NSC LMV1024URX Pdm output with pre-amplifier for electret microphone Datasheet

LMV1024/LMV1026 (Stereo)
PDM Output with Pre-Amplifier for Electret Microphones
General Description
Features
National’s LMV1024 and LMV1026 stereo amplifiers are
solutions for the new generation of voice enrichment capabilities. National has integrated sigma delta modulation and
analog cores to improve the voice quality and the performance and to support designer’s choices.
Each 20 kHz preamplifier drives a Pulse Density Modulated
(PDM) signal at an over sampled 60 bit stream, offering
versatility. These solutions provide immediate conversion to
high performance audio spectrum, thus completing a high
quality audio system. The LMV1024 and LMV1026 operate
from 1.6V to 3V.
(Typical VDD = 1.8V, CLOCK = 1.2 MHz, fINPUT = 1 kHz,
VINPUT = 18 mVPP, unless otherwise specified)
n Enhanced high-performance, full PDM output from the
element
n Stereo chipset and array routing
n SNR A-weighted
59 dB
n Digital A-weighted noise floor
−89 dBFS
n Supply current
518 µA
n Clock frequency
400 kHz to 2.4 MHz
n Total harmonic distortion
0.03%
n Power supply rejection ratio
100 dB
n Adhesion technology > 1 kg
n Highly integrated stereo or mono signaling
n Maximized system performance
n Reduced components and layout
n RF (buzz noise) managed with 4 wire signaling
n Thinnest 0.35 mm micro SMD packaging
National’s new adjustable clock frequency technology is designed for stereo, performance and ease of use. The stereo
function is either a rising or falling edge clock command. The
high drive, robust PDM signal directly from an ECM (Electret
Condenser Microphone), upgrades existing lower quality,
low-level signal constraints.
These advanced mixed signal preamplifiers cleanly, accurately and creatively eliminate older, poorer quality systems,
which restricted performance and versatility. The 4-wire
PDM signaling reduces RF noise and simplifies layout. Using National’s PDM solutions is the choice for higher quality
mono or stereo and multi-array applications.
National’s 518uA circuits deliver stereo portability with audio
quality bandwidth. These solutions enable rapid system
evaluation and enhance consumer satisfaction. National provides the LMV1024 and LMV1026 in 6-bump micro SMD
packages with 1 kg adhesion properties.
Applications
n
n
n
n
n
n
Digital output audio subsystems and stereo arrays
Electret condenser microphones with all digital output
Portable communications and small form factor
Digital audio computing or voice security
Automotive or array systems
Headphone and Headset accessories
Typical Application
20133475
For a stereo application, see STEREO OPERATION in the Application Section.
© 2005 National Semiconductor Corporation
DS201334
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LMV1024/LMV1026 PDM Output with Pre-Amplifier for Electret Microphones
November 2005
LMV1024/LMV1026
Absolute Maximum Ratings (Note 1)
Junction Temperature (Note 6)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Mounting Temperature
Infrared or Convection (20 sec.)
ESD Tolerance (Note 2)
Human Body Model
Supply Voltage
250V
1.6V to 3.0V
Input Clock Frequency
Supply Voltage
VDD - GND
400 kHz to 2.4 MHz
Duty Cycle
3.3V
Storage Temperature Range
235˚C
Operating Ratings (Note 1)
2500V
Machine Model
150˚C max
40% to 60%
Operating Temperature Range
−65˚C to 150˚C
−40˚C to 85˚C
1.8V Electrical Characteristics
(Note 3)
Unless otherwise specified, all limits are guaranteed for TJ = 25˚C, VDD = 1.8V, VIN = 18 mVPP, fCLK = 1.2 MHz, Duty Cycle =
50% and 100 nF capacitor between VREF and GND. Boldface limits apply at the temperature extremes.
