ordering Information
Order Number
Package
Package Drawing
Number
Device Marking
LMV1091TM
25 Bump µSMD
TMD25AAA
ZA4
250 units on tape and reel
LMV1091TMX
25 Bump µSMD
TMD25AAA
ZA4
3000 units on tape and reel
3
Transport Media
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LMV1091
TABLE 1. Pin Name and Function
Bump Number
Pin Name
Pin Function
Pin Type
A1
A2
MIC BIAS
Microphone Bias
Analog Output
MIC2+
Microphone 2 positive input
Analog Input
A3
MIC2–
Microphone 2 negative input
Analog Input
A4
MIC1+
Microphone 1 positive input
Analog Input
A5
MIC1–
Microphone 1 negative input
Analog Input
B1
MODE0
Mic mode select pin
Digital Input
B2
MODE1
Mic mode select pin
Digital Input
B3
GA0
Pre-Amplifier Gain select pin
Digital Input
B4
GA1
Pre-Amplifier Gain select pin
Digital Input
B5
GND
Ground
Ground
C1
MUTE2
Mute select pin
Digital Input
C2
GB0
Post-Amplifier Gain select pin
Digital Input
C3
NC
No Connect
C4
GA2
Pre-Amplifier Gain select pin
C5
REF
Reference voltage de-coupling
Analog Ref
D1
MUTE1
Mute select pin
Digital Input
D2
GB1
Post-Amp Gain select pin
Digital Input
D3
GB2
Post-Amp Gain select pin
Digital Input
D4
GA3
Pre-Amp Gain select pin
Digital Input
D5
VDD
Power Supply
Supply
E1
LPF+
Low pass Filter for positive output
Analog Input
E2
OUT+
Positive optimized audio output
Analog Output
E3
OUT-
Negative optimized audio output
Analog Output
E4
LPF-
Low pass Filter for negative output
Analog Input
E5
SD
Chip enable
Digital Input
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Digital Input
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Supply Voltage
Storage Temperature
Power Dissipation (Note 3)
ESD Rating (Note 4)
ESD Rating (Note 5)
CDM
Junction Temperature (TJMAX)
235°C
70°C/W
θJA (microSMD)
Soldering Information See AN-1112 “microSMD Wafer Level
Chip Scale Package.”
6.0V
-85°C to +150°C
Internally Limited
2000V
200V
500V
150°C
Operating Ratings
(Note 1)
2.7V ≤ VDD ≤ 5.5V
Supply Voltage
TMIN ≤ TA ≤ TMAX
−40°C ≤ TA ≤ +85°C
Electrical Characteristics 3.3V (Note 1, Note 2)
Unless otherwise specified, all limits guaranteed for TA = 25°C, VDD = 3.3V, VIN = 18mVP-P, f = 1kHz, SD = VDD, Pre Amp gain =
20dB, Post Amp gain = 6dB, RL = 100kΩ, and CL = 4.7pF, f = 1kHz pass through mode.
LMV1091
Symbol
Parameter
Conditions
Typical Limits
(Note 6) (Note 7)
Units
(Limits)
VIN = 18mVP-P, A-weighted, Audio band
63
dB
Signal-to-Noise Ratio
VOUT = 18VP-P,
voice band (300–3400Hz)
65
dB
eN
Input Referred Noise level
A-Weighted
VIN
Maximum Input Signal
THD+N < 1%, Pre Amp Gain = 6dB
SNR
VOUT
Maximum AC Output Voltage
DC Level at Outputs
THD+N Total Harmonic Distortion + Noise
ZIN
Differential Out+, Out-
μVRMS
5
880
820
mVP-P (min)
1.2
1.1
VRMS (min)
0.2
% (max)
THD+N < 1%
Out+, Out-
820
Differential Out+ and Out-
0.1
mV
Input Impedance
142
kΩ
ZOUT
Output Impedance
220
Ω
ZLOAD
Load Impedance (Out+, Out-) (Note 9)
RLOAD
CLOAD
AM
Microphone Preamplifier Gain Range
Minimum
Maximum
AMR
Microphone Preamplifier Gain Adjustment
Resolution
AP
Post Amplifier Gain Range
APR
Post Amplifier Gain Resolution
10
100
6
36
2
Minimum
Maximum
dB
dB
1.7
2.3
6
18
3
kΩ (min)
pF (max)
dB (min)
dB (max)
dB
dB
2.6
3.4
dB (min)
dB (max)
FFNSE Far Field Noise Suppression Electrical
f = 1kHz (See Test Method)
f = 300Hz (See Test Method)
34
42
26
SNRIE Signal-to-Noise Ratio Improvement Electrical
f = 1kHz (See Test Method)
f = 300Hz (See Test Method)
26
33
18
fRIPPLE = 217Hz (VRIPPLE = 100mVP-P)
