Datasheet

A-WEIGHTED FILTER
5 pF
LMV1012
www.ti.com SNAS194H –NOVEMBER 2002–REVISED MAY 2013
MEASURING NOISE AND SNR
The overall noise of the LMV1012 is measured within the frequency band from 10 Hz to 22 kHz using an A-
weighted filter. The input of the LMV1012 is connected to ground with a 5 pF capacitor, as in Figure 23. Special
precautions in the internal structure of the LMV1012 have been taken to reduce the noise on the output.
Figure 23. Noise Measurement Setup
The signal to noise ratio (SNR) is measured with a 1 kHz input signal of 18 mV
PP
using an A-weighted filter. This
represents a sound pressure level of 94 dB SPL. No input capacitor is connected for the measurement.
SOUND PRESSURE LEVEL
The volume of sound applied to a microphone is usually stated as a pressure level referred to the threshold of
hearing of the human ear. The sound pressure level (SPL) in decibels is defined by:
Sound pressure level (dB) = 20 log P
m
/P
O
where
• P
m
is the measured sound pressure
• P
O
is the threshold of hearing (20 μPa). (1)
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).
The conversion is given by:
dBPa = dB SPL + 20*log 20 μPa (2)
dBPa = dB SPL - 94 dB (3)
Translation from absolute sound pressure level to a voltage is specified by the sensitivity of the microphone. A
conventional microphone has a sensitivity of -44 dBV/Pa.
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