Rainbow Electronics GM862-GPS User Manual

Page 44

GM862-GPS Hardware User Guide

1vv0300728 Rev. 8 - 20/09/07

page 44 of 73

(*) WARNING. AC means that the signals from microphone has to be connected to input lines
of the module by a CAPACITOR , which value must be ≥ 100nF . Not respecting this constraint,
the input stage will be damaged.

TIP: definition of the nominal sensitivity of the microphone lines .

The nominal sensitivity of the microphone lines indicates the voltage level on the GM862-GPS pins
present during "normal spoken" conditions.
For a handset , the "normal spoken” conditions take place when the talker mouth is 7cm far from the
microphone ; under these conditions the voice will produce an acoustic pressure of -4,7dBPa @1kHz
on the microphone membrane .

TIP: electrical equivalent signal and operating voice levels.

At "normal spoken" conditions, a microphone having the suggested nominal sensitivity of -
45dBV

rms

/Pa

, will produce

the electrical equivalent signal :

that means :

During a call, this level varies according to the volume of the talker voice; usually the following rough
thumb rule for the dynamic range may be used :

1) the talker is screaming . This is the strongest voice level condition: the signal increases by

+20dB ;

2) the talker is whispering. This is the lowest voice level condition: the voice level decreases by –

50dB .

These changes must be considered for designing the external microphone amplifier.

TIP: example of external microphone amplifier calculation .

Let’s suppose to use the 1

differential

microphone path .In this case the maximum differential input

voltage to “Mic_MT” lines is 365mV

rms

(1,03V

) corresponding to –8,76dBV.

Now we can calculate the maximum voltage gain of an external microphone amplifier G

MicLevel = ( -45) + (-4.7) = -49.7 dB

Vrms

MicVoltage = 10

( -49.7 / 20 )

= 3.3* 10

-3

rms