Symbol
Parameter
Conditions
Min
(Note 4)
Typ
(Note 5)
SNR
Signal to Noise Ratio
fIN = 1 kHz, A-Weighted
59
eN
Digital Noise Floor (Integrated)
f = 20 Hz to 10 kHz, A-Weighted, 4.7
pF Capacitor Connected from Input
to GND to Simulate ECM, No Signal
−89
THD
Total Harmonic Distortion
fIN = 1 kHz, VIN = 18 mVPP
0.03
Supply Current
VIN = GND, CLK = ON, High
Impedance Load (Note 7)
518
VIN = GND, CLK = OFF
503
CLOCK Input Logic Low Level
VIH
CLOCK Input Logic High Level
Units
dB
dBFS(A)
IDD
VIL
Max
(Note 4)
%
µA
600
0.3
1.5
V
V
VOL
DATA Output Logic Low Level
VOH
DATA Output Logic High Level
VIN
Max Input Signal
fIN = 1 kHz, THD < 1%
243
mVPP
VOUT
Max Output Signal
fIN = 1 kHz, THD < 1%
−6.8
dBFS
PSRR
Power Supply Rejection Ratio
VIN = GND, Test Signal on VDD =
217 Hz, 100 mVPP
100
dB
tA
Time from CLOCK Transition to
DATA Becoming High
Impedance (See also Figure 10,
Application Section)
LMV1024: On Rising Edge of the
CLOCK
65
ns
Time from CLOCK Transition to
DATA Becoming Valid (See also
Figure 10, Application Section)
LMV1024: On Falling Edge of the
CLOCK
90
ns
tB
CIN
Input Capacitance
RIN
Input Impedance
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0.1
1.7
LMV1026: On Falling Edge of the
CLOCK
LMV1026: On Rising Edge of the
CLOCK
2
V
V
2
pF
1000
MΩ
(Note 3)
Unless otherwise specified, all limits are guaranteed for TJ = 25˚C, VDD = 2.7V, VIN = 18 mVPP, fCLK = 1.2 MHz, Duty Cycle =
50% and 100 nF capacitor between VREF and GND. Boldface limits apply at the temperature extremes.
Symbol
Parameter
Conditions
Min
(Note 4)
Typ
(Note 5)
Max
(Note 4)
Units
SNR
Signal to Noise Ratio
fIN = 1 kHz, A-Weighted
59
dB
en
Digital Noise Floor (Integrated)
f = 20 Hz to 10 kHz, A-Weighted, 4.7
pF Capacitor Connected from Input
to GND to Simulate ECM, No Signal
−89
dBFS(A)
THD
Total Harmonic Distortion
fIN = 1 kHz, VIN = 18 mVPP
0.03
%
IDD
Supply Current
VIN = GND, CLK = ON, High
Impedance Load (Note 7)
535
µA
VIN = GND, CLK = OFF
519
VLOW
CLOCK Logic Low Level
VHIGH
CLOCK Logic High Level
VOL
DATA Output Logic Low Level
VOH
DATA Output Logic High Level
VIN
Max Input Signal
650
0.3
V
0.1
V
2.4
V
2.6
V
fIN = 1 kHz, THD < 1%
249
mVPP
VOUT
Max Output Signal
fIN = 1 kHz, THD < 1%
−6.6
dBFS
PSRR
Power Supply Rejection Ratio
VIN = GND, Test Signal on VDD =
217 Hz, 100 mVPP
100
dB
tA
Time from CLOCK Transition to
DATA Becoming High
Impedance (See also Figure 10,
Application Section)
LMV1024: On Rising Edge of the
CLOCK
65
ns
Time from CLOCK Transition to
DATA Becoming Valid (See also
Figure 10, Application Section)
LMV1024: On Falling Edge of the
CLOCK
90
ns
2
pF
1000
MΩ
tB
CIN
Input Capacitance
RIN
Input Impedance
LMV1026: On Falling Edge of the
CLOCK
LMV1026: On Rising Edge of the
CLOCK
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is
intended to be functional, but specific performance is not guaranteed. For guaranteed specifications and the test conditions, see the Electrical Characteristics.
Note 2: The Human Body Model (HBM) is 1.5 kΩ in series with 100 pF. The Machine Model is 0Ω in series with 200 pF.
Note 3: Electrical table values apply only for factory testing conditions at the temperature indicated. Factory testing conditions result in very limited self-heating of
the device such that TJ = TA. No guarantee of parametric performance is indicated in the electrical tables under conditions of internal self-heating where TJ > TA.