99
85
dB (min)
fRIPPLE = 1kHz (VRIPPLE = 100mVP-P)
95
80
dB (min)
Input referred
60
2.0
dB
dB
Input Referred, Input AC grounded
PSRR Power Supply Rejection Ratio
CMRR Common Mode Rejection Ratio
VBM
Microphone Bias Supply Voltage
IBIAS = 1.2mA
eVBM
Mic bias noise voltage on VREF pin
A-Weighted, CB = 10nF
IDDQ
Supply Quiescent Current
VIN = 0V
0.60
Supply Current
VIN = 25mVP-P both inputs
Noise cancelling mode
0.60
IDD
5
dB
1.85
2.15
V (min)
V (max)
μVRMS
7
0.8
mA (max)
mA
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LMV1091
Mounting Temperature
Infrared or Convection (20 sec.)
Thermal Resistance
Absolute Maximum Ratings (Note 1)
LMV1091
0.7
μA (max)
Turn-On Time (Note 9)
40
ms (max)
Turn-Off Time (Note 9)
60
ms (max)
ISD
Shut Down Current
TON
TOFF
SD pin = GND
0.1
VIH
Logic High Input Threshold
GA0, GA1, GA2, GA3, GB0, GB1, GB2,
Mute1, Mute2,
Mode 0, Mode 1, SD
1.4
V (min)
VIL
Logic Low Input Threshold
GA0, GA1, GA2, GA3, GB0, GB1, GB2,
Mute1, Mute2,
Mode 0, Mode 1, SD
0.4
V (max)
Electrical Characteristics 5.0V (Note 1)
Unless otherwise specified, all limits guaranteed for TA = 25°C, VDD = 5V, VIN = 18mVP-P, SD = VDD, Pre Amp gain = 20dB, Post
Amp gain = 6dB, RL = 100kΩ, and CL = 4.7pF, f = 1kHz pass through mode.
Symbol
Parameter
Conditions
LMV1091
Typical
Limit
Units
(Limits)
(Note 6) (Note 7)
VIN = 18mVP-P, A-weighted, Audio band
63
dB
Signal-to-Noise Ratio
VOUT = 18mVP-P,
voice band (300–3400Hz)
65
dB
eN
Input Referred Noise level
A-Weighted
5
μVRMS
VIN
Maximum Input Signal
THD+N < 1%
880
820
mVP-P (min)
Maximum AC Output Voltage
f = 1kHz, THD+N < 1%
between differential output
1.2
1.1
VRMS (min)
SNR
VOUT
DC Output Voltage
THD+N Total Harmonic Distortion + Noise
ZIN
ZOUT
820
Differential Out+ and Out-
Input Impedance
Microphone Preamplifier Gain Range
AMR
Microphone Preamplifier Gain Adjustment
Resolution
AP
Post Amplifier Gain Range
APR
Post Amplifier Gain Adjustment Resolution
0.2
142
Output Impedance
AM
0.1
mV
Minimum
Maximum
kΩ
220
Ω
6
36
dB
dB
2
Minimum
Maximum
% (max)
1.7
2.3
6
18
3
dB (min)
dB (max)
dB
dB
2.6
3.4
dB (min)
dB (max)
FFNSE Far Field Noise Suppression Electrical
f = 1kHz (See Test Method)
f = 300Hz (See Test Method)
34
42
26
SNRIE Signal-to-Noise Ratio Improvement Electrical
f = 1kHz (See Test Method)
f = 300Hz (See Test Method)
26
33
18
fRIPPLE = 217Hz (VRIPPLE = 100mVP-P)
99
85
dB (min)
fRIPPLE = 1kHz (VRIPPLE = 100mVP-P)
95
80
dB (min)
Input referred
60
2.0
dB
dB
Input Referred, Input AC grounded
PSRR Power Supply Rejection Ratio
CMRR Common Mode Rejection Ratio
dB
1.85
2.15
V ( min)
V (max)
VBM
Microphone Bias Supply Voltage
IBIAS = 1.2mA
eVBM
Microphone bias noise voltage on VREF pin
A-Weighted, CB = 10nF
Supply Quiescent Current
VIN = 0V
0.60
IDD
Supply Current
VIN = 25mVP-P both inputs
Noise cancelling mode
0.60
ISD
Shut Down Current
SD pin = GND
0.1
TON
Turn On Time
40
ms (max)
TOFF
Turn Off Time
60
ms (max)
IDDQ
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6
μVRMS
7
0.8
mA (max)
mA
μA
Parameter
Conditions
LMV1091
Typical
Limit
Units
(Limits)
VIH
Logic High Input Threshold
GA0, GA1, GA2, GA3, GB0, GB1, GB2,
Mute1, Mute2,
Mode 0, Mode 1, SD
1.4
V (min)
VIL
Logic Low Input Threshold
GA0, GA1, GA2, GA3, GB0, GB1, GB2,
Mute1, Mute2,
Mode 0, Mode 1, SD
0.4
V (max)
Note 1: “Absolute Maximum Ratings” indicate limits beyond which damage to the device may occur, including inoperability and degradation of device reliability
