AVR-Chat

Graham, thanks for the great explanation.   It is fairly obvious when you
draw out the circuit diagram.   People on this list are a wonderful
resource!   Steve

 

From: AVR-Chat@yahoogroups.com [mailto:AVR-Chat@yahoogroups.com] On Behalf
Of ecros_technology
Sent: Saturday, October 24, 2009 11:35 AM
To: AVR-Chat@yahoogroups.com
Subject: [AVR-Chat] Re: pcb layout of SOIC packages

 

  

--- In AVR-Chat@yahoogroups.com <mailto:AVR-Chat%40yahoogroups.com> , "Steve
Hodge" <steve@...> wrote:
> <snip>
> Your mention of hi and lo in parallel ...
> I am using 68 uF ... goes off to the various
> IC's, each with their own 0.1 uF decoupling
> capacitor. Is this the situation you are
> referring to?

No. That's perfect. The 68 uF isn't really a "decoupling" capacitor. It is
there to a) stabilize the regulator and b) soak up medium-speed changes in
load current. Because it has high parasitic resistance and inductance, it is
of very little help in decoupling noise.

Also because of its high parasitic resistance, it doesn't stimulate the
problem you get with two different values of ceramic capacitor.

The problem occurs when you have, for example, 0.1 uF and 1 nF decoupling
capacitors in parallel. The (incorrect) thinking behind this is that the 0.1
uF takes care of high frequencies and the 1 nF takes care of very high
frequencies. But, the high frequency performance of modern multi-layer
ceramic capacitors is limited by their parasitic inductance, which is a
function of the package design, not the capacitance. So, just use 0.1 uF in
a good package.

To understand the problem, draw the circuit diagram of the two parallel
capacitors, including their parasitic inductance. Now consider frequencies
at which the larger capacitance is basically a short circuit. Short it out
on your diagram. What's left? A parallel tuned circuit! The smaller
capacitance resonates with the combined parasitic inductances, the impedance
goes up and the effectiveness for decoupling goes all to hell.

Admittedly, what I'm describing is not an effect you'll run into with
miorocontrollers. However, it pays to get into good habits and why pay for
two capacitors when one is actually better?

Graham.



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