LMV1091
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SNAS481C – OCTOBER 2009 – REVISED MAY 2013
LMV1091 Dual Input, Far Field Noise Suppression Microphone Amplifier
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FEATURES
DESCRIPTION
•
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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.
1
2
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 DSBGA Package
APPLICATIONS
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Mobile Headset
Mobile and Handheld Two-way Radios
Bluetooth and Other Powered Headsets
Hand-held Voice Microphones
KEY SPECIFICATIONS
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Far Field Noise Suppression Electrical (FFNSE
at f = 1kHz): 34dB (typ)
SNRIE: 26dB (typ)
Supply Voltage: 2.7V to 5.5V
Supply Current: 600μA (typ)
Standby Current: 0.1μA (typ)
Signal-to-Noise Ratio (Voice band): 65dB (typ)
Total Harmonic Distortion + Noise: 0.1% (typ)
PSRR (217Hz): 99dB (typ)
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 far-field 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.
1
2
Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of
Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
All trademarks are the property of their respective owners.
PRODUCTION DATA information is current as of publication date.
Products conform to specifications per the terms of the Texas
Instruments standard warranty. Production processing does not
necessarily include testing of all parameters.
Copyright © 2009–2013, Texas Instruments Incorporated
LMV1091
SNAS481C – OCTOBER 2009 – REVISED MAY 2013
www.ti.com
System Diagram
Far-field noise, > 50 cm
Near-Field Voice
Tra
ffic
N
Up to 4 cm
ois
e
Lo ud
LMV1091
Pure analog solution
provides superior
performance over DSP
solutions
Music
Analog
Noise
Canceling
Block
Crowd Noise
Anno
unce
men
ts
e
Ma
in
ch
Near-Field Voice
eN
ois
Far field noise reduced
by up to 20 dB in properly
configured and using
+/-0.5 dB matched
microphones
+/-0.5 dB
matched
omnidirectional
microphones
Typical Application
VDD
CVREF
C1
10 nF
1 PF
REF
VDD
Mic
Bias
Bias
Mute 2
RIN3
1.1 k:
RIN1
1.1 k:
Mic
CNTRL
CIN1
470 nF
CIN2
LPF+
Mute 1
*
Mic2-
470 nF
CIN4
LPF-
+
470 nF
CIN3
Mic1+
*
OUT-
Mic1-
470 nF
RIN4
RIN2
1.1 k:
1.1 k:
Optimized
Audio
Ouput
OUT+
Mic2+
Pre-Amp Gain
(6 - 36 dB)
GND
GA0
GA1
GA2
Optimized
Audio
Ouput
Post-Amp Gain
(6-18 dB)
Shutdown
Mode
SD
Mode 0 Mode 1
GA3
GB0
GB1
GB2
* The value of the low-pass filter capacitor is application dependent, see the application section for additional information.
Figure 1. Typical Dual Microphone Far Field noise Cancelling Application
2
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Connection Diagram
1
2
3
4
5
A
Mic
Bias
Mic2+
Mic2-
Mic1+
Mic1-
B
Mode0
Mode1
GA0
GA1
GND
C
Mute2
GB0
NC
GA2
REF
D
Mute1
GB1
GB2
GA3
VDD
E
LPF+
OUT+
OUT-
LPF-
_SD
Figure 2. 25-Bump DSBGA (Top View)
See YFQ0025 Package
PIN NAME AND FUNCTION
Bump
Numbe
r
Pin Name
A1
A2
Pin Function
Pin Type
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
Digital Input
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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These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam
during storage or handling to prevent electrostatic damage to the MOS gates.
Absolute Maximum Ratings (1) (2)
Supply Voltage
6.0V
Storage Temperature
-85°C to +150°C
Power Dissipation (3)
Internally Limited
ESD Rating
(4)
2000V
ESD Rating (5)
200V
CDM
500V
Junction Temperature (TJMAX)
150°C
Mounting Temperature
Infrared or Convection (20 sec.)