and/or performance. Functional operation of the device and/or non-degradation at the Absolute Maximum Ratings or other conditions beyond those indicated in
the Recommended Operating Conditions is not implied. The Recommended Operating Conditions indicate conditions at which the device is functional and the
device should not be operated beyond such conditions. All voltages are measured with respect to the ground pin, unless otherwise specified.
Note 2: The Electrical Characteristics tables list guaranteed specifications under the listed Recommended Operating Conditions except as otherwise modified
or specified by the Electrical Characteristics Conditions and/or Notes. Typical specifications are estimations only and are not guaranteed.
Note 3: The maximum power dissipation must be de-rated at elevated temperatures and is dictated by TJMAX, θJC, and the ambient temperature TA. The maximum
allowable power dissipation is PDMAX = (TJMAX – TA) / θJA or the number given in the Absolute Maximum Ratings, whichever is lower. For the LMV1091, TJMAX =
150°C and the typical θJA for this microSMD package is 70°C/W and for the LLP package θJA is 64°C/W. Refer to the Thermal Considerations section for more
information.
Note 4: Human body model, applicable std. JESD22-A114C.
Note 5: Machine model, applicable std. JESD22-A115-A.
Note 6: Typical values represent most likely parametric norms at TA = +25°C, and at the Recommended Operation Conditions at the time of product
characterization and are not guaranteed.
Note 7: Datasheet min/max specification limits are guaranteed by test, or statistical analysis.
Note 8: Default value used for performance measurements.
Note 9: Guaranteed by design.
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LMV1091
Symbol
LMV1091
Test Methods
30092212
FIGURE 2. FFNSE, NFSLE, SNRIE Test Circuit
two microphones (see Figure 9). In this configuration the
speech signal at the microphone closest to the sound source
will have greater amplitude than the microphone further away.
Additionally the signal at microphone further away will experience a phase lag when compared with the closer microphone. To simulate this, phase delay as well as amplitude
shift was added to the NFSLE test. The schematic from Figure
3 is used with the following procedure to measure the NFSLE.
1. A 25mVP-P and 17.25mVP-P (0.69*25mVP-P) sine wave is
applied to Mic1 and Mic2 respectively. Once again, a
signal generator is used to delay the phase of Mic2 by
15.9° when compared with Mic1.
2. Measure the output level in dBV (X)
3. Mute the signal from Mic2
4. Measure the output level in dBV (Y)
5. NFSLE = Y - X dB
FAR FIELD NOISE SUPPRESSION (FFNSE)
For optimum noise suppression the far field noise should be
in a broadside array configuration from the two microphones
(see Figure 8). Which means the far field sound source is
equidistance from the two microphones. This configuration
allows the amplitude of the far field signal to be equal at the
two microphone inputs, however a slight phase difference
may still exist. To simulate a real world application a slight
phase delay was added to the FFNSE test. The block diagram
from Figure 3 is used with the following procedure to measure
the FFNSE.
1. A sine wave with equal frequency and amplitude
(25mVP-P) is applied to Mic1 and Mic2. Using a signal
generator, the phase of Mic 2 is delayed by 1.1° when
compared with Mic1.
