A-WEIGHTED FILTER

5 pF

LMV1012

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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 mVPP 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 Pm/PO

where

•

Pm is the measured sound pressure

•

PO 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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