Note 4: All limits are guaranteed by design or statistical analysis.
Note 5: Typical values represent the most likely parametric norm.
Note 6: The maximum power dissipation is a function of TJ(MAX) , θJA and TA. The maximum allowable power dissipation at any ambient temperature is
PD = (TJ(MAX) - TA)/θJA. All numbers apply for packages soldered directly into a PC board.
Note 7: The Supply Current depends on the applied Clock Frequency and the load on the DATA output.
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LMV1024/LMV1026
2.7V Electrical Characteristics
LMV1024/LMV1026
Connection Diagram
Large Dome 6-Bump Ultra Thin micro SMD
20133427
Top View
Pin Description
Pin
Name
Description
A2
VDD
Positive supply voltage
C1
GND
Ground
Input
C2
Input
The microphone is connected to this input pin.
Reference
B1
VREF
A capacitor of 100 nF is connected between VREF and ground. This capacitor is used to
filter the internal converter reference voltage.
Clock Input
A1
Clock
The user adjustable clock frequency ranges from 400 kHz to 2.4 MHz.
Data Output
B2
Data
Over sampled bitstream output. Data is valid if clock is LOW (LMV1024). The data of
the LMV1026 data is valid when clock is HIGH. When the data is not valid the data
output is Hi-Z. For exact specifications see application section.
Power Supply
Ordering Information
Package
Part Number
Package Marking
LMV1024UR
6-Bump Ultra Thin micro SMD
lead free only
IE
LMV1024URX
LMV1026UR
IF
LMV1026URX
Transport Media
3k Units Tape and Reel
250 Units Tape and Reel
3k Units Tape and Reel
Block Diagram
20133426
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4
NSC Drawing
250 Units Tape and Reel
URA06GGA
Supply Current vs. Supply Voltage
@ CLOCK = 2.4 MHz
Supply Current vs. Supply Voltage
@ CLOCK = 1.2 MHz
20133446
20133430
Supply Current vs. Supply Voltage
CLOCK = OFF
SNR, SINAD and -THD vs. Input Amplitude
@ CLOCK = 408 kHz, 3.4 kHz Audio BW
20133431
20133447
SNR, SINAD and -THD vs. Input Amplitude
@ CLOCK = 960 kHz, 8 kHz Audio BW
SNR, SINAD and -THD vs. Input Amplitude
@ CLOCK = 1.2 MHz, 10 kHz Audio BW
20133448
20133449
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LMV1024/LMV1026
Typical Performance Characteristics Unless otherwise specified, measurements are performed on
an LMV1024 with VDD = 1.8V, Clock Duty Cycle = 50% and a 100 nF capacitor is placed between VREF and GND, TJ = 25˚C.
LMV1024/LMV1026
Typical Performance Characteristics Unless otherwise specified, measurements are performed on
an LMV1024 with VDD = 1.8V, Clock Duty Cycle = 50% and a 100 nF capacitor is placed between VREF and GND, TJ =
25˚C. (Continued)
SNR, SINAD and -THD vs. Input Amplitude
@ CLOCK = 1.92 MHz, 16 kHz Audio BW
SNR, SINAD and -THD vs. Input Amplitude
@ CLOCK = 2.4 MHz, 20 kHz Audio BW
20133450
20133432
SNR, SINAD and -THD vs. Output
SNR, SINAD and -THD vs. Output
@ CLOCK = 408 kHz, 3.4 kHz Audio BW
@ CLOCK = 960 kHz, 8 kHz Audio BW
20133451
20133452
SNR, SINAD and -THD vs. Output
SNR, SINAD and -THD vs. Output
@ CLOCK = 1.2 MHz, 10 kHz Audio BW
@ CLOCK = 1.92 MHz, 16 kHz Audio BW
20133453
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20133454
6
SNR, SINAD and -THD vs. Output
SINAD vs. Frequency
@ CLOCK = 2.4 MHz, 20 kHz Audio BW
@ CLOCK = 408 kHz, 3.4 kHz Audio BW
20133433
20133455
SINAD vs. Frequency
SINAD vs. Frequency
