NSC LMV1088 Dual input, far field noise suppression microphone amplifier with automatic calibration ability Datasheet

LMV1088
Dual Input, Far Field Noise Suppression Microphone
Amplifier with Automatic Calibration Ability
General Description
Key Specifications
The LMV1088 is a fully analog dual input microphone array
amplifier designed to reduce background acoustic noise,
while delivering superb speech clarity in voice communications applications.
The LMV1088 incorporates calibration circuitry which may be
initiated by either an I2C command or by a logic level control
on a separate input pin. The calibration sequence compensates for gain and frequency response variations of the microphones used with the LMV1088, eliminating the need to
use expensive matched microphone sets. The calibration data is stored in the internal EEPROM memory. The LMV1088
has two differential input microphone amplifier channels plus
far field noise suppression (FFNS) processing circuitry. The
amplifiers and FFNS circuitry are adjustable for gain differences in the MIC channels of +/- 3dB. The frequency response variations of the microphones over the voice band
frequency range can also be adjusted for differences of
+/-3dB.
The compensation or calibration function is achieved via
memory stored coefficients. These are determined when the
FFNS calibration fuction is activated. The purpose of the calibration sequence is to choose the optimized coefficients for
the FFNS circuitry for the given microphones, spacing, and
acoustical environment.
(3.3V supply, unless otherwise specified)
■ Supply voltage
■ Supply current
■ Signal to noise ratio (A-weighted)
■ Total harmonic distortion (A-weighted)
■ Temperature range
2.7V to 5.5V
1mA (typ)
60dB (typ)
0.1% (typ)
−40°C to 85°C
Features
■
■
■
■
■
Low power consumption
Neglectable noise suppression processing delay
Automatic Calibration
Three microphone usage modes
Space-saving 36 Bump micro SMD package
Applications
■ Cellular phones
■ Mobile and handheld two-way radios
■ Bluetooth and other powered headsets
Application of the LMV1088
20213040
© 2007 National Semiconductor Corporation
202130
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LMV1088 Dual Input, Far Field Noise Suppression Microphone Amplifier with Automatic
Calibration Ability
December 10, 2007
LMV1088
Typical Application
20213001
FIGURE 1. Typical Dual Microphone Far Field noise Cancelling Application
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2
LMV1088
Connection Diagrams
36 Bump micro SMD package
20213030
Top View
Order Number LMV1088RL
See NS Package Number RLA36VVA
36 Bump micro SMD Marking
micro SMD Package View
20213031
Top View
X = Plant Code
YY = Date Code
TT= Die Tracability
ZA1 = LMV1088RL
20213033
Bottom View
3
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LMV1088
TABLE 1. Pin Name and Function
Bump Number
Pin Name
Pin Function
A1
NC
No Connect (Note 1)
A2
T7
Connect to GND
A3
PE
Program Enable EEPROM
A4
MIC2–
microphone 2 input —
A5
MIC2+
microphone 2 input +
A6
Mic Bias
Bias for Microphones
B1
NC
No Connect (Note 1)
B2
NC
No Connect (Note 1)
B3
T5
Float(Note 2)
B4
GND
Amplifier ground
B5
T1
Float(Note 2)
B6
MIC1+
microphone 1 input +
C1
NC
No Connect (Note 1)
C2
NC
No Connect (Note 1)
C3
T6
Float(Note 2)
C4
T3
Float(Note 2)
C5
GND
Amplifier ground
C6
MIC1–
microphone 1 input —
D1
ADR
I2C Address select
D2
NC
No Connect (Note 1)
D3
GND
Amplifier ground
D4
T4
Float(Note 2)
D5
T2
Connect to GND
D6
REF
Reference Voltage De-coupling
E1
SCL
I2C Clock
E2
T8
Connect to GND
E3
NC
No Connect (Note 1)
E4
NC
No Connect (Note 1)
E5
NC
No Connect (Note 1)
E6
NC
No Connect (Note 1)
F1
SDA
F2
I2CV
I2C Data
I2C
DD
VDD
Power Supply
F4
OUT
Optimized Audio Out
F5
LPF
Lowpasss Filter Capacitor
F6
CAL
Calibration Start
Note 1: Connect NC pins to GND for optimum noise performance
Note 2: Grounding the float pins can result in excessive currents
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power supply
F3
4
70°C/W
θJA (microSMD)
Soldering Information See AN-112 “microSMD Wafers Level
Chip Scale Package.”