2. Measure the output level in dBV (X)
3. Mute the signal from Mic2
4. Measure the output level in dBV (Y)
5. FFNSE = Y - X dB
SIGNAL TO NOISE RATIO IMPROVEMENT ELECTRICAL
(SNRIE)
The SNRIE is the ratio of FFNSE to NFSLE and is defined as:
SNRIE = FFNSE - NFSLE
NEAR FIELD SPEECH LOSS (NFSLE)
For optimum near field speech preservation, the sound
source should be in an endfire array configuration from the
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The overall noise of the LMV1091 is measured within the frequency band from 10Hz to 22kHz using an A-weighted filter.
30092211
FIGURE 11: Noise Measurement Setup
For the signal to noise ratio (SNR) the signal level at the outtal gain (20dB preamplifier and 6dB postamplifier) with only
put is measured with a 1kHz input signal of 18mVP-P using an
Mic1 or Mic2 used.
A-weighted filter. This voltage represents the output voltage
The input signal is applied differentially between the Mic+ and
of a typical electret condenser microphone at a sound presMic-. Because the part is in Pass Through mode the low-pass
sure level of 94dB SPL, which is the standard level for these
filter at the output of the LMV1091 is disabled.
measurements. The LMV1091 is programmed for 26dB of to-
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LMV1091
The Mic+ and Mic- inputs of the LMV1091 are AC shorted
between the input capacitors, see Figure 11.
Measuring Noise and SNR
LMV1091
Typical Performance Characteristics
Unless otherwise specified, TJ = 25°C, VDD = 3.3V, Input Voltage
= 18mVP-P, f = 1kHz, pass through mode, Pre Amp gain = 20dB, Post Amp gain = 6dB, RL = 100kΩ, and CL = 4.7pF.
THD+N vs Frequency
Mic1 = AC GND, Mic2 = 36mVP-P
Noise Canceling Mode
THD+N vs Frequency
Mic2 = AC GND, Mic1 = 36mVP-P
Noise Canceling Mode
30092257
30092258
THD+N vs Frequency
Mic1 = 36mVP-P
Mic1 Pass Through Mode
THD+N vs Frequency
Mic2 = 36mVP-P
Mic2 Pass Through Mode
30092259
30092260
THD+N vs Input Voltage
Mic1 = AC GND, f = 1kHz
Mic2 Noise Canceling Mode
THD+N vs Input Voltage
Mic2 = AC GND, f = 1kHz
Mic1 Noise Canceling Mode
30092261
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30092262
10
LMV1091
THD+N vs Input Voltage
f = 1kHz
Mic1 Pass Through Mode
THD+N vs Input Voltage
f = 1kHz
Mic2 Pass Through Mode
30092263
30092264
PSRR vs Frequency
Pre Amp Gain = 20dB, Post Amp Gain = 6dB
VRIPPLE = 100mVP-P, Mic1 = Mic2 = AC GND
Mic1 Pass Through Mode
PSRR vs Frequency
Pre Amp Gain = 20dB, Post Amp Gain = 6dB
VRIPPLE = 100mVP-P, Mic1 = Mic2 = AC GND
Mic2 Pass Through Mode
30092265
30092266
PSRR vs Frequency
Pre Amp Gain = 20dB, Post Amp Gain = 6dB
VRIPPLE = 100mVP-P, Mic1 = Mic2 = AC GND
Noise Canceling Mode
Far Field Noise Suppression Electrical vs Frequency
30092268
30092267
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LMV1091
Signal-to-Noise Ratio Electrical vs Frequency
30092269
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INTRODUCTION
The LMV1091 is a fully analog single chip solution to reduce
the far field noise picked up by microphones in a communi-
30092224
FIGURE 3. Simplified Block Diagram of the LMV1091
The output signal of the microphones is amplified by a preamplifier with adjustable gain between 6dB and 36dB. After
the signals are matched the analog noise cancelling suppresses the far field noise signal. The output of the analog
noise cancelling processor is amplified in the post amplifier
with adjustable gain between 6dB and 18dB. For optimum
noise and EMI immunity, the microphones have a differential
connection to the LMV1091 and the output of the LMV1091
is also differential. The adjustable gain functions can be controlled via GA0–GA3 and GB0–GB2 pins.
