User's Manual

Telit GE863-QUAD / GE863-PY
Hardware User guide
1vv0300715, Rev. ISSUE#0, - 21/02/06
Reproduction forbidden without Telit Communication written authorization – All Right reserved – Right of modification reserved page 33 of 71
6.3.2 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- pad (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 2kΩ biasing resistance, so try to keep the value of R2 around 2kΩ. For mic.
characteristics refer to the manufacturer.