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Supply Voltage
Storage Temperature
ESD Rating (Note 6)
ESD Rating (Note 7)
Junction Temperature (TJMAX)
Mounting Temperature
Infrared or Convection (20 sec.)
6.0V
-85°C to +150°C
2000V
200V
150°C
235°C
Operating Ratings
(Note 4)
Supply Voltage
I2CVDD (Note 12)
Temperature Range
2.7V to 5.5V
1.8V to 5.5V
−40°C to 85°C
Electrical Characteristics 3.3V
(Note 3)
Unless otherwise specified, all limits guaranteed for TJ = 25°C, VDD = 3.3V, VIN = 18mVPP, pass through mode (Note 10), preamplifier
gain = 20 dB, postamplifier gain = -2.5dB, RL = 100kΩ, and CL = 4.7pF.
Symbol
SNR
VIN
Vout
Parameter
ZOUT
Typical (Note 8) Limits (Note 9)
Units (Limits)
f = 1kHz, , VIN = 18mVPP, A-Weighted
60
dB
Max Input Signal
f = 1kHz and THD+N < 1%
97
mVPP
AC Output Voltage
f = 1kHz
500
mVRMS
800
mV
DC Output Voltage
f = 1kHz, VIN = 18mVPP
Input Impedance
Output Impedance
0.1
%
100
kΩ
AM
Microphone Pre Amplifier Gain Range f = 1kHz
AMR
Microphone Pre Amplifier Gain
Adjustment Resolution
Post Amplifier Gain Range
Ω
150
RLOAD
CLOAD
ZLOAD
AP
LMV1088
Signal-to-Noise Ratio
THD+N Total Harmonic Distortion + Noise
ZIN
Conditions
10
10
f = 1kHz
f = 1kHz Pass Through Mode and
Summing Mode
f = 1kHz Noise Cancelling Mode
(Note 11)
kΩ (min)
pF (max)
6 – 36
dB
2
dB
-2.5 – 9.5
dB
0 – 12
dB
3
dB
APR
Post Amplifier Gain Adjustment
Resolution
f = 1kHz
ACR
Gain Compensation Range
f = 300Hz — f = 3400Hz
±3
dB (max)
AMD
Gain Matching Difference After
Calibration
f = 300Hz
f = 1kHz
f = 3kHz
0.5
0.5
0.5
dB (max)
dB (max)
dB (max)
TCAL
Calibration Duration
770
ms (max)
Input Referred, Input AC grounded
PSRR Power Supply Rejection Ratio
CMRR Common Mode Rejection Ratio
f = 217Hz (100mVPP)
85
dB
f = 1kHz (100mVPP)
80
dB
f = 1kHz,
60
dB
VBM
Microphone Bias Supply Voltage
IBIAS = 1mA
2.0
V
εVBM
Microphone Bias Supply Noise
A-Weighted
10
μVRMS
IBM
Total available Microphone Bias
Current
IDDQ
Supply Quiescence Current
IDDCP
Supply Current during Calibration and Calibrating or Programming
Programming
EEPROM
IDD
Supply Current
VIN = 0V
Vin = 25mVPP both inputs, Noise
canceling mode
5
1.2
mA (min)
1
1.5
mA (max)
28
50
mA (max)
1
1.5
mA (max)
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LMV1088
Thermal Resistance
Absolute Maximum Ratings (Note 3)
LMV1088
Electrical Characteristics 5.0V
(Note 3)
Unless otherwise specified, all limits guaranteed for TJ = 25°C, VDD = 5V, VIN = 18mVPP, pass through mode (Note 10), preamplifier
gain = 20dB, postamplifier gain = –2.5dB, RL = 100kΩ, and CL = 4.7pF.
