Telit GM862-QUAD/-QUAD-PY
Hardware User Guide
1vv0300692, Rev. ISSUE#0, - 04/10/2005
Reproduction forbidden without DAI Telecom written authorization – All Right reserved – Right of modification reserved
page 30 of 69
636
*
606
*
2
1
637
*
604
*
2
1
.
C
R
C
R
freq
π
π
=
=
[Hz]
For example:
- Let's assume you have a microphone with these characteristics:
- sensitivity -45 dB
Vrms/Pa
and you want to use it in the internal mic. audio path.
With the mic. having nominal sensitivity -45dB
Vrms/Pa
at the "normal spoken" conditions: -4.7dB
Pa
at 7 cm from the mouth of the talker.
At that acoustic pressure the voltage output from the microphone is:
Mic Voltage Output (dB) = ( -45) + (-4.7) = -49.7 dB
Vrms
corresponding to:
Mic_Voltage_Output = 10
( -49.7 / 20 )
= 3.3* 10
-3
V
rms
in order to have a signal of 50 mV
rms
on the GM862-QUAD internal mic. inputs then the buffer must
have a gain of:
Voltage @ GM862-QUAD_intmic/ Mic_Voltage_Output = (50 * 10
-3
)/( 3.3 * 10
-3
) = 15
Hence in these conditions the signal level on the input pins of the internal mic. of the GM862-
QUAD is 24 dB (15 times) higher than the microphone output and therefore the buffer has to gain
24 dB.
The corresponding values for the resistors on the buffer could be ( if we keep the input resistance
10k
Ω
)
R604 = R606 = gain* R603= gain* R605 = 15* 10 = 150 k
Ω
The commercial values of 150k
Ω
& 10k
Ω
are then chosen.
As a consequence the values of the capacitors C636 and C637 shall be:
C636=C637= 1/ (2
π
*8000*R606)= 132 *10
-12
F
A commercial value of 100pF is then chosen.
