LMV1091TMXNOPB - NATIONAL SEMICONDUCTOR

LMV1091

LMV1091 Dual Input, Far Field Noise Suppression Microphone Amplifier

Literature Number: SNAS481B

LMV1091

January 13, 2011

Dual Input, Far Field Noise Suppression Microphone

Amplifier

General Description

The LMV1091 is a fully analog dual differential input, differ-

ential 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 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 pro-

viding 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 mi-

crophones are biased with an internal low-noise bias supply.

Key Specifications

■Far Field Noise Suppression Electrical * 34dB (typ)

■SNRIE26dB (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)

*FFNSE at f = 1kHz

Features

■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

■Mobile headset

■Mobile and handheld two-way radios

■Bluetooth and other powered headsets

■Hand-held voice microphones

System Diagram

30092240

LMV1091 Dual Input, Far Field Noise Suppression Microphone Amplifier

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

30092231

Top View

X = Plant Code

YY = Date Code

TT = Die Traceability

ZA4 = LMV1091TM

micro SMD Package View

30092216

Bottom View

Ordering Information

Order Number Package Package

Drawing Number

Device

Marking Transport Media

LMV1091TM 25 Bump µSMD TMD25AAA ZA4 250 units on tape and reel

LMV1091TMX 25 Bump µSMD TMD25AAA ZA4 3000 units on tape and reel

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LMV1091

TABLE 1. Pin Name and Function

Bump Number Pin Name Pin Function Pin Type

A1 MIC BIAS Microphone Bias Analog Output

A2 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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LMV1091

Absolute Maximum Ratings (Note 1)

If Military/Aerospace specified devices are required,

please contact the National Semiconductor Sales Office/

Distributors for availability and specifications.

Supply Voltage 6.0V

Storage Temperature -85°C to +150°C

Power Dissipation (Note 3) Internally Limited

ESD Rating (Note 4) 2000V

ESD Rating (Note 5) 200V

CDM 500V

Junction Temperature (TJMAX)150°C

Mounting Temperature

Infrared or Convection (20 sec.) 235°C

Thermal Resistance

θJA (microSMD) 70°C/W

Soldering Information See AN-1112 “microSMD Wafer Level

Chip Scale Package.”

Operating Ratings (Note 1)

Supply Voltage 2.7V ≤ VDD ≤ 5.5V

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.

Symbol Parameter Conditions

LMV1091 Units

(Limits)

Typical

(Note 6)

Limits

(Note 7)

SNR Signal-to-Noise Ratio

VIN = 18mVP-P, A-weighted, Audio band 63 dB

VOUT = 18VP-P,

voice band (300–3400Hz) 65 dB

eNInput Referred Noise level A-Weighted 5 μVRMS

VIN Maximum Input Signal THD+N < 1%, Pre Amp Gain = 6dB 880 820 mVP-P (min)

VOUT

Maximum AC Output Voltage Differential Out+, Out-

THD+N < 1% 1.2 1.1 VRMS (min)

DC Level at Outputs Out+, Out- 820 mV

THD+N Total Harmonic Distortion + Noise Differential Out+ and Out- 0.1 0.2 % (max)

ZIN Input Impedance 142 kΩ

ZOUT Output Impedance 220 Ω

ZLOAD Load Impedance (Out+, Out-) (Note 9)RLOAD

CLOAD

100

kΩ (min)

pF (max)

AMMicrophone Preamplifier Gain Range Minimum

Maximum

36 dB

AMR

Microphone Preamplifier Gain Adjustment

Resolution 21.7

2.3

dB (min)

dB (max)

APPost Amplifier Gain Range Minimum

Maximum

18 dB

APR Post Amplifier Gain Resolution 3 2.6

3.4

dB (min)

dB (max)

FFNSEFar Field Noise Suppression Electrical f = 1kHz (See Test Method)

f = 300Hz (See Test Method)

26 dB

SNRIESignal-to-Noise Ratio Improvement Electrical f = 1kHz (See Test Method)

f = 300Hz (See Test Method)

18 dB

PSRR Power Supply Rejection Ratio

Input Referred, Input AC grounded

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 dB

VBM Microphone Bias Supply Voltage IBIAS = 1.2mA 2.0 1.85

2.15

V (min)

V (max)

eVBM Mic bias noise voltage on VREF pin A-Weighted, CB = 10nF 7 μVRMS

IDDQ Supply Quiescent Current VIN = 0V 0.60 0.8 mA (max)

IDD Supply Current VIN = 25mVP-P both inputs

Noise cancelling mode 0.60 mA

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LMV1091

ISD Shut Down Current SD pin = GND 0.1 0.7 μA (max)

TON Turn-On Time (Note 9) 40 ms (max)

TOFF Turn-Off Time (Note 9) 60 ms (max)

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 Units

(Limits)

Typical Limit

(Note 6) (Note 7)

SNR Signal-to-Noise Ratio

VIN = 18mVP-P, A-weighted, Audio band 63 dB

VOUT = 18mVP-P,

voice band (300–3400Hz) 65 dB

eNInput Referred Noise level A-Weighted 5 μVRMS

VIN Maximum Input Signal THD+N < 1% 880 820 mVP-P (min)

VOUT

Maximum AC Output Voltage f = 1kHz, THD+N < 1%

between differential output 1.2 1.1 VRMS (min)

DC Output Voltage 820 mV

THD+N Total Harmonic Distortion + Noise Differential Out+ and Out- 0.1 0.2 % (max)

ZIN Input Impedance 142 kΩ

ZOUT Output Impedance 220 Ω

AMMicrophone Preamplifier Gain Range Minimum

Maximum

AMR

Microphone Preamplifier Gain Adjustment

Resolution 2 1.7

2.3

dB (min)

dB (max)

