lmv1024 National Semiconductor Corporation, lmv1024 Datasheet - Page 12

• Manufacturer Part Number: lmv1024
• Description: Ic Pre Amp Mic Mono Ab Adc 6usmd
• Manufacturer: National Semiconductor Corporation

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A-WEIGHTED FILTER
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-to-noise ratio
perceived by the human ear.
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
FIGURE 4. A-weighted Filter
201334 Version 3 Revision 4
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Print Date/Time: 2009/07/20 17:12:49
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to conventional microphones. This section compares the var-
ious microphone types and their sensitivity. Examples are
given to calculate the resulting output for a given sound pres-
sure.
Sound Pressure Level
The volume of sound applied to a microphone is usually stat-
ed as a sound pressure in dB SPL. This unity of dB SPL refers
to the threshold of hearing of the human ear. The sound pres-
sure in decibels is defined by:
SPL = 20 log (P
Where,SPL is the Sound Pressure in dB SPLP
sured absolute sound pressure in PaP
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 P
pressure in decibels referred to 1 Pa instead of 20 µPa.
The absolute sound pressure P
SPL (dB SPL) + P
SPL - 94 dB
JFET Microphone
Translation from the absolute sound pressure level to a volt-
age 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 SPLC is the dB SPL
to dBPa conversion (−94 dB)S is the Sensitivity in dB(V/Pa)
Microphone Output = 70 – 94 – 44 = −68 dBVThis is equiva-
lent to 1.13 mV
The analog output signal is so low that it can easily be dis-
torted by interference from outside the microphone. Addition-
al gain is desirable to make the signal less sensitive to
interference.
PP
M
/P
M
.
O
must be known. This is the absolute sound
O
)
(dBPa)P
M
= SPL + 20*log 20 µPaP
M
in dBPa is given by:P
O
is the threshold of
M
is the mea-
M
M
=
=