Re: gEDA-user: High speed decoupling (was: Simulation of ceramic capacitors, pairs and groups)

[John Doty <jpd-atMUNGEDwispertel_dot_net>]

On Mar 10, 2006, at 11:03 PM, Karel Kulhavy wrote:

>
> I think the ESR and ESL doesn't depend on if the capacitor is in  
> system
> or alone. What we need is just a reliable source of numbers or someone
> measuring it. I don't know how I can measure ESR and ESL on a scope.

For ESL you are wrong: when most of the magnetic energy is in stray  
fields outside the part, the way the part is attached to the system  
is what determines ESL. Where does the return current flow? For a  
part like a capacitor that doesn't concentrate magnetic energy you  
can compute circuit inductance by assuming the part is a slug of metal.

ESR is more useful as a spec, but for realistic capacitors it is  
still very dependent on test conditions. How you should measure it  
depends on what the circuit details and requirements are: if ESR  
cause some bad effect, look for that bad effect in a test circuit  
that uses the part in a similar way to your real system. You'll find  
this difficult: in most practical circuits the dominant dissipation  
isn't in capacitors, so the effects of ESR are often difficult to  
measure. If you don't understand the effect you're worried about well  
enough to measure it you generally can't do engineering with it.

John Doty              Noqsi Aerospace, Ltd.
jpd@wispertel.net