User Manual

GM862-QUAD / PY Hardware User Guide

1vv0300748 Rev. 5 - 20/09/07

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.

Let’s assume you have a microphone with a sensitivity of -45dBV

rms

/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:

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

corresponding to

In order to have a signal of 1 mV

rms

at the “Mic_HF” inputs , as suggested at TIP “environment

consideration “,

the buffer must have a gain or +10 dB

Keeping in mind that “balancing the line will double the signal”, to calculate the resistor values assign

half of required gain G

to each amplifier section . And therefore G

=1,5times (or +3,52dB) .

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

be :

R711 = G

* R708= 1.5*10 =15 kΩ

R719 = (G

-1) * R720 = (1.5 -1)*10 =5 kΩ

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

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

rms

MicVoltage = 10

(

69.7 / 20 )

= 0,33* 10

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

rms

= “Mic_HF /MicVoltage = (1*10

)/(0,33*10