www.piclist.com/techref/index.htm?key=logic+bypass+caps

All,

Capacitors exhibit reactance when excited by alternating current.

This reactance opposes changes in voltage. If the capacitance

value remains constant, and frequency is increased, the amount of

this reactance goes down. Likewise, the opposite is true, that

if the capacitance remains constant, and frequency is decreased,

reactance goes up. This shows that Capacitive Reactance is