Bias microphone supply output pin depends on the noise voltage on the internal the reference node. The de-coupling
capacitor on the VREF pin determines the noise voltage on this
internal reference. This capacitor should be larger than 1nF;
having a larger capacitor value will result in a lower noise
voltage on the Mic Bias output.
Gain Balance and Gain Budget
In systems where input signals have a high dynamic range,
critical noise levels or where the dynamic range of the output
voltage is also limited, careful gain balancing is essential for
the best performance. Too low of a gain setting in the preamplifier can result in higher noise levels while too high of a gain
setting in the preamplifier will result in clipping and saturation
in the noise cancelling processor and output stages.
The gain ranges and maximum signal levels for the different
functional blocks are shown in Figure 4. Two examples are
given as a guideline on how to select proper gain settings.
Power Supply Circuits
A low drop-out (LDO) voltage regulator in the LMV1091 allows
the device to be independent of supply voltage variations.
The Power On Reset (POR) circuitry in the LMV1091 requires
the supply voltage to rise from 0V to VDD in less than 100ms.
The Mic Bias output is provided as a low noise supply source
for the electret microphones. The noise voltage on the Mic
30092241
FIGURE 4. Maximum Signal Levels
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LMV1091
cation system. A simplified block diagram is provided in
Figure 3.
Application Data
LMV1091
6.
Calculating the new gain for the preamp will result in
<23.5dB gain.
7. The nearest lower gain will be 22dB.
So using preamp gain = 22dB and postamp gain = 6dB is the
optimum for this application.
Example 1
An application using microphones with 50mVP-P maximum
output voltage, and a baseband chip after the LMV1091 with
1.5VP-P maximum input voltage.
For optimum noise performance, the gain of the input stage
should be set to the maximum.
1. 50mVP-P +36dB = 3.1VP-P.
2. 3.1VP-P is higher than the maximum 1.5VP-P allowed for
the Noise Cancelling Block (NCB). This means a gain
lower than 29.5dB should be selected.
3. Select the nearest lower gain from the gain settings
shown in Table 2,28dB is selected. This will prevent the
NCB from being overloaded by the microphone. With this
setting, the resulting output level of the Pre Amplifier will
be 1.26VP-P.
4. The NCB has a gain of 0dB which will result in 1.26VP-P
at the output of the LMV1091. This level is less than
maximum level that is allowed at the input of the post amp
of the LMV1091.
5. The baseband chip limits the maximum output voltage to
1.5VP-P with the minimum of 6dB post amp gain, this
results in requiring a lower level at the input of the post
amp of 0.75VP-P. Now calculating this for a maximum
preamp gain, the output of the preamp must be no more
than 0.75mVP-P.
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Example 2
An application using microphones with 10mVP-P maximum
output voltage, and a baseband chip after the LMV1091 with
3.3VP-P maximum input voltage.
For optimum noise performance we would like to have the
maximum gain at the input stage.
1. 10mVP-P + 36dB = 631mVP-P.
2. This is lower than the maximum 1.5VP-P, so this is OK.
3. The NCB has a gain of 0dB which will result in 1.5VP-P at
the output of the LMV1091. This level is lower than the
maximum level that is allowed at the input of the Post
Amp of the LMV1091.
4. With a Post Amp gain setting of 6dB the output of the
Post Amp will be 3VP-P which is OK for the baseband.
5. The nearest lower Post Amp gain will be 6dB.
So using preamp gain = 36dB and postamp gain = 6dB is
optimum for this application.
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The Pre-amplifier gain of the LMV1091TM can be controlled
using the GA0-GA3 pins. See table 2 below for Pre-amplifier
TABLE 2. Mic Pre-Amp Gain Settings
GA3
GA2
GA1
GA0
Pre-Amplifier Gain
0
0
0
0
6dB
0
0
0
1
8dB
0
0
1
0
10dB
0
0
1
1
12dB
0
1
0
0
14dB
0
1
0
1
16dB
0
1
1
0
18dB
0
1
1
1
20dB
1
0
0
0
22dB
1
0
0
1
24dB
1
0
1
0
26dB
1
0
1
1
28dB
1
1
0
0
30dB
1
1
0
1
32dB
1
1
1
0
34dB
1
1
1
1
36dB
TABLE 3. Post-Amp Gain Settings
GB2
GB1
GB0
Post-Amplifier Gain
0
0
0
6dB
0
0
1
9dB
0
1
0
12dB
0
1
1
15dB
1
0
0
18dB
1
0
1
18dB
1
1
0
18dB
1
1
1
18dB
Noise Reduction Mode Settings
The LMV1091TM has four mode settings. It can be placed in noise cancellation mode, mic 1 on with mic 2 off, mic 1 off with mic
2 on, and mic1 and mic2. See table 4 for control settings.
TABLE 4. Noise Reduction Mode Settings
Mode 1
Mode 0
Noise Reduction Mode Selection
0
0
Noise cancelling mode
0
1
Only Mic 1 On
1
0
Only Mic 2 On
1
1
Mic 1 + Mic 2
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LMV1091
gain control. The Post-Amp gain can be controlled using the
GB0-GB2 pins. See table 3 below for Post-amplifier gain control.
Pre-Amp/Post-Amp Gains
LMV1091
Mute Section
Mic 1 and Mic 2 can be muted independently, using the Mute 1 and Mute 2 pins. See Table 5 for control settings.
TABLE 5. Noise Reduction Mode Settings
Mute 2
Mute 1
Mute Mode Selection
0
0
Mic 1 an Mic 2 on
0
1
Mic 1 mute
1
0
Mic 2 mute
1
1
Mic 1 and Mic 2 mute
large, the far field noise reduction performance will be degraded. The optimum spacing between Mic 1 and Mic 2 is
1.5-2.5cm. This range provides a balance of minimal near
field speech loss and maximum far field noise reduction. The
microphones should be in line with the desired sound source
'near speech' and configured in an endfire array (see Figure
9) orientation from the sound source. If the 'near speech' (desired sound source) is equidistant to the source like a broadside array (see Figure 8) the result will be a great deal of near
field speech loss.
Microphone Placement
Because the LMV1091 is a microphone array Far Field Noise
Reduction solution, proper microphone placement is critical
for optimum performance. Two things need to be considered:
The spacing between the two microphones and the position
of the two microphones relative to near field source
If the spacing between the two microphones is too small near
field speech will be canceled along with the far field noise.
Conversely, if the spacing between the two microphones is
30092243
FIGURE 8: Broadside Array (WRONG)
30092242
FIGURE 9: Endfire Array (CORRECT)
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16
A-Weighted Filter
At the output of the LMV1091 there is a provision to create a
1st order low-pass filter (only enabled in 'Noise Cancelling'
mode). This low-pass filter can be used to compensate for the
change in frequency response that results from the noise
cancellation process. The change in frequency response resembles a first-order high-pass filter, and for many of the
applications it can be compensated by a first-order low-pass
filter with cutoff frequency between 1.5kHz and 2.5kHz.
The transfer function of the low-pass filter is derived as:
The human ear is sensitive for acoustic signals within a frequency range from about 20Hz to 20kHz. Within this range
the sensitivity of the human ear is not equal for each frequency. To approach the hearing response, weighting filters are
introduced. One of those filters is the A-weighted filter.
The A-weighted filter is used in signal to noise measurements,
where the wanted audio signal is compared to device noise
and distortion.
The use of this filter improves the correlation of the measured
values to the way these ratios are perceived by the human
ear.
This low-pass filter is created by connecting a capacitor between the LPF pin and the OUT pin of the LMV1091. The
value of this capacitor also depends on the selected output
gain. For different gains the feedback resistance in the lowpass filter network changes as shown in Table 6.
This will result in the following values for a cutoff frequency of
2000 Hz:
TABLE 6. Low-Pass Filter Capacitor For 2kHz
Post Amplifier Gain Setting (dB)
Rf (kΩ)
Cf (nF)
6
20
3.9
9
29
2.7
12
40
2.0
15
57
1.3
18
80
1.0
30092210
FIGURE 10: A-Weighted Filter
17
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LMV1091
Low-Pass Filter At The Output
LMV1091
Revision History
Rev
Date
1.0
10/28/09
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Description
Initial released.
18
LMV1091
Physical Dimensions inches (millimeters) unless otherwise noted
25 Bump micro SMD Technology
NS Package Number TMD25AAA
X1 = 2015mm X2 = 2015mm X3 = 600mm
19
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LMV1091 Dual Input, Far Field Noise Suppression Microphone Amplifier
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