@ CLOCK = 960 kHz, 8 kHz Audio BW
@ CLOCK = 1.2 MHz, 10 kHz Audio BW
20133456
20133457
SINAD vs. Frequency
SINAD vs. Frequency
@ CLOCK = 1.92 MHz, 16 kHz Audio BW
@ CLOCK = 2.4 MHz, 20 kHz Audio BW
20133458
20133434
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LMV1024/LMV1026
Typical Performance Characteristics Unless otherwise specified, measurements are performed on
an LMV1024 with VDD = 1.8V, Clock Duty Cycle = 50% and a 100 nF capacitor is placed between VREF and GND, TJ =
25˚C. (Continued)
LMV1024/LMV1026
Typical Performance Characteristics Unless otherwise specified, measurements are performed on
an LMV1024 with VDD = 1.8V, Clock Duty Cycle = 50% and a 100 nF capacitor is placed between VREF and GND, TJ =
25˚C. (Continued)
Noise vs. Frequency
Noise vs. Frequency
@ CLOCK = 408 kHz, 3.4 kHz Audio BW
@ CLOCK = 960 kHz, 8 kHz Audio BW
20133459
20133460
Noise vs. Frequency
Noise vs. Frequency
@ CLOCK = 1.2 MHz, 10 kHz Audio BW
@ CLOCK = 1.92 MHz, 16 kHz Audio BW
20133461
20133462
PSRR vs. Frequency
@ CLOCK = 1.2 MHz
Noise vs. Frequency
@ CLOCK = 2.4 MHz, 20 kHz Audio BW
20133444
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20133463
8
PSRR vs. Frequency
@ CLOCK = 2.4 MHz
Output vs. Input Amplitude
@ CLOCK = 408 kHz, 3.4 kHz Audio BW
20133436
20133464
Output vs. Input Amplitude
Output vs. Input Amplitude
@ CLOCK = 960 kHz, 8 kHz Audio BW
@ CLOCK = 1.2 MHz, 10 kHz Audio BW
20133465
20133466
Output vs. Input Amplitude
Output vs. Input Amplitude
@ CLOCK = 1.92 MHz, 16 kHz Audio BW
@ CLOCK = 2.4 MHz, 20 kHz Audio BW
20133467
20133437
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LMV1024/LMV1026
Typical Performance Characteristics Unless otherwise specified, measurements are performed on
an LMV1024 with VDD = 1.8V, Clock Duty Cycle = 50% and a 100 nF capacitor is placed between VREF and GND, TJ =
25˚C. (Continued)
LMV1024/LMV1026
Typical Performance Characteristics Unless otherwise specified, measurements are performed on
an LMV1024 with VDD = 1.8V, Clock Duty Cycle = 50% and a 100 nF capacitor is placed between VREF and GND, TJ =
25˚C. (Continued)
Output vs. Frequency
Output vs. Frequency
@ CLOCK = 408 kHz, 3.4 kHz Audio BW
@ CLOCK = 960 kHz, 8 kHz Audio BW
20133468
20133469
Output vs. Frequency
Output vs. Frequency
@ CLOCK = 1.2 MHz, 10 kHz Audio BW
@ CLOCK = 1.92 MHz, 16 kHz Audio BW
20133470
20133471
Output vs. Frequency
@ CLOCK = 2.4 MHz, 20 kHz Audio BW
20133438
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10
Figure 1 depicts a typical application, where the LMV1024 or
LMV1026 is built inside the ECM canister. This ECM can be
directly connected to a DSP in a digital audio system, like a
baseband chip in a cell phone. Connecting is easy because
of the digital LMV1024/LMV1026 interface. A digital filter in
the DSP or Baseband decimates the audio signal.
The LMV1024/LMV1026 consist of a pre-amplifier and sigma
delta converter for placement inside an electret condenser
microphone (ECM). The output of the LMV1024/LMV1026 is
a robust digital serial bit stream eliminating the sensitive
low-level analog signals of conventional JFET microphones.
This application section describes, among others, a typical
application, a sensitivity comparison between different ECM
types, stereo operation and layout recommendations on the
ECM PCBs.