Symbol
SNR
VIN
Vout
Parameter
ZOUT
Units (Limits)
f = 1kHz,, VIN = 18mVPP, A-Weighted
60
dB
Max Input Signal
f = 1kHz and THD+N < 1%
97
mVPP
AC Output Voltage
f = 1kHz
500
mVRMS
800
mV
0.1
%
100
kΩ
DC Output Voltage
f = 1kHz VIN = 18mVPP
Input Impedance
Output Impedance
AM
Microphone Pre Amplifier Gain Range f = 1kHz
AMR
Microphone Pre Amplifier Gain
Adjustment Resolution
Post Amplifier Gain Range
Ω
150
RLOAD
CLOAD
ZLOAD
AP
LMV1088
Typical (Note 8) Limit (Note 9)
Signal-to-Noise Ratio
THD+N Total Harmonic Distortion + Noise
ZIN
Conditions
10
10
f = 1kHz
f = 1kHz Pass Through Mode and
Summing Mode
f = 1kHz Noise Cancelling Mode (Note
11)
kΩ (min)
pF (max)
6 – 36
dB
2
dB
-2.5 – 9.5
dB
0 – 12
dB
3
dB
APR
Post Amplifier Gain Adjustment
Resolution
f = 1kHz
ACR
Gain Compensation Range
f = 300Hz — f = 3400Hz
±3
dB (max)
AMD
Gain Matching Difference After
Calibration
f = 300Hz
f = 1kHz
f = 3kHz
0.5
0.5
0.5
dB (max)
dB (max)
dB (max)
TCAL
Calibration Duration
770
ms
Input Referred, Input AC grounded
PSRR Power Supply Rejection Ratio
CMRR Common Mode Rejection Ratio
f = 217Hz (100mVPP)
85
dB
f = 1kHz (100mVPP)
80
dB
f = 1kHz
60
dB
VBM
Microphone Bias Supply Voltage
IBIAS = 1mA
2.0
V
εVBM
Microphone Bias Supply Noise
A-Weighted
10
μVRMS
IBM
Total Available Microphone Bias
Current
IDDQ
Supply Quiescence Current
IDDCP
Supply Current during Calibration and
Calibrating or Programming EEPROM
Programming
IDD
Supply Current
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VIN = 0V
Vin = 25mVPP both inputs, Noise
canceling mode
6
1.2
mA (min)
1
1.5
mA (max)
28
50
mA(max)
1
1.5
mA (max)
LMV1088
Digital Interface Characteristics
(Notes 3, 12)
Unless otherwise specified, all limits guaranteed for TJ = 25°C, I2CVDD within the Operating Rating (Note 12)
LMV1088
Symbol
Parameter
Conditions
Typical
(Note 8)
Limits (Note
9)
Units
(Limits)
VIH
Logic High Input Level
SCL, SDA, ADR, CAL, PE pins
0.6xI2CVDD
V (min)
VIL
Logic Low Input Level
SCL, SDA, ADR, CAL, PE pins
0.4xI2CVDD
V (max)
tsCAL
CAL Setup Time
thCAL
CAL Hold time until calibration is
finished
tsPEC
PE Setup Time
thPEC
PE Hold until calibration is finished
2
ms
770
ms (min)
2
ms
770
ms (min)
Note 3: “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 4: 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 5: 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
LMV1088, 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 6: Human body model, applicable std. JESD22-A114C.
Note 7: Machine model, applicable std. JESD22-A115-A.
Note 8: 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 9: Datasheet min/max specification limits are guaranteed by test, or statistical analysis.
Note 10: In Pass Though mode, only one microphone input is active. See also I2C Compatible Interface for more information how to configure the LMV1088
Note 11: In Noise Cancelling Mode there is 2.5 dB additional gain before calibration when compared to the other operating modes to compensate for the gain
reduction that is caused by the noise cancelling effect
Note 12: The voltage at I2CVDD must not exceed the voltage on VDD
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LMV1088
Typical Performance Characteristics Unless otherwise specified, TJ = 25°C, VDD = 3.3V, VIN =
18mVPP, pass through mode (Note 10), preamplifier gain = 20dB, postamplifier gain = –2.5dB, RL = 100kΩ, and CL = 4.7pF.
