LMV1024 datasheet(13/17 Pages) NSC

13 / 17 page

Application Section (Continued)

Microphone with Additional Gain

When gain is added to the electret element, the analog

signal becomes larger and therefore more robust. This can

be accomplished by using a pre-amplifier with a higher gain

then the JFET. The sensitivity of the microphone consists of

the sensitivity of the electret plus the gain of the pre-

amplifier.

When

choosing

National

Semiconductor’s

LMV1015-15 for instance, a gain of 15 dB is added by the

pre-amplifier. This results in a sensitivity of −29 dB(V/Pa)

with a typical electret element of −44 dB(V/Pa). National

Semiconductor has a wide range of pre-amplifiers with dif-

ferent gain factors, which can be used to replace the JFET

inside the microphone canister. Please visit www.national-

.com for more information on the LMV1015 and LMV1032

pre-amplifier series. A block diagram with the LMV1015 pre-

amplifier inside an ECM is given in Figure 5.

When taking the same example of busy traffic (70 dB SPL),

the output voltage of the microphone with the LMV1015 is:

Microphone Output = SP +C+S

Where,

SP is the Sound Pressure in dB SPL

C is the dB SPL to dBPa conversion (−94 dB)

S is the Sensitivity in dB(V/Pa)

Microphone output = 70 - 94 - 29 = −53 dBV.

This is equivalent to 6.33 mV

PP.

The pre-amplifier with additional gain reduces the impact of

noise on the wiring / traces from the microphone to the

baseband chip significantly. To minimize interference ulti-

mately, an Analog-to-Digital converter is integrated in both

the LMV1024/LMV1026, realizing a digital interface between

the microphone and the baseband.

Digital Microphone

By integrating the Analog-to-Digital converter (ADC) in the

LMV1024/LMV1026 all analog signals are kept within the

“shielded” microphone canister. The output is a digital inter-

face that is robust and insensitive to interference and noise

from outside the canister. The output is expressed in dBFS

and therefore the sensitivity is also stated in dB(FS/Pa)

instead of dB(V/Pa). To calculate the digital output (Data) in

dBFS the following equation can be written for the LMV1024/

LMV1026:

(1)

20133445

FIGURE 5. Microphone Sensitivity

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