20133424
FIGURE 1. Typical Application
BUILT-IN PRE-AMPLIFIER / ADC
The LMV1024/LMV1026 are offered in a space saving small
6-bump micro SMD package in order to fit inside small ECM
canisters. The LMV1024 or LMV1026 IC is placed on the
PCB. This PCB forms the bottom of the microphone, which is
placed in the cell phone.
20133443
20133474
FIGURE 2. Built-in Pre-Amplifier / ADC
FIGURE 3. Cross section of a Microphone
Figure 3 depicts a cross section of a microphone with the IC
inside the ECM canister. The PCB of the microphone has 4
pads that connects VDD, Ground, DATA and the CLOCK.
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LMV1024/LMV1026
TYPICAL APPLICATION
Application Section
LMV1024/LMV1026
Application Section
sensitivity. Examples are given to calculate the resulting
output for a given sound pressure.
(Continued)
A-WEIGHTED FILTER
Sound Pressure Level
The human ear has a frequency range from about 20 Hz to
20 kHz. Within this range the sensitivity of the human ear is
not equal for each frequency. In order to approach a natural
hearing response, weighting filters are introduced. One of
these filters is the A-weighted filter. The A-weighted filter is
commonly used in signal-to-noise ratio measurements,
where sound is compared to device noise. The filter improves the correlation of the measured data to the signal-tonoise ratio perceived by the human ear.
The volume of sound applied to a microphone is usually
stated as a sound pressure in dB SPL. This unity of dB SPL
refers to the threshold of hearing of the human ear. The
sound pressure in decibels is defined by:
SPL = 20 log (PM/PO)
Where,
SPL is the Sound Pressure in dB SPL
PM is the measured absolute sound pressure in Pa
PO is the threshold of hearing (20 µPa)
In order to calculate the resulting output voltage of the
electret element for a given sound pressure in dB SPL, the
absolute sound pressure PM must be known. This is the
absolute sound pressure in decibels referred to 1 Pa instead
of 20 µPa.
The absolute sound pressure PM in dBPa is given by:
PM = SPL (dB SPL) + PO (dBPa)
PM = SPL + 20*log 20 µPa
PM = SPL - 94 dB
JFET Microphone
Translation from the absolute sound pressure level to a
voltage can be done when the electrets sensitivity is known.
A typical electret element has a sensitivity of −44 dB(V/Pa).
This is also the typical sensitivity number for the JFET microphone, since a JFET usually has a gain of about 1x (0
dB). A block diagram of a microphone with a JFET is given in
Figure 5.
Example: Busy traffic has a Sound Pressure of 70 dB SPL.
Microphone Output = SPL + C + S
Where,
SPL is the Sound Pressure in dB SPL
C is the dB SPL to dBPa conversion (−94 dB)
S is the Sensitivity in dB(V/Pa)
Microphone Output = 70 – 94 – 44 = −68 dBV
This is equivalent to 1.13 mVPP.
The analog output signal is so low that it can easily be
distorted by interference from outside the microphone. Additional gain is desirable to make the signal less sensitive to
interference.
20133440
FIGURE 4. A-weighted Filter
SENSITIVITY
Sensitivity is a measure for the transfer from the applied
acoustic signal to the output of the microphone. Conventional JFET microphones and microphones with built-in gain
have a sensitivity that is expressed in dB(V/Pa), where 0dB
= 1V/Pa. A certain pressure on the electret of the microphone gives a certain voltage at the output of the microphone. Since the LMV1024 microphone has a digital output,
the sensitivity will be stated in dB(Full Scale/Pascal) or
dB(FS/Pa) as opposed to conventional microphones. This
section compares the various microphone types and their
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12
LMV1024/LMV1026
Application Section
(Continued)
20133445
FIGURE 5. Microphone Sensitivity
Microphone with Additional Gain
When gain is added to the electret element, the analog
signal becomes larger and therefore more robust. This can
be accomplished by using a pre-amplifier with a higher gain
then the JFET. The sensitivity of the microphone consists of
the sensitivity of the electret plus the gain of the preamplifier. When choosing National Semiconductor’s
LMV1015-15 for instance, a gain of 15 dB is added by the
pre-amplifier. This results in a sensitivity of −29 dB(V/Pa)
with a typical electret element of −44 dB(V/Pa). National
Semiconductor has a wide range of pre-amplifiers with different gain factors, which can be used to replace the JFET
inside the microphone canister. Please visit www.national.com for more information on the LMV1015 and LMV1032
pre-amplifier series. A block diagram with the LMV1015 preamplifier inside an ECM is given in Figure 5.
