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 29 of 69
5.4 Microphone buffering
As seen previously, a microphone shall be connected to the input pins of the GM862-QUAD
through a buffer amplifier that boosts the signal level to the required value.
Again the buffered microphone circuitry can be balanced or unbalanced. Where possible it is
always preferable a balanced mic. solution. The buffering circuit shall be placed close to the
microphone or close to the microphone wire connector.
5.4.1 Buffered Balanced Mic.
A sample circuit can be:
This circuit has a gain of 15 times (+24 dB), and is therefore suited for the internal mic. input,
having a microphone with a sensitivity close to the suggested one (-45 dB
Vrms/Pa
), if the used
microphone has a different sensitivity, or if the buffer is connected to the external mic. input, then a
gain adjustment shall be done by changing resistors R604-R605 and R606-R607 and as a
consequence the capacitors C636 and C637 to maintain the bandwidth 150-8000 (-3dB).
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 8 kHz in order to have -1dB at 4Khz.
The cutoff frequency is given by the formula:
