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 28 of 69
5.3.2 Unbalanced Microphone biasing
The unbalanced microphone biasing voltage should be obtained from a dedicated voltage regulator,
in order to eliminate the noise present on the power lines. This regulator can be the same for all the
audio paths. The microphone should be supplied from a capacitor multiply circuit.
For example a circuit for the unbalanced microphone biasing can be:
NOTE: In the unbalanced application the capacitor C3 shall be > 200nF otherwise the frequency
response will be cut at low band frequencies (down to 300Hz). This capacitor can be placed close to
the MIC- pin (EXT_MIC- or INT_MIC- depending on the audio path chosen) or if possible it should be
placed close to the shielded cable connector. If the ground return path is well designed, then it is
possible to eliminate the C3 capacitor, provided the buffer is close to the mic. input.
NOTE: The cable to the microphone should be shielded.
NOTE: The microphone sensitivity changes with the value of R2. Usually the microphones are
characterized with 2k
Ω
biasing resistance, so try to keep the value of R2 around 2k
Ω
. For mic.
characteristics refer to the manufacturer.