When taking the same example of busy traffic (70 dB SPL),
the output voltage of the microphone with the LMV1015 is:
Microphone Output = SP + C + S
Where,
SP is the Sound Pressure in dB SPL
C is the dB SPL to dBPa conversion (−94 dB)
S is the Sensitivity in dB(V/Pa)
Microphone output = 70 - 94 - 29 = −53 dBV.
This is equivalent to 6.33 mVPP.
The pre-amplifier with additional gain reduces the impact of
noise on the wiring / traces from the microphone to the
baseband chip significantly. To minimize interference ultimately, an Analog-to-Digital converter is integrated in both
the LMV1024/LMV1026, realizing a digital interface between
the microphone and the baseband.
Digital Microphone
By integrating the Analog-to-Digital converter (ADC) in the
LMV1024/LMV1026 all analog signals are kept within the
“shielded” microphone canister. The output is a digital interface that is robust and insensitive to interference and noise
from outside the canister. The output is expressed in dBFS
and therefore the sensitivity is also stated in dB(FS/Pa)
instead of dB(V/Pa). To calculate the digital output (Data) in
dBFS the following equation can be written for the LMV1024/
LMV1026:
(1)
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LMV1024/LMV1026
Application Section
the LMV1024. The sensitivity of a typical digital microphone
is therefore: −44 + 15.2 = −28.8 dB(FS/Pa).
(Continued)
Where,
Digital Output = SP + C + S
Where,
SP is the Sound Pressure in dB SPL
C is the dB SPL to dBPa conversion (−94 dB)
S is the Sensitivity in dB(V/Pa)
PREF is the reference power, which is defined as the maximum allowed input power (Full Scale). PINPUT is the applied
power on the input pin and “A” is the gain of the pre-amplifier
in decibels.
Written into voltages, the equation is:
Taking the example of busy traffic (70 dB SPL) again results
in the following digital output (dBFS):
Digital Output (dBFS) = SP - C + S
Digital Output (dBFS) = 70 - 94 - 28.8 = −52.8 dBFS
(2)
ANALOG-TO-DIGITAL CONVERTER
Or in decibels:
Digital Output (dBFS) = Input (dBV) - Reference (dB) + A
Where,
Input = 20 Log VINPUT (VRMS)
Ref = 20 Log VREF (VRMS)
A is the Gain (dB)
The ADC used in the LMV1024/LMV1026 is an one bit sigma
delta converter with a Pulse Density Modulated output signal
(PDM). The output of this ADC can be either High (one) or
Low (zero). Assume that the LMV1024/LMV1026 input is at
the minimum level. In that case the DATA output will produce
almost only “zeros”. When the input increases, the amount of
“ones” increases too. At mid-point, where the input is 0V, the
number of “zeros” will equal the number of “ones”. At the
time that the input approaches the maximum level, the DATA
output produces a majority of “ones”. Figure 6 shows the
resulting DATA output as function of the input.
For the LMV1024/LMV1026 the reference voltage VREF is
1.5VP (1.06 VRMS) and the Gain A is 15.7 dB. These parameters are fixed inside the device. Knowing this, Equation (2)
can be simplified:
Digital Output (dBFS) = VINPUT (dBV) - 0.5 + 15.7
Digital Output (dBFS) = VINPUT (dBV) + 15.2
The sensitivity of the digital microphone is the sensitivity of a
conventional microphone plus the input to output transfer of
20133472
FIGURE 6. DATA Output versus Input Amplitude
The high corner of the band of interest (knee) is determined
by the clock frequency divided by 2 times the OSR. The
factor of two comes from the Nyquist theorem. The over
sampling ratio (OSR) of this particular ADC is chosen at 60.
