Telit Wireless Solutions GE863-PY, GE863-QUAD manual Freq . = 2π * R719* C726 2π * R711* C727

GE863-QUAD

GE863-PY

1vv0300715 Rev. 1 - 19/09/06

freq. =	1	=	1	[Hz]
	2π * R719*C726		2π * R711*C727

The buffer bandwidth at -3dB shall be 4KHz.

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 amplifier the biasing is given by a different divider R715-R717 with the capacitor C720 and through a series resistor R718 of 470KΩ.

TIP: example of calculation.

Llet's assume you have a microphone with a sensitivity of -45dBVrms/Pa and you want to use it in 2nd differential microphone path (“Mic_HF” inputs) in "normal spoken" conditions at acoustic pressure of -4.7dBPa.

As reported at page XX , the electrical level output from the microphone will be :

MicLevel = ( -45) + (-4.7) = -49.7 dBVrms

but we have to consider 20dB loss due to the higher distance from the mouth of the talker ( 50cm ) .

MicLevel = ( -49.7) + (-20) = -69.7 dBVrms

corresponding to

MicVoltage = 10 ( -69.7 / 20 ) = 0,33* 10 -3

In order to have a signal of 1 mVrms at the “Mic_HF” inputs , as suggested at TIP “environment consideration “,

the buffer must have a gain

GA= “Mic_HF /MicVoltage = (1*10 -3)/(0,33*10 -3)=3

or +10 dB

Keeping in mind that “ balancing the line will double the signal”, to calculate the resistor values assign half of required gain GA to each amplifier section . And therefore GS =1,5times (or +3,52dB) .

Choosing as 10kΩ as the input resistance , the corresponding values for the resistors on the buffer will be :

R711 = GS * R708= 1.5*10 =15 kΩ

R719 = (GS -1) * R720 = (1.5 -1)*10 =5 kΩ

The commercial values of 15kΩ and 5.6kΩ be accepted .

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