LMV1024 datasheet(13/17 Pages) NSC

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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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