LMV1032UP-25 National Semiconductor Corporation, LMV1032UP-25 Datasheet - Page 8

• Manufacturer Part Number: LMV1032UP-25
• Description: Amplifiers For 3 Wire Analog Electret Microphones
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Section
LOW CURRENT
The LMV1032 has low supply current for a longer battery life.
The low supply current makes this amplifier suitable for
microphone applications which need to be always on.
BUILT IN GAIN
The LMV1032 is offered in space saving small micro SMD
package in order to fit in the metal can of a microphone. The
LMV1032 is placed on the PCB inside the microphone.
The bottom side of the PCB has the pins that connect the
supply voltage to the amplifier and make the output avail-
able. The input of the amplifier is connected inside the metal
can via the PCB to the microphone.
A-WEIGHTED FILTER
The human ear has a frequency range from 20 Hz to about
20 kHz. Within this range the sensitivity of the human ear is
not equal for each frequency. To approach the hearing re-
sponse weighting filters are introduced. One of those filters
is the A-weighted filter.
The A-weighted filter is usually used in signal to noise ratio
measurements, where sound is compared to device noise. It
improves the correlation of the measured data to the signal
to noise ratio perceived by the human ear.
FIGURE 1. Built-in Gain
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MEASURING NOISE AND SNR
The overall noise of the LMV1032 is measured within the
frequency band from 10 Hz to 22 kHz using an A-weighted
filter. The input of the LMV1032 is connected to ground with
a 5 pF capacitor.
Signal to noise ratio (SNR) is measured with a 1 kHz input
signal of 18 mV
a sound pressure level of 94 dB SPL. No input capacitor is
connected.
SOUND PRESSURE LEVEL
The volume of sound applied to a microphone is usually
stated as a pressure level referred to the threshold of hear-
ing of the human ear. The sound pressure level (SPL) in
decibels is defined by:
Sound pressure level (dB) = 20 log P
Where,
P
P
In order to be able to calculate the resulting output voltage of
the microphone for a given SPL, the sound pressure in dB
SPL needs to be converted to the absolute sound pressure
in dBPa. This is the sound pressure level in decibels referred
to 1 Pascal (Pa).
m
O
is the threshold of hearing (20µPa)
is the measured sound pressure
FIGURE 3. Noise Measurement Setup
FIGURE 2. A-Weighted Filter
PP
using an A-weighted filter. This represents
m
/P
O
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