AN1534 - APPLICATION NOTE

4/4

This represents the gain of the amplifier in the

be taken into consideration as they establish a

fy the gain without being forced to change the val-

ues of the other components (apart from the high

What we refer to as the "gain of the microphone" is

the gain we talk about in the "Microphone consid-

erations" chapter. You need to consider the signal

fed to the microphone to avoid saturation or insuf-

ficient gain.

have to consider that it creates together with the

4- SPECIAL NOTE ON EMI

As you can notice on Figure 1, we have suggested

an optional Electro Magnetic Interference filter

(cutting frequencies above 33.8MHz). With cellu-

lar phones being a constant company to our daily

life, high power RF interferences need to be prop-

erly managed into sensitive amplification devices.

This filter has to be implemented close enough to

the microphone and we also have to pay attention

on not having too long connecting wires (anten-

nas!). Ideally this application should be wired on a

double sided PCB with one side acting as ground

plane.

5 - COMPONENTS CALCULATION

So, in this example, we have a gain value of 90 or

+39dB and we have globally a band pass filter

the overall gain, we also have also to consider the

stage after the pre-amplifier. If it’s a CODEC or A/

D converter, it will most probably have its own am-

plification (usually +20dB gain). All together, this

gives the below chain for the chapter 1 example:

This configuration is well adapted to battery pow-

ered equipment as the overall maximum con-

sumption of this application is 3.3mA (0.5mA for

the microphone and 2.8mA max for TS971).

at 1 meter

at 5 cm

Microphone Output

0.13mV

(126

µV)

2.5mV

TS971 Output

(+39dB)

0.11mV

(11.34mV)

0.23V

Codec Output

(+20dB)

0.11V

2.27V

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