Supply Current vs. Supply Voltage
THD+N vs Frequency, pass trough mode Mic1
VIN = 36mVpp
20213021
20213003
THD+N vs Frequency, pass trough mode Mic2
VIN = 36mVpp
THD+N vs Frequency, Noise canceling mode
signal at Mic1, Mic2 AC shorted, VIN = 36mVpp
20213004
20213005
THD+N vs Frequency, Noise cancelling mode
Mic1 AC shorted, signal at Mic2, VIN = 36mVpp
THD+N vs Vin, pass trough mode Mic1
20213016
20213006
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LMV1088
THD+N vs Vin, pass trough mode Mic2
THD+N vs Vin, Noise canceling mode
signal at Mic1, Mic2 AC shorted
20213015
20213014
THD+N vs Vin, Noise canceling mode
Mic1 AC shorted, signal at Mic2
PSRR vs Frequency, pass trough mode Mic1,
Mic1+ Mic2 AC shorted
20213018
20213017
PSRR vs Frequency, pass trough mode Mic2,
Mic1+ Mic2 AC shorted
PSRR vs Frequency, Noise canceling mode ,
Mic1+ Mic2 AC shorted
20213019
20213020
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LMV1088
PSRR vs Frequency, Microphone Bias ,
Mic1+ Mic2 AC shorted
20213022
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10
I2C Compatible Interface
I2C SIGNALS
The LMV1088 pin SCL is used for the I2C clock SCL and the
pin SDA is used for the I 2C data signal SDA. Both these sig-
TABLE 2. Chip Address
D7
D6
D5
D4
D3
D2
D1
D0
Chip Address
I2C Adress='0'
1
1
0
0
1
1
0
W/R
2nd Chip Address
I2C Adress='1'
1
1
0
0
1
1
1
W/R
1st
Note 13: The master should issue STOP after no acknowledgement.
I2C DATA VALIDITY
The data on SDA line must be stable during the HIGH period
of the clock signal (SCL). In other words, state of the data line
can only be changed when SCL is LOW.
TRANSFERRING DATA
Every byte put on the SDA line must be eight bits long, with
the most significant bit (MSB) being transferred first. Each
byte of data has to be followed by an acknowledge bit. The
acknowledge related clock pulse is generated by the master.
The transmitter releases the SDA line (HIGH) during the acknowledge clock pulse. The receiver must pull down the SDA
line during the 9th clock pulse, signifying an acknowledge. A
receiver which has been addressed must generate an acknowledge after each byte has been received.
After the START condition, the I2C master sends a chip address. This address is seven bits long followed by an eighth
bit which is a data direction bit (R/W). The LMV1088 address
is 110011002or 110011102. For the eighth bit, a “0” indicates
a WRITE and a “1” indicates a READ. The second byte selects the register to which the data will be written. The third
byte contains data to write to the selected register.
202130q1
I2C Signals: Data Validity
I2C START AND STOP CONDITIONS
START and STOP bits classify the beginning and the end of
the I2C session. START condition is defined as SDA signal
transitioning from HIGH to LOW while SCL line is HIGH.
STOP condition is defined as the SDA transitioning from LOW
to HIGH while SCL is HIGH. The I2C master always generates
START and STOP bits. The I2C bus is considered to be busy
after START condition and free after STOP condition. During
data transmission, I2C master can generate repeated START
conditions. First START and repeated START conditions are
equivalent, function-wise.(Note 13)
202130q3
I2C Chip Address
Register changes take effect at the SCL rising edge during
the last ACK from slave.
In Figure 2 there is a write example shown, for a device at a
random chosen address'001101002'.
202130q2
I2C Start Stop Conditions
11
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LMV1088
nals need a pull-up resistor according to I2C specification. The
LMV1088 can be controlled on two slave addresses depending on the logical level at the I2C address pin. The two I2C
slave address for LMV1088 are given inTable 2 .
