Telit Wireless Solutions GM862-GPS - Manual (Page 35)

GM862-GPS Hardware User Guide
1vv0300728 Rev. 0 - 27/04/06
Reproduction forbidden without Telit Communications S.p.A. written authorization - All Right reserved page 35 of 55
The buffer gain is given by the formula:
607
606
605
604
R
R
R
R
Gain = =
The C636 and C637 capacitors are placed in order to cut off the gain at higher frequencies than the
transmitted GSM band, the cutoff frequency (-3dB) should be 3500Hz in order to have -1dB at 3kHz.
The cutoff frequency is given by the formula:
636 * 606 * 2
1
637 * 604 * 2
1
.
C R C R
freq
π π
= = [Hz]
TIP: example of calculation .
Let's assume you have a microphone with a sensitivity of -45 dBV
rms
/Pa and you want to use it in 1st
differential microphone path (“Mic_MT” inputs) in "normal spoken" conditions at acoustic pressure of
-4.7dBPa.
As reported at page 33 , the electrical level output from the microphone will be :
corresponding to:
When the talker is screaming ,we will have a signal of 330 mV
rms
on the “Mic_MT “ inputs for a buffer
gain G
A
:
G
A
=20 log (AmplifierOutput / MicVoltage) =20 log (330 * 10 -3 )/( 3.3 * 10 -3 ) = 20 log 10=20dB
The corresponding values for the resistors on the buffer could be ( if we keep the input resistance
10kΩ )
R604 = R606 = gain* R603= gain* R605 = 10* 15 = 150 kΩ
The commercial values of 150kΩ & 15kΩ are then chosen.
As a consequence the values of the capacitors C636 and C637 shall be:
C636=C637= 1/ (2π*4000*R606)= 265 *10 -12 F
A commercial value of 270pF gives a cutoff frequency of 3931Hz with an errorless than 1,8% .
MicLevel = ( -45) + (-4.7) = -49.7 dB
Vrms
MicVoltage = 10 ( -49.7 / 20 ) = 3.3* 10 -3 V
rms