inductor help in STM32

[yassin.kraouch]

has anyone an idea of the role of the inductor in the circuit in the attachement?? it is located in the STM32 eval board,

 

[keith1200rs]

To help minimise high frequency noise on the analogue Vdd from the digital.

Keith.

 

[yassin.kraouch]

but how do you know that VDD is an analog signal??, and there is not high frequency in this circuit

 

[keith1200rs]

but how do you know that VDD is an analog signal??, and there is not high frequency in this circuit

The node labelled VDD powers the micro and so is "dirty" - a digital VDD. The 3.3V goes through the inductor to VDDA which is normally the name used for analogue VDD (or AVDD) and would be used for analogue circuitry. The high frequency noise will come from the micro itself.

Keith.

 

[yassin.kraouch]

thank you keith, but as you see the VDDA also power the micro, and it can be a digital signal?? no ?? yes?

 

[keith1200rs]

Correct, but the VDDA will only power the analogue sections of the micro - I assume it has ADC/DAC or some other analogue functions. The digital parts of the micro will get their power from the VDD pins, not the VDDA pins.

Keith.

 

[yassin.kraouch]

yes, right and the last question, how the inductor minimise hight frequency noise??

 

[keith1200rs]

The impedance of an inductor increases with frequency. The impedance of a capacitor reduces with increasing frequency. So the inductor L1 and the the capacitors C46/C50 form a potential divider. The attenuation will depend on the impedance of the inductors and capacitors. At high frequencies the inductance will have a high impedance and the capacitors will have a low impedance so the attenuation is high. If you left out the inductor then the only resistance in series with the power supply will be the PCB track resistance/inductance so the attenuation of high frequencies will not be very good. Sometimes a small resistor is used instead of an inductor, but then you get a larger voltage drop compared to an inductor.

Keith.