Rainbow Electronics GM862-GPS - Manual (Page 44)

GM862-GPS Hardware User Guide
1vv0300728 Rev. 8 - 20/09/07
Reproduction forbidden without Telit Communications S.p.A. written authorization - All Rights Reserved 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
st differential microphone path .In this case the maximum differential input
voltage to “Mic_MT” lines is 365mV
rms
(1,03V
pp
) corresponding to –8,76dBV.
Now we can calculate the maximum voltage gain of an external microphone amplifier G
A:
MicLevel = ( -45) + (-4.7) = -49.7 dB
Vrms
MicVoltage = 10 ( -49.7 / 20 ) = 3.3* 10 -3 V
rms