AN4598 datasheet(4/12 Pages) STMICROELECTRONICS

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Noise

AN4598

4/12

DocID026988 Rev 1

The maximum gain setting (Gmax) is calculated using Equation 2 by considering the

maximum sensitivity of the microphone (Somax), the maximum output signal amplitude

needed after the amplification stage (Voutrms_max), and the maximum acoustic pressure

(Pmax).

Equation 2

If Voutrms = 1 Vrms, Somax = -35 dBV/Pa, and Pmax = 1 Pa, Gmax = 56.

Noise

Overall noise includes the noise generated by the microphone and the noise generated by

the amplification stage.

The noise level (erms) on the output of the microphone is calculated using Equation 3 by

considering the signal-to-noise ratio (SNR) as given in the MP23AB02B datasheet.

Equation 3

Where erms is expressed in Vrms and SNR in dBA.

erms is shown as the noise floor in Figure 1.

Note that the SNR is A-weighted in order to consider noise as a human ear would hear it.

SNR is the difference (in dB) between a 94 dB SPL signal (whose amplitude is So (dBV/Pa)

* 1 (Pa) = So (dBV)) and the noise level (dBV).

The typical noise level in the MP23AB02B is 7 µVrms.

This is equivalent to a white noise source (eo), over a 20 kHz bandwidth, as calculated using

Equation 4.

Equation 4

Where eo is expressed in nV/√Hz.

In case of the MP23AB02B, eo is 56 nV/√Hz.

Gmax

Voutrms_max

Somax

---------------

Pmax

---------------------------------

erms

So SNR

–

--------------------------

erms

20000

--------------------

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