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 32 of 69
The bandwidth (-3dB) is given by the approximated formula (considering C725 >> C726) :
727
*
711
*
2
1
726
*
719
*
2
1
.
C
R
C
R
freq
π
π
=
=
[Hz]
The buffer bandwidth at -3dB shall be 8kHz.
Note that the biasing of the operational amplifier is given for the inverting amplifier by the series
divider R714-R715. The 100nF capacitor C719 is needed to filter the noise that could be coupled to
that divider. For the not inverting operational the biasing is given by a different divider R715-R717
with the capacitor C720 and through a series resistor R718 of 470K
Ω
.
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 external mic. audio path.
With the mic. having nominal sensitivity -45dB
Vrms/Pa
at the "normal spoken" conditions: -4.7dB
Pa
but at 50 cm from the mouth of the talker an additional 20 dB loss shall be considered.
At that acoustic pressure the voltage output from the microphone is:
Mic Voltage Output (dB) = ( -45) + (-4.7) -20 = -69.7 dB
Vrms
corresponding to:
Mic_Voltage_Output = 10
( -69.7 / 20 )
= 3.3* 10
-4
V
rms
in order to have a signal of 1 mV
rms
(10 dB lower than the nominal input for the GM862-QUAD
external mic. path) on the GM862-QUAD internal mic. inputs then the buffer must have a gain of:
Voltage @ GM862-QUAD_intmic/ Mic_Voltage_Output = (1 * 10
-3
)/( 3.3 * 10
-4
) = 3
Hence in these conditions the signal level on the input pins of the internal mic. of the GM862-
QUAD is 10 dB (3 times) higher than the microphone output and therefore the buffer has to gain 10
dB.
To calculate the resistor values it must be kept in mind that balancing the line will double the signal
and hence already add +6 dB, therefore the buffer must gain only 1.5 times.
The corresponding values for the resistors on the buffer could be ( if we keep the input resistance
10k
Ω
)
R711 = gain* R708= 1.5* 10 = 15 k
Ω
R719 = (gain -1) * R720 = (1.5 -1) * 10 = 5 k
Ω
The commercial values of 15k
Ω
& 5.6k
Ω
are then chosen.