Application Data
LMV1088
202130q5
w = write (SDA = “0”)
r = read (SDA = “1”)
ack = acknowledge (SDA pulled down by slave)
rs = repeated start
FIGURE 2. Example I2C Write Cycle
When a READ function is to be accomplished, a WRITE function must precede the READ function, as shown in the Read
Cycle waveform.
In Figure 3, there is a read example shown, for a device at a
random chosen address'001101012'.
202130q6
FIGURE 3. Example I2C Read Cycle
202130q9
FIGURE 4. I2C Timing Diagram
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12
Symbol
Limit
Parameter
Min
Max
Units
1
Hold Time (repeated) START Condition
0.6
µs
2
Clock Low Time
1.3
µs
3
Clock High Time
600
ns
4
Setup Time for a Repeated START Condition
600
5
Data Hold Time (Output direction, delay generated by LMV1088)
300
900
ns
5
Data Hold Time (Input direction, delay generated by the Master)
0
900
ns
6
Data Setup Time
100
7
Rise Time of SDA and SCL
20
300
ns
8
Fall Time of SDA and SCL
15
300
ns
ns
ns
9
Set-up Time for STOP condition
600
10
Bus Free Time between a STOP and a START Condition
1.3
ns
Cb
Capacitive Load for Each Bus Line
10
200
µs
pF
NOTE: Data guaranteed by design
TABLE 4. Register Map
Address Reg.
7
6
5
4
3
2
1
0
A
Men[2]
Men[1]
M[2]
M[1]
MPA[3]
MPA[2]
MPA[1]
MPA[0]
0x02h
B
0
0
0
MicSel[1]
MicSel[0]
Gpa[2]
Gpa[1]
Gpa[0]
0x12h
R
0
0
0
0
0
0
0
CAL
0x01h
TABLE 5. I2C Register Description
Reg.
Bits
Description
Default
Microphone preamplifier gain from 6dB up to 36dB in 2dB steps.
0000
A
[3:0]
6dB
0001
8dB
0010
10dB
0011
12dB
0100
14dB
0101
16dB
0110
18dB
0111
20dB
1000
22dB
1001
24dB
1010
26dB
1011
28dB
1100
30dB
1101
32dB
1110
34dB
1111
36dB
default
0111
A
[5:4]
A4 = Mute mic1 and A 5 = mute mic2.
( 0 = microphone on)
00(on)
A
[7:6]
Mic enable bits, A6 = enable Mic1, A7 = enable Mic2
(1 = enable)
11(on)
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LMV1088
TABLE 3. I2C Timing Paramters
LMV1088
Reg.
Bits
Description
Default
Gain setting for the post amplifier from (3dB steps) (Note 11).
Pass Through
mode
B
[2:0]
Noise Canceling
mode
000
-2.5dB
0db
001
0.5dB
3dB
010
3.5dB
6dB
011
6.5dB
9dB
100
9.5dB
12dB
101
9.5dB
12dB
110
9.5dB
12dB
111
9.5dB
12dB
default
000
Mic select bits
B
B
[4:3]
[7:5]
R
[0]
R
[7:1]
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00
Noise canceling mode
01
Only Mic1 on
10
Only Mic2 on
11
Mic1 + Mic2
Not Used
00
000
Start Calibration via I2C
'0' to '1'= start calibration (keep '1' during calibration)
internal test
0
0000000
14
The full automatic calibration should only be required once,
when the product containing the LMV1088 has completed
manufacture, and prior to application packaging. The product
containing the LMV1088 will be calibrated to the microphones, the microphone spacings, and the acoustical properties of the final manufactured product containing the
LMV1088.
The compensation or calibration technology is achieved via
memory stored coefficients when the FFNS circuitry activates
the calibration sequence. The purpose of the calibration sequence is to choose the optimized coefficients for the FFNS
circuitry for the given microphones, spacing, and acoustical
environment of the product containing the LMV1088
A basic calibration can be performed with a single 1kHz tone,
however to take full advantage of this calibration feature a
three tone calibration (See the section PERFORMING A
THREE TONE CALIBRATION) is preferred .
The automatic calibration process can be initiated from either
a digital interface CALIBRATE pin (CAL) or via the I2C interface.
