Question...

Tricky Nekro · 2006-03-02 21:06

Hello to everyone...

I just wanted to know what it means to connect a pin to Vcc (+5v) through a low - pass filter...
I'm degnining a microcontroller circuit and it just say this in the docs...

Thank you, Provas, Greece

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-Rule your Destiny-
--Be Good. Be Bad. Be Provas--

PJAllen · 2006-03-03 03:38

· A low-pass filter in this situation, being on V_CC, is intended to suppress impulse noise: high frequencies, spikes.
· A low-pass filter can be made from a simple R-C combination (that's Resistor-Capacitor.)
· However, since this filter has system power running through it, presumably, then you won't want to have a resistor in series; you'll want to make an L-C (inductor-capacitor) low-pass filter.· An inductor has a lot less resistance and consequently less voltage drop across it.
· Hope you are OK with this explanation and remember -- The Library is your friend.
·

Tricky Nekro · 2006-03-03 23:03

Thanks for the help...

By inductor you mean a small coil... If yes... How can I find out it's diameter?

Thanks, Provas, Greece

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
-Rule your Destiny-
--Be Good. Be Bad. Be Provas--

PJAllen · 2006-03-04 12:50

· Well, yes, an inductor is a coil.· But, what is required here is what is known as a choke (or a molded choke.)· The required inductance, indeed·all values of L, C, or R·for any filter, are relative to the frequencies to be suppressed (attenuated.)
·
· It is not like one could provide all of the answers here.· There isn't a magic answer that I am withholding from you.· Filter Design is a field of study all its own.· Volumes have been written on the subject.· Remember -- The Library is your friend.

· [noparse][[/noparse] Here is a wild guess: maybe all you need is a ferrite bead placed on the V_CC output.· Anticipating your next question -- I do not know which would be best. ]

LoopyByteloose · 2006-03-05 13:48

How can you find its diameter?

Of all the electronic devices that are designed, coils are the most complex. Just knowing the diameter really won't help.
Basically, you want to know how many Henrys and what watt rating it will handle.

If you get into the math, you will go deeply into trying to understand magnetic fields and how they are affected by the space around them and the kinds of material at their core.

There are sites with Filter Calculators and their are many kinds of filters, but only three catagories [noparse][[/noparse]low-pass; high-pass; and band-pass]. The Art of Electronics has a good introduction and may simplify your choices.· You really don't need to know everything unless you have a huge complex·problem.

Ferrite beads are very popular because they seem to easily trap a lot of high frequency garbage. My monitor cable has one on each end. They are also very easy to remove it you have doubts about what they are doing.· We see more and more of them on power supplies at the point of connection.
They seem to be keeping unwanted EMI from using cables as a point of entry.· Still, I have no idea is bigger is better or if you should be winding around them or not.· Maybe someone can answer that for all of us.

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"When all think alike, no one is thinking very much.' - Walter Lippmann (1889-1974)

······································································ Warm regards,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan

Post Edited (Kramer) : 3/5/2006 1:57:50 PM GMT

Beau Schwabe · 2006-03-05 15:01

Provas,

Coils are probably the most misunderstood electronic component there is. To completely cover
what a coil is, is out of the scope of this forum.

In answer to your question "what it means to connect a pin to Vcc (+5v) through a low - pass filter".

To effectively create a LPF (Low Pass Filter), you also need a capacitor in conjunction with the coil.
Sometimes this "capacitor" is implied through parasitics within the circuitry (i.e. within the coil there
exists a parasitic capacitance between windings), or You can choose a specific value of capacitor to
be used with a coil.

In order to choose the correct coil value, it is important to know what frequency you want to filter.

LOW-PASS filter:

   Input >-----Coil-----o------> Output
                        |
                    Capacitor
                        |
                        |
                       GND


HIGH-PASS filter:

   Input >-----Capacitor-----o------> Output
                             |
                            Coil
                             |
                             |
                            GND


NOTCH filter:

    Input >----o----Capacitor----o----Output
               |                 |
               o------Coil-------o

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

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