UC864-G Hardware User Guide
DRAFT
- 29/10/07
Reproduction forbidden without Telit Communications S.p.A. written authorization - All Rights Reserved
page 47 of 66
727
*
711
*
2
1
726
*
719
*
2
1
.
C
R
C
R
freq
π
π
=
=
[Hz]
The buffer bandwidth at -3dB shall be 4KHz.
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 amplifier the biasing is given by a different divider R715-R717
with the capacitor C720 and through a series resistor R718 of 470K
Ω
.
TIP: Example of calculation.
Llet's assume you have a microphone with a sensitivity of -45dBV
rms
/Pa and you want to use it in 2nd
differential microphone path (“
Mic_HF
” inputs) in "normal spoken" conditions at acoustic pressure of
-4.7dBPa.
As reported at page XX,, the electrical level output from the microphone will be :
but we have to consider 20dB loss due to the higher distance from the mouth of the talker ( 50cm ) .
corresponding to
In order to have a signal of 1 mV
rms
at the
“Mic_HF”
inputs , as suggested at
TIP
“
environment
consideration
“,
the buffer must have a gain
or +
10 dB
Keeping in mind that “balancing the line will double the signal”, to calculate the resistor values assign
half of required gain
G
A
to each amplifier section . And therefore,
G
S
=1,5times (or +3,52dB).
Choosing as 10k
Ω
as the input resistance, the corresponding values for the resistors on the buffer will
be:
R711
=
G
S
* R708= 1.5*10 =
15 k
Ω
R719
= (
G
S
-1) * R720 = (1.5 -1)*10 =
5 k
Ω
The commercial values of
15k
Ω
and
5.6k
Ω
be accepted.
As a consequence of the assigned values of the resistors, the nominal values of C726 and C727 are:
MicLevel = ( -45) + (-4.7) = -49.7 dBV
rms
MicVoltage = 10
( -69.7 / 20 )
= 0,33* 10
-3
MicLevel = ( -49.7) + (-20) = -69.7 dBV
rms
G
A
=
“
Mic_HF
/MicVoltage = (1*10
-3
)/(0,33*10
-3
)=3