The logic level at the PROGRAM ENABLE (PE) pin determines if the result of the calibration is volatile or permanent.
AUTOMATIC CALIBRATION VIA I2C COMMAND
To initiate the automatic calibration via the CAL pin, the following procedure is required:
• From the initial condition where both PE and CAL are at
'low' level
• bring PE to a 'high' level (enable EEprom write)
• bring CAL to a 'high' level to start Calibration
• Apply Audio stimulus (single tone 1kHz or three tone
sequence as described in PERFORMING A THREE
TONE CALIBRATION)
• Hold CAL 'high' for at least 770ms
• Remove Audio stimulus
• bring CAL to a 'low' level to stop Calibration
• bring PE to a 'low' level (disable EEprom write)
A tone may be applied prior to the rising of CAL and PE. Signals applied to the microphone inputs before rising of CAL
and PE are ignored by the calibration system.
202130r1
FIGURE 5. Automatic Calibration via CAL pin
Note: When the I2C is operated, make sure that register 'R' (address 0x12)
bit 0 is '0' before operating the CAL pin (default value for this bit).
When this bit is set '1' the calibration engine of the LMV1088 is started
and will remain active with a higher supply current than normal operation. The state of the calibration remains active until this bit is reset,
'0”. With the bit set the 'low' to' high' transfer of the CAL pin will be
ignored.
•
Apply Audio stimulus (single tone 1kHz or three tone
sequence as described in PERFORMING A THREE
TONE CALIBRATION)
• Wait at least 770ms
• Remove Audio stimulus
• Write '0' into I2C to finish calibration
• Bring PE to a 'low' level (disable EEprom write)
A tone may be applied prior to the rising of CAL and PE. Signals applied to the microphone inputs before rising of CAL
and PE are ignored by the calibration system. .
AUTOMATIC CALIBRATION VIA CAL PIN
To initiate the automatic calibration via the I2 interface, the
following procedure is required:
• From the initial condition where PE is 'low' level
• Bring PE to a 'high' level (enable EEprom write)
• Write '1' into I2C register 'R' (address 0x12) bit 0 to start
calibration
202130r2
FIGURE 6.
15
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LMV1088
To make the result of the calibration permanent (stored in the
EEPROM) the PROGRAM ENABLE (PE) pin must be high
during the automatic calibration process.
Calibration
LMV1088
• A second tone with a frequency of 300Hz
• A third tone with a frequency of 3kHz
A tone may be applied prior to the rising of CAL and PE. Signals applied to the microphone inputs before rising of CAL
and PE are ignored by the calibration system. .
Between each tone pair there is a small time, indicated by a
cross, to change the frequency. During that time the input tone
is ignored by the calibration system.
The total calibration sequence requires less then 770ms.
PERFORMING A THREE TONE CALIBRATION
In a system with two microphones in an enclosure there will
always be a difference in the transfer function in both gain and
frequency response. The LMV1088 has the capability to perform an automatic calibration function to minimize these differences. To perform this calibration, a test sequence of three
tones is required right after the PE and CAL inputs are brought
to a logic high level. At the end of this sequence the calibration
data is automatically stored in the internal EEPROM.
The three tones have to be applied as follows:
• A first tone with a frequency of 1kHz
202130r3
FIGURE 7. Three Tone Calibration Timing
TABLE 6. Automatic Calibration Timing Parameters
Symbol
Limits
Parameter
Min
Max
Unitis
tST1
Calibration Start Tone 1
tET1
Calibration End Tone 1
tST2
Calibration Start Tone 2
tET2
Calibration End Tone 2
tST3
Calibration Start Tone 3
tET3
Calibration End Tone 3
600
ms
tCC
Calibration Complete
770
ms
10
200
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ms
215
400
16
ms
ms
415
NOTE: Data guaranteed by design
ms
ms
20213035
FIGURE 8. Three Tone Calibration Test setup
SUPPLY CURRENT DURING CALIBRATION
The Calibration function performs two main tasks in a sequence. First the AC characteristics of the microphones are
matched. Then in the second stage, if the PE pin is high, the
on-chip EEPROM is programmed.
