ATmega328P decoupling Vcc using Ferrite Bead

Go To Last Post
7 posts / 0 new
Author
Message
#1
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

I tried decoupling Vcc as recommended in AN2519 : Figure 2-2 

 

my PCB is as recommended in appnote, MCU is decoupled from Vcc using ferrite bead and 100nF cap close to MCU

I never used ferrite beads before and dont know how to calculate right value of ferrite bead for my PCB

I did 5 trial with different parts (all 1206 footprint) placing between reg and MCU

 

1) Ferrite bead 1k/100MHz

2) Ferrite bead 600R/100MHz

3) Ferrite bead 120R/100MHz

4) Inductor 10uH

5) Resistor 10R

 

as I understand from appnote the aim of ferrite bead is to prevent spread noise from MCU to other devices

but it seems using ferrite beads - in my trials - causes increase ripples on MCU side

without ferrite bead/Inductor there is no ripples on MCU side

 

is it normal to have some ripple in MCU side for the sake of not affecting other devices by MCU ?

 

I attached schematic and test results

 

I appreciate anyone sharing experiences

 

Attachment(s): 

Majid

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

m.majid wrote:

...and dont know how to calculate right value of ferrite bead for my PCB

 

A ferrite bead is a ferrite bead...

 

 

...what you have tried are inductors (and for some strange reason a resistor).

 

What is important is that the DC resistance must be of a low value. A ferrite bead is placed onto a piece of wire to make a small value inductor with a low value of resistance.

"This forum helps those that help themselves."

"If you think you need floating point to solve the problem then you don't understand the problem. If you really do need floating point then you have a problem you do not understand."

 

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

First, I want to commend you for actually doing the experiment. But, along with that, there IS a caution. It is hard, I mean REALLY hard, to make meaningful measurements in this situation. Been there, done that, too many times. 

 

It is not at all surprising that the 10R has the least ripple nor is it surprising that the straight 10uH inductor has the most. That is because the combination of the inductor and the bypass capacitor make a parallel resonant circuit as seen from the load (the MCU). Ferrite suppressor beads add an equivalent parallel resistance to the inductor and this tends to damp the ringing. 

 

Now, to the question of whether or not it is "good" or "bad". The answer is an unequivocal "it all depends". It might have a poor effect if you are using the comparator or ADC. Most of the rest of the time, it is pretty much "who cares?".

 

So, why go to the effort of even adding isolation components? Well, especially if you are trying to meet radiated emissions limits for CE or FCC certification, it is a REALLY big deal. That is because the MCU creates power supply current spikes on every clock edge. These current spikes can be quite large (my long-ago back of the envelope computation suggested 100mA) and very narrow (equal to the logic level rise time, which is in the range of 5ns to 10ns). These large and narrow spikes can cause traces through which those currents flow to radiate at much higher frequencies than any clock in the system. Those radiated signals can make it impossible to meet radiated emissions limits. So, what you are doing by adding a lossy low-pass filter is to lower both the amplitude and the frequency of the currents into the rest of the system. You pay for that, of course. Part of the "pay" is more ripple on the load (MCU) side. 

 

Part of the challenge, here, is that those high amplitude, very fast spikes, are REALLY hard to see with an oscilloscope. First, you need a scope with at least 200MHz bandwidth. Then you need a GOOD probe. Then you need to connect the probe in a way that gives you accurate signals, and that usually means zero length ground connection at the probe tip. If you don't do all of these things, then you simply won't see the fast spikes (or you will see a false representation of them) and you won't understand what they are really doing in your system. 

 

Now, this does not fully answer the original question. But, I do hope that it helps you to understand what you saw and helps you to determine whether or not it is a problem. 

 

If it were me, I would use a cap larger than 100nf and I would use any of the ferrite suppressor beads, and be done with it.

 

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

For Brian, "ferrite bead" is the generic name applied to ferrite suppressor elements whether or not the shape is that of a classic "bead". Even SMT parts, that look nothing like a bead are called ferrite beads. 

 

Majid used 1206 SMT parts. 

 

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

There are SMD types like:
.
BLM31PG121SN1L
1206 footprint
120 ohm @ 100MHz
20 mili ohm DC
3.5A
.
https://www.digikey.com/product-detail/en/murata-electronics-north-america/BLM31PG121SN1L/490-1056-2-ND/584265

Majid

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Thank you very much for complete very useful reply
.
My circuit is only Rx Tx data I think 100 mv ripple no harms, as you mentioned its important in ADC,
But I think I should do the same for other part of my circuit : SD voltage regulator and Rx Tx driver
.
There is no requirement for now I just did trial the appnote for experiment, maybe I skip using ferrite bead
I got my answer, I will consider your guides in case of required.

Cheers

Majid

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Serial driver (MAX232 and similar) are a different challenge. They typically include a switched capacitor charge pump. That charge pump can reflect noise into the local power system but the frequency is usually lower than the MCU clock. So, filtering requirements can be quite different. 

 

A linear voltage regulator will have its own requirements for load-side capacitor and input capacitor. These, however, are usually determined for stability rather than generated noise. A switch mode regulator will also have load and input capacitors, but these are usually defined for particular circuit operation.

 

The short answer is that you should check on a circuit by circuit basis.

 

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net