APPost Amplifier Gain Range Minimum

Maximum

APR Post Amplifier Gain Adjustment Resolution 3 2.6

3.4

dB (min)

dB (max)

FFNSEFar Field Noise Suppression Electrical f = 1kHz (See Test Method)

f = 300Hz (See Test Method)

26 dB

SNRIESignal-to-Noise Ratio Improvement Electrical f = 1kHz (See Test Method)

f = 300Hz (See Test Method)

18 dB

PSRR Power Supply Rejection Ratio

Input Referred, Input AC grounded

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 dB

VBM Microphone Bias Supply Voltage IBIAS = 1.2mA 2.0 1.85

2.15

V ( min)

V (max)

eVBM Microphone bias noise voltage on VREF pin A-Weighted, CB = 10nF 7 μVRMS

IDDQ Supply Quiescent Current VIN = 0V 0.60 0.8 mA (max)

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)

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LMV1091

Symbol Parameter Conditions LMV1091 Units

(Limits)

Typical Limit

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

Test Methods

30092212

FIGURE 2. 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 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

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 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 expe-

rience a phase lag when compared with the closer micro-

phone. 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 NF-

SLE.

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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LMV1091

Measuring Noise and SNR

The overall noise of the LMV1091 is measured within the fre-

quency band from 10Hz to 22kHz using an A-weighted filter.

The Mic+ and Mic- inputs of the LMV1091 are AC shorted

between the input capacitors, see Figure 11.

30092211

FIGURE 11: Noise Measurement Setup

For the signal to noise ratio (SNR) the signal level at the out-

put 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 pres-

sure level of 94dB SPL, which is the standard level for these

measurements. The LMV1091 is programmed for 26dB of to-

tal 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.

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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

30092257

THD+N vs Frequency

Mic2 = AC GND, Mic1 = 36mVP-P

Noise Canceling Mode

30092258

THD+N vs Frequency

Mic1 = 36mVP-P

Mic1 Pass Through Mode

30092259

THD+N vs Frequency

Mic2 = 36mVP-P

Mic2 Pass Through Mode

30092260

THD+N vs Input Voltage

Mic1 = AC GND, f = 1kHz

Mic2 Noise Canceling Mode

30092261

THD+N vs Input Voltage

Mic2 = AC GND, f = 1kHz

Mic1 Noise Canceling Mode

30092262

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LMV1091

THD+N vs Input Voltage

f = 1kHz

Mic1 Pass Through Mode

30092263

THD+N vs Input Voltage

f = 1kHz

Mic2 Pass Through Mode

30092264

PSRR vs Frequency

Pre Amp Gain = 20dB, Post Amp Gain = 6dB

VRIPPLE = 100mVP-P, Mic1 = Mic2 = AC GND

Mic1 Pass Through Mode

30092265

PSRR vs Frequency

Pre Amp Gain = 20dB, Post Amp Gain = 6dB

VRIPPLE = 100mVP-P, Mic1 = Mic2 = AC GND

Mic2 Pass Through Mode

30092266

PSRR vs Frequency

Pre Amp Gain = 20dB, Post Amp Gain = 6dB

VRIPPLE = 100mVP-P, Mic1 = Mic2 = AC GND

Noise Canceling Mode

30092267

Far Field Noise Suppression Electrical vs Frequency

30092268

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LMV1091

Signal-to-Noise Ratio Electrical vs Frequency

30092269

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LMV1091

Application Data

INTRODUCTION

The LMV1091 is a fully analog single chip solution to reduce

the far field noise picked up by microphones in a communi-

cation system. A simplified block diagram is provided in

Figure 3.

30092224

FIGURE 3. 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 sup-

presses 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 con-

trolled 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 volt-

age 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 pream-

plifier 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.

30092241

FIGURE 4. Maximum Signal Levels

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LMV1091

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.

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 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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LMV1091

Pre-Amp/Post-Amp Gains

The Pre-amplifier gain of the LMV1091TM can be controlled

using the GA0-GA3 pins. See table 2 below for Pre-amplifier

gain control. The Post-Amp gain can be controlled using the

GB0-GB2 pins. See table 3 below for Post-amplifier gain con-

trol.

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

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

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

large, the far field noise reduction performance will be de-

graded. 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' (de-

sired sound source) is equidistant to the source like a broad-

side array (see Figure 8) the result will be a great deal of near

field speech loss.

30092243

FIGURE 8: Broadside Array (WRONG)

30092242

FIGURE 9: Endfire Array (CORRECT)

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LMV1091

Low-Pass Filter At The Output

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 re-

sembles 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:

This low-pass filter is created by connecting a capacitor be-

tween 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 low-

pass 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

A-Weighted Filter

The human ear is sensitive for acoustic signals within a fre-

quency range from about 20Hz to 20kHz. Within this range

the sensitivity of the human ear is not equal for each frequen-

cy. 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.

30092210

FIGURE 10: A-Weighted Filter

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LMV1091

Revision History

Rev Date Description

1.0 10/28/09 Initial released.

1.01 05/17/10 Changed the unit measure of the X1, X2, and X3 (under the Physical

Dimension) from mm to μm.

1.02 01/13/11 Fixed typos on Figure 1 (Typical Application diagram).

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LMV1091

Physical Dimensions inches (millimeters) unless otherwise noted

25 Bump micro SMD Technology

NS Package Number TMD25AAA

X1 = 2015μm X2 = 2015μm X3 = 600μm

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LMV1091

LMV1091 Dual Input, Far Field Noise Suppression Microphone Amplifier

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