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| MCU - 100nf cap to GND before and/or after the inductor AVCC pin |
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| Psi:
I've had some AVR's blow up and I have a suspicion as to why. Currently there's a 10uH inductor feeding VCC into the AVCC pin (VCC for ADC) There's a 100nF cap to GND on the VCC side of the inductor but not on the MCU side. The inductor connects directly to the AVCC pin Could this be resonating and created a high voltage spike on the AVCC pin at turn-on which is damaging the mcu? |
| daqq:
Makes sense, if nothing else this should give bad performance from the ADC. There's a cap there recommended AFTER the inductor in any case, see https://ww1.microchip.com/downloads/en/DeviceDoc/Atmel-7810-Automotive-Microcontrollers-ATmega328P_Datasheet.pdf , page 213. |
| Psi:
yeah, i think I was just an idiot and forgot to add it. |
| Kleinstein:
There should be a cap directly at the AVCC pin. The inductor is optional and may better be a ferrite "bead", so an inductor with damping. In some cases also a resistor (e.g. 10 ohm) instead of the inductor is good, especially if there is no significant load to the AVCC powered pins. |
| tom66:
The purpose of decoupling capacitors is to counter the inductive PCB tracks and planes, which would reduce the energy available to the MCU at high switching frequencies. Placing the inductor there is almost the opposite of what you want to do. Now, an analog pin is likely to have less switching current on it, but it will probably still have some. The correct solution is to use a Pi network of C-L-C components, with a small value cap right next to the MCU pins, fed by a series L or FB, with another cap to GND on the other side. In some designs I've seen a resistor of 10-22 ohms in series as well: the current drawn by the pin is quite minimal, so this works to add further high-frequency filtering (using the 'R' and 'C' as a simple lowpass.) |
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