TLV1012 datasheet(7/12 Pages) TI

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Measuring Noise and SNR

5 pF

A-Weighted

Filter

Sound Pressure Level

Absolute

Sound

Pressure

(dBPa)

–94 dB

Sensitivity

(dBV/Pa)

Sound

Pressure

(dBSPL)

Voltage

(dBV)

TLV1012

www.ti.com .......................................................................................................................................... SLCS154A – OCTOBER 2008 – REVISED NOVEMBER 2008

The overall noise of the TLV1012 is measured within the frequency band from 10 Hz to 22 kHz using an

A-weighted filter. The input of the TLV1012 is connected to ground with a 5-pF capacitor.

Figure 4. Noise Measurement

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 dBSPL. No input capacitor is connected.

The volume of sound applied to a microphone is usually stated as the pressure level with respect to the threshold

of hearing of the human ear. The sound pressure level in decibels is defined by:

Sound pressure level (dB) = 20 log Pm/PO

Where Pm is the measured sound pressure, and PO is the threshold of hearing (20 µPa).

To calculate the resulting output voltage of the microphone for a given sound pressure level, the sound pressure

in dBSPL needs to be converted to the absolute sound pressure in dBPa. This is the sound pressure level in

decibels, which is referred to as 1 Pascal (Pa).

The conversion is given by:

dBPa = dBSPL + 20 log 20

µPa

dBPa = dBSPL – 94 dB

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.

Figure 5. dB SPL to dBV Conversion

For example, busy traffic is 70 dBSPL:

VOUT = 70 – 94 – 44 = –68 dBV

This is equivalent to 1.13 mVPP.

Because the TLV1012-15 has a gain of 6 (15.6 dB) over the JFET, the output voltage of the microphone is

6.78 mVPP. By replacing the JFET with the TLV1012-15, the sensitivity of the microphone is –28.4 dBV/Pa (–44 +

15.6).

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