During the first stage of this sequence the supply current on
the LMV1088 will increase to about 2.5 mA. During the writing
of the EEPROM the supply current will rise for about 215ms
to about 30 mA. This increased current is used for the on chip
charge pump which generates the high voltages that are required for programming the EEPROM.
20213036
FIGURE 9. Supply current during calibration and
programming
17
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LMV1088
The sound will travel with the limited speed of 300m/s from
the loudspeaker source to the microphones. When creating
the calibration signals this time should not be ignored, 30cm
distance will cause 1ms delay.
THREE TONE CALIBRATION SETUP
A calibration test setup consist of a test room (acoustical box)
with a loudspreaker (acoustical source) driven with the test
tone sequence from Figure 7. The test setup is shown in Figure 8. The distance between the source and microphone 1
and microphone 2 must be equal and the sound must travel
without any obstacle from source to both microphones.
LMV1088
Low-Pass Filter At The Output
Measurement Setup
At the output of the LMV1088 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 approximately compensated by a firstorder low-pass filter with cutoff frequency between 1.5kHz
and 2.5kHz.
The transfer function of the low pass filter is derived as:
Because of the nature of the calibration system it is not possible to predict the absolute gain in the two microphone
channels of the Far Field Noise Cancelling System. This is
because, after the calibration function has been operated, the
noise cancelling circuit will compensate for the difference in
gain between the microphones. In Noise Cancelling mode,
this can result in a final gain offset of max 3dB between the
gain set in the registers (RA[3:0] and RB[2:0]) and the actual
measured gain between input and output of the LMV1088.
After performing a calibration the frequency characteristic of
the microphone channels will be matched for the two microphones. As a result of this matching there can be a slight slope
in the frequency characteristic in one or both amplifiers.
A-WEIGHTED FILTER
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
and THD+N 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 LMV1088. 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 7.
TABLE 7. Low-pass Filter internal impedance
Post Amplifier Gain
Setting (dB) (Note 14)
Feedback resistance Rif
0
20
3
29
6
40
9
57
12
80
(kΩ)
This will result in the following values for a cutoff frequency of
2000 Hz:
TABLE 8. Low—pass Filter Capacitor for 2kHz
Post Amplifier Gain Setting (dB)
(Note 14)
Rif (kΩ)
Cf (nF)
0
20
3.9
3
29
2.7
6
40
2.0
9
57
1.3
12
80
1.0
20213010
Note 14: Noise Cancelling Mode
FIGURE 10. A-Weighted Filter
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18
20213011
FIGURE 11. Noise Measurement Setup
For the signal to noise ratio (SNR) the signal level at the output is measured with a 1kHz input signal of 18mVPP using an
A-weighted filter. This voltage represents the output voltage
of a typical electret condenser microphone at sound pressure
level of 94dB SPL, which is the standard level for these measurements. The LMV1088 is programmed for 17.5dB of total
gain (20dB pre-amplifier and -2.5dB post-amplifier) with only
Mic1 or Mic2 on. (See also I2C Compatible Interface)
The input signal is applied differential between the corresponding Mic+ and Mic- . Because the part is in Pass Through
mode the Low-pass Filter at the output of the LMV1088 is
disabled.
19
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LMV1088
The Mic+ and Mic- inputs of the LMV1088 are shorted for AC
signals via a short between the input capacitors , see Figure
11.
MEASURING NOISE AND SNR
The overall noise of the LMV1088 is measured within the frequency band from 10Hz to 22kHz using an A-weighted filter.
LMV1088
Revision History
Rev
Date
Description
1.0
09/26/07
Initial release.
1.01
12/10/07
Few text edits (changed TL to RL).
www.national.com
20
LMV1088
Physical Dimensions inches (millimeters) unless otherwise noted
36 Bump micro SMD Technology
NS Package Number RLA36VVA
X1 = 3.51±0.03, X2 = 3.51±0.03, X3 = 0.6±0.075,
21
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LMV1088 Dual Input, Far Field Noise Suppression Microphone Amplifier with Automatic
Calibration Ability
Notes
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