235°C
Thermal Resistance
θJA (DSBGA)
70°C/W
Soldering Information See SNVA009A “microSMD Wafer Level Chip Scale Package.”
(1)
(2)
(3)
(4)
(5)
“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.
If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/Distributors for availability and
specifications.
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 DSBGA package is 70°C/W. Refer to the Thermal
Considerations section for more information.
Human body model, applicable std. JESD22-A114C.
Machine model, applicable std. JESD22-A115-A.
Operating Ratings (1)
Supply Voltage
2.7V ≤ VDD ≤ 5.5V
TMIN ≤ TA ≤ TMAX
−40°C ≤ TA ≤ +85°C
(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.
Electrical Characteristics 3.3V (1) (2)
Unless otherwise specified, all limits ensured 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.
Symbol
SNR
eN
(1)
(2)
(3)
(4)
4
Parameter
LMV1091
Conditions
Typical (3)
Limits (4)
Units
(Limits)
VIN = 18mVP-P, A-weighted, Audio band
63
dB
Signal-to-Noise Ratio
VOUT = 18VP-P,
voice band (300–3400Hz)
65
dB
Input Referred Noise level
A-Weighted
5
μVRMS
“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.
The Electrical Characteristics tables list ensured 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 ensured.
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 ensured.
Datasheet min/max specification limits are specified by test, or statistical analysis.
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Electrical Characteristics 3.3V(1)(2) (continued)
Unless otherwise specified, all limits ensured 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.
VIN
VOUT
THD+N
ZIN
Maximum Input Signal
THD+N < 1%, Pre Amp Gain = 6dB
880
820
mVP-P (min)
Maximum AC Output Voltage
Differential Out+, OutTHD+N < 1%
1.2
1.1
VRMS (min)
DC Level at Outputs
Out+, Out-
820
Total Harmonic Distortion + Noise
Differential Out+ and Out-
0.1
0.2
% (max)
mV
Input Impedance
142
kΩ
ZOUT
Output Impedance
220
Ω
ZLOAD
Load Impedance (Out+, Out-) (5)
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 Methods)
f = 300Hz (See Test Methods)
34
42
26
SNRIE
Signal-to-Noise Ratio Improvement
Electrical
f = 1kHz (See Test Methods)
f = 300Hz (See Test Methods)
26
33
18
PSRR
Power Supply Rejection Ratio
fRIPPLE = 217Hz (VRIPPLE = 100mVP-P)
99
85
dB (min)
fRIPPLE = 1kHz (VRIPPLE = 100mVP-P)
95
80
dB (min)
Input referred
60
1.85
2.15
V (min)
V (max)
dB
dB
Input Referred, Input AC grounded
CMRR
(5)
(6)
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
IDD
Supply Current
VIN = 25mVP-P both inputs
Noise cancelling mode
0.60
SD pin = GND
0.1
ISD
Shut Down Current
TON
Turn-On Time (6)
TOFF
Turn-Off Time (6)
VIH
Logic High Input Threshold
GA0, GA1, GA2, GA3, GB0, GB1, GB2,
Mute1, Mute2,
Mode 0, Mode 1, SD
VIL
Logic Low Input Threshold
GA0, GA1, GA2, GA3, GB0, GB1, GB2,
Mute1, Mute2,
Mode 0, Mode 1, SD
2.0
dB
μVRMS
7
0.8
mA (max)
mA
0.7
μA (max)
40
ms (max)
60
ms (max)
1.4
V (min)
0.4
V (max)
Specified by design.
Specified by design.
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Electrical Characteristics 5.0V (1)
Unless otherwise specified, all limits ensured 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
LMV1091
Conditions
Typical (2)
Limit (3)
Units
(Limits)
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)
0.2
% (max)
SNR
VOUT
DC Output Voltage
THD+N
ZIN
ZOUT
Total Harmonic Distortion + Noise
820
Differential Out+ and Out-
0.1
mV
Input Impedance
142
kΩ
Output Impedance
220
Ω
6
36
dB
dB
AM
Microphone Preamplifier Gain Range
AMR
Microphone Preamplifier Gain
Adjustment Resolution
AP
Post Amplifier Gain Range
APR
Post Amplifier Gain Adjustment
Resolution
Minimum
Maximum
2
Minimum
Maximum
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 Methods)
f = 300Hz (See Test Methods)
34
42
26
SNRIE
Signal-to-Noise Ratio Improvement
Electrical
f = 1kHz (See Test Methods)
f = 300Hz (See Test Methods)
26
33
18
PSRR
Power Supply Rejection Ratio
fRIPPLE = 217Hz (VRIPPLE = 100mVP-P)
99
85
dB (min)
fRIPPLE = 1kHz (VRIPPLE = 100mVP-P)
95
80
dB (min)
CMRR
Common Mode Rejection Ratio
Input referred
60
1.85
2.15
V ( min)
V (max)
dB
dB
Input Referred, Input AC grounded
(1)
(2)
(3)
6
2.0
dB
VBM
Microphone Bias Supply Voltage
IBIAS = 1.2mA
eVBM
Microphone bias noise voltage on VREF
pin
A-Weighted, CB = 10nF
IDDQ
Supply Quiescent Current
VIN = 0V
0.60
IDD
Supply Current
VIN = 25mVP-P both inputs
Noise cancelling mode
0.60
mA
ISD
Shut Down Current
SD pin = GND
0.1
μA
TON
Turn On Time
40
ms (max)
TOFF
Turn Off Time
60
ms (max)
1.4
V (min)
0.4
V (max)
μVRMS
7
VIH
Logic High Input Threshold
GA0, GA1, GA2, GA3, GB0, GB1, GB2,
Mute1, Mute2,
Mode 0, Mode 1, SD
VIL
Logic Low Input Threshold
GA0, GA1, GA2, GA3, GB0, GB1, GB2,
Mute1, Mute2,
Mode 0, Mode 1, SD
0.8
mA (max)
“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.
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 ensured.
Datasheet min/max specification limits are specified by test, or statistical analysis.
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Test Methods
LMV1091
Mic2+
Osc2
LPF
470 nF
Mic2-
OUT+
AC Voltmeter
470 nF
Mic1+
Osc1
470 nF
Mic1-
OUT-
470 nF
Figure 3. FFNSE, NFSLE, SNRIE Test Circuit
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 20). 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 18 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
NEAR FIELD SPEECH LOSS (NFSLE)
For optimum near field speech preservation, the sound source should be in an endfire array configuration from
the two microphones (see Figure 21). 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 18 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
SIGNAL TO NOISE RATIO IMPROVEMENT ELECTRICAL (SNRIE)
The SNRIE is the ratio of FFNSE to NFSLE and is defined as:
SNRIE = FFNSE - NFSLE
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Measuring Noise and SNR
The overall noise of the LMV1091 is measured within the frequency band from 10Hz to 22kHz using an Aweighted filter. The Mic+ and Mic- inputs of the LMV1091 are AC shorted between the input capacitors, see
Figure 4.
LMV1090
Mic2+
short
470 nF
Mic2-
LPF
OUT+
A-WEIGHTED
FILTER
AC Voltmeter
470 nF
Mic1+
short
470 nF
Mic1-
OUT-
470 nF
Figure 4. Noise Measurement Setup
For the signal to noise ratio (SNR) the signal level at the output is measured with a 1kHz input signal of 18mVP-P
using an A-weighted filter. This voltage represents the output voltage of a typical electret condenser microphone
at a sound pressure level of 94dB SPL, which is the standard level for these measurements. The LMV1091 is
programmed for 26dB of total gain (20dB preamplifier and 6dB postamplifier) with only Mic1 or Mic2 used.
The input signal is applied differentially between the Mic+ and Mic-. Because the part is in Pass Through mode
the low-pass filter at the output of the LMV1091 is disabled.
8
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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
Mic2 = AC GND, Mic1 = 36mVP-P
Noise Canceling Mode
10
10
1
1
THD+N (%)
THD+N (%)
THD+N
vs
Frequency
Mic1 = AC GND, Mic2 = 36mVP-P
Noise Canceling Mode
0.1
0.1
0.01
0.01
0.001
20
100
0.001
20
10k 20k
1k
100
10k 20k
Figure 5.
Figure 6.
THD+N
vs
Frequency
Mic1 = 36mVP-P
Mic1 Pass Through Mode
THD+N
vs
Frequency
Mic2 = 36mVP-P
Mic2 Pass Through Mode
10
10
1
1
0.1
0.1
0.01
0.01
0.001
20
100
1k
FREQUENCY (Hz)
10k 20k
0.001
20
100
10k 20k
1k
FREQUENCY (Hz)
Figure 7.
Figure 8.
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
100
100
10
10
THD+N (%)
THD+N (%)
1k
FREQUENCY (Hz)
THD+N (%)
THD+N (%)
FREQUENCY (Hz)
1
0.1
0.01
0.001
1
0.1
0.01
0.1
0.01
0.001
1
0.01
0.1
INPUT VOLTAGE (VP-P)
INPUT VOLTAGE (VP-P)
Figure 9.
Figure 10.
1
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Typical Performance Characteristics (continued)
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
Input Voltage
f = 1kHz
Mic1 Pass Through Mode
THD+N
vs
Input Voltage
f = 1kHz
Mic2 Pass Through Mode
10
10
THD+N (%)
100
THD+N (%)
100
1
1
0.1
0.1
0.01
0.001
0.01
0.1
0.01
0.001
1
0.01
0.1
1
INPUT VOLTAGE (VP-P)
Figure 11.
Figure 12.
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
+0
+0
-10
-10
-20
-20
-30
-30
-40
-40
PSRR (dB)
PSRR (dB)
INPUT VOLTAGE (VP-P)
-50
-60
-70
-80
-50
-60
-70
-80
-90
-90
-100
-100
-110
20
100
1k
FREQUENCY (Hz)
10k 20k
-110
20
100
1k
FREQUENCY (Hz)
10k 20k
Figure 13.
Figure 14.
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
60
+0
-10
50
-20
40
FFNSE (Hz)
PSRR (dB)
-30
-40
-50
-60
-70
30
20
-80
10
-90
-100
-110
20
100
1k
FREQUENCY (Hz)
0
100
10k 20k
Figure 15.
10
1k
10k
FREQUENCY (Hz)
Figure 16.
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SNAS481C – OCTOBER 2009 – REVISED MAY 2013
Typical Performance Characteristics (continued)
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.
Signal-to-Noise Ratio Electrical
vs
Frequency
35
30
SNRIE (Hz)
25
20
15
10
5
0
100
1k
10k
FREQUENCY (Hz)
Figure 17.
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11
LMV1091
SNAS481C – OCTOBER 2009 – REVISED MAY 2013
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APPLICATION DATA
INTRODUCTION
The LMV1091 is a fully analog single chip solution to reduce the far field noise picked up by microphones in a
communication system. A simplified block diagram is provided in Figure 18.
Post Amp Gain
(6 dB - 18 dB)
Preamp Gain
(6 dB - 36 dB)
Mic1
Analog
Noise
Cancelling
Block
OUT+
Optimized
Audio
Ouput
OUT-
Mic2
Figure 18. Simplified Block Diagram of the LMV1091
The output signal of the microphones is amplified by a pre-amplifier 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.
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 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 19. Two
examples are given as a guideline on how to select proper gain settings.
12
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SNAS481C – OCTOBER 2009 – REVISED MAY 2013
Pre Amp
Gain
(6 dB - 36 dB)
Gain
(Max. 0 dB)
Post Amp Gain
(6 dB - 18 dB)
OUT+
Analog
Noise
Cancelling
Block
Mic1
or
Mic2
Maximum
AC Input
Voltage