This sets the high corner of the band at the clock frequency
divided by 120. For instance when a bandwidth of 10 kHz is
desired, the clock frequency needs to be 1.2 MHz or higher.
Figure 7 depicts the noise shaping effect in a frequency
spectrum plot, where a 1 kHz signal is applied.
An important characteristic of the sigma delta converter is
that the noise is shifted out of the band to frequencies above
the band of interest. The band that can be used (Audio
Bandwidth) relates directly the applied clock frequency.
Table 1 shows the relation between the Clock Frequency
and a couple of common Audio Bandwidths.
TABLE 1. Audio Bandwidth vs. Clock Frequency
Clock Frequency
Audio Bandwidth
408 kHz
3.4 kHz
960 kHz
8 kHz
1.2 MHz
10 kHz
1.92 MHz
16 kHz
2.4 MHz
20 kHz
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LMV1024/LMV1026
Application Section
(Continued)
20133425
FIGURE 9. Stereo Application
Both microphones produce valid data in only one half of a
clock cycle to allow the two microphones to operate on the
same I/O lines (Data and Clock). To avoid overlap between
the drivers of the microphones, one microphone always
goes into a high impedance state before the second microphone starts driving the data-line. The LMV1024 is positive
edge triggered while the LMV1026 is negative edge triggered. The timing between the two microphones is shown in
Figure 10. For exact timing values, please see the Electrical
Characteristics table.
20133461
FIGURE 7. Frequency Spectrum
To eliminate the noise above the band of interest a low pass
decimation filter is implemented in the baseband chip or
DSP. The resulting frequency spectrum contains only the
frequency components left within the band of interest. Figure
8 depicts the frequency spectrum after filtering.
20133429
FIGURE 10. Timing
LAYOUT CONSIDERATIONS
To obtain the best possible performance from the microphone, special care needs to be taken for the design of the
PCB. Especially the VIN trace is very sensitive as it is connected to the high impedance electret element. It is essential
to isolate and shield the VIN trace as much as possible from
the digital signal traces (DATA and CLOCK). This needs to
be done to avoid any switching noise coupling directly into
the input of the IC. An example of a PCB layout is given in
Figure 11. The microphone PCB has two capacitors. One
capacitor (100 nF) is connected to the reference pin of the
LMV1024/LMV1026. The other capacitor (10 nF) is used as
decoupling for high frequencies on the supply. No capacitors
should be placed on the data output of the LMV1024/
LMV1026 since it will only load the output driver and would
degrade the performance. This is opposite to the regular
analog phantom biased microphones, where capacitors are
needed to improve RFI.
20133473
FIGURE 8. Frequency Spectrum after Filtering
STEREO OPERATION
The LMV1024 and the LMV1026 are designed to operate
together in a stereo solution with two microphones. One
microphone will have a LMV1024 built-in and the other will
have a LMV1026 built-in. These two microphones share the
same interface lines to minimize wiring (Figure 9).
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LMV1024/LMV1026
Application Section
(Continued)
20133428
FIGURE 11. PCB Layout
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16
inches (millimeters) unless otherwise noted
NOTE: UNLESS OTHERWISE SPECIFIED.
1. FOR SOLDER BUMP COMPOSITION, SEE “SOLDER INFORMATION” IN THE PACKAGING SECTION OF THE NATIONAL SEMICONDUCTOR WEB
PAGE (www.national.com).
2. RECOMMEND NON-SOLDER MASK DEFINED LANDING PAD.
3. PIN A1 IS ESTABLISHED BY LOWER LEFT CORNER WITH RESPECT TO TEXT ORIENTATION.
4. XXX IN DRAWING NUMBER REPRESENTS PACKAGE SIZE VARIATION WHERE X1 IS PACKAGE WIDTH, X2 IS PACKAGE LENGTH AND X3 IS
PACKAGE HEIGHT.
5. NO JEDEC REGISTRATION AS OF MARCH 2003
6-Bump Ultra Thin micro SMD
NS Package Number URA06GGA
X1 = 1.128 mm, X2 = 1.628 mm, X3 = 0.35 mm
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LMV1024/LMV1026 PDM Output with Pre-Amplifier for Electret Microphones
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