STG3699_ Ver la hoja de datos (PDF) - STMicroelectronics
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STG3699_ Datasheet PDF : 4 Pages
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AN1743 - APPLICATION NOTE
For correct power supply stability, it is necessary to apply a capacitance of 10µF to the STG Analog
Switch and 1µF to the Audio Amplifier between the VCC pin signal and GND to filter all possible noise
from the Power Supply voltage.
To use the audio amplifier TS4990 correctly in differential mode, it is important to apply two resistors with
different values as a function of the gain and the right power coupling between possible loads applied to
the Analog Switch.
The values of the resistors can be found using the formula 2*Rfeed/R1 or 2*Rfeed/R2.
As the TS4990 does not have common mode feedback, an input-coupling capacitor before resistors R1
and R2 is mandatory. The capacitor value is calculated by the formula C = 1/2π*R1* FC
or C = 1/2π*R2*FC where FC is the lowest cut off frequency signal required.
Usually the hands free phone jack circuitry presents a capacitance that when coupled with the analog
switch inputs (D1 or D2 for example) creates cross talk between channel OFF and channel ON.
For instance a 100pF capacitance present on the D1 input generates a noise between channels of
145mV as shown in the following picture.
Figure 3: Noise Present on Analog Switch Channel OFF
VCC = 3.8V
Vin = 3.6V
In Frequ. = 20ΚΗz
Com Load =100pF
Xtalk = 27.80dB
Using the formula: XTALK (dB) = 20log Vout /Vin, it is possible to get back to the cross talk value. If this
value is low a possible signal interference can induce noise between two different loads applied to the
outputs of the Analog Switch, therefore causing the system to function badly.
4. CONCLUSION
These pages describe possible problems applying the STG3699 Analog Switch when the operating
conditions stated in the specifications are not adhered to, resulting in less than the guaranteed
functionality for these devices being achieved.
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