GE863 datasheet(35/69 Pages) ETC1

35 / 69 page

Telit GE863-QUAD / GE863-PY

Hardware User guide

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

page 35 of 69

As shown in the balanced buffered mic. the gain adjustments can be done by changing R719 -

R720 and R708 - R711 and as a consequence the capacitors C726 and C727.

The bandwidth (-3dB) is given by the approximated formula (considering C725 >> C726) :

727

711

726

719

freq

[Hz]

The buffer bandwidth at -3dB shall be 8kHz.

Note that the biasing of the operational amplifier is given for the inverting amplifier by the series

divider R714-R715. The 100nF capacitor C719 is needed to filter the noise that could be coupled

to that divider. For the not inverting operational the biasing is given by a different divider R715-

R717 with the capacitor C720 and through a series resistor R718 of 470KΩ.

For example:

- Let's assume you have a microphone with these characteristics:

- sensitivity -45 dBVrms/Pa

and you want to use it in the external mic. audio path.

With the mic. having nominal sensitivity

-45dBVrms/Pa at the "normal spoken" conditions: -

4.7dB Pa but at 50 cm from the mouth of the talker an additional 20 dB loss shall be considered.

At that acoustic pressure the voltage output from the microphone is:

Mic Voltage Output (dB) = ( -45) + (-4.7) -20 = -69.7 dBVrms

corresponding to:

Mic_Voltage_Output = 10 ( -69.7 / 20 ) = 3.3* 10 -4 Vrms

in order to have a signal of 1 mVrms (10 dB lower than the nominal input for the GE863-QUAD

external mic. path) on the GE863-QUAD internal mic. inputs then the buffer must have a gain

of:

Voltage @ GE863-QUAD_intmic/ Mic_Voltage_Output = (1 * 10 -3 )/( 3.3 * 10 -4 ) = 3

Hence in these conditions the signal level on the input pads of the internal mic. of the GE863-

QUAD is 10 dB (3 times) higher than the microphone output and therefore the buffer has to gain

10 dB.

To calculate the resistor values it must be kept in mind that balancing the line will double the

signal and hence already add +6 dB, therefore the buffer must gain only 1.5 times.

The corresponding values for the resistors on the buffer could be ( if we keep the input resistance

10kΩ )

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