Electronics > Beginners

Connecting VDD to VDDA rail

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ghamille:
Hi all,

I have seen a lot of designs around STM32 microcontrollers (blogs and videos) where they mention doing “additional filtering” for the VDDA pin. I think I understand why :)
Usually though, they will also mention the power comes from the already filtered VDD rail. As a newbie, I am wondering how to connect the VDD rail to the VDDA one.

In the attached diagram I am working on, would I connect 1 --> 3 or rather 2 --> 3. I would love to also understand why it has to be connected a certain way. Any pointer to learning resources would be appreciated.

Thanks a lot for your help!

selcuk:
You may connect circle 1 to circle 3. Since circle 2 is GND net, you cannot connect to it. I guess circle 3 continues to VDDA through the ferrite bead.

The purpose is to prevent the noise on VDD net from propagating to the VDDA net. But if you don't place the ferrite bead intentionally between the VDD and VDDA, it is better to not use it (i.e. short VDD to VDDA). If you know the noise frequency and characteristics and can calculate a bead value then you can use it. Otherwise you cannot be sure about the effectiveness or signal degradation.

You may read a book about EMC or signal integrity to have better knowledge about filters. This is an open access one.

Filters:
https://link.springer.com/chapter/10.1007/978-3-031-14186-7_15

Ferrite beads:
https://link.springer.com/chapter/10.1007/978-3-031-14186-7_11

ghamille:
Thank you for the resources.

For the decoupling, I was going by the datasheet for my specific microcontroller (STM32L052K6T6: https://www.st.com/resource/en/datasheet/stm32l052k6.pdf).
So what you are saying that if I don’t calculate the exact value for the ferrite bead, might as well connect all of those capacitors (both for VDD and VDDA) in parallel, and use that to power both pins?

wasedadoc:
Are you using the any of the ADC or DAC?  If not the benefit of a filter is irrelevant and you can connect VDDA and VDD directly.

ataradov:

--- Quote from: ghamille on April 21, 2024, 04:12:29 pm ---So what you are saying that if I don’t calculate the exact value for the ferrite bead, might as well connect all of those capacitors (both for VDD and VDDA) in parallel, and use that to power both pins?

--- End quote ---
If you know that your application will potentially have lots of switching noise and you care about performance of the analog blocks, then place the ferrite to have options to experiment later.

You may not  be able to always calculate the exact value you need, sometimes you just build the board and try different values. And in the end you may end up with just a 0 Ohm resistor instead of a bead.

But generally, if you are not optimizing the cost to the cents, having some generic bead like 470 Ohm @ 100 MHz will at least not hurt anything.

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