GE863 datasheet(31/69 Pages) ETC1

31 / 69 page

Telit GE863-QUAD / GE863-PY

Hardware User guide

1vv0300697, Rev. ISSUE#0, - 21/11/05

page 31 of 69

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.

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