Rainbow Electronics GM862-GPS - Manual (Page 50)

GM862-GPS Hardware User Guide
1vv0300728 Rev. 8 - 20/09/07
Reproduction forbidden without Telit Communications S.p.A. written authorization - All Rights Reserved page 50 of 73
8.5 Microphone Buffering
As seen previously, a microphone shall be connected to the input pins of the GM862-GPS 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 solution. The buffering circuit shall be placed close to the microphone or close
to the microphone wire connector.
8.5.1 Buffered Balanced Mic
A sample circuit can be:
This circuit has a gain of 10 times (+20 dB), and is therefore suited for the “Mic_MT “ input if you have
a microphone with a sensitivity close to the suggested one (-45 dBV
rms/
Pa ). If your microphone has a
different sensitivity or if the buffer is connected to the “Mic_HF “ inputs , then a gain adjustment shall
be done by changing resistors R604 and R606 ( if the required value is not a standard one , you can
change R605 e R607 ) and as a consequence the capacitors C636 and C637 to maintain the
bandwidth 150-4000Hz (at -3dB).
270pF
270pF
+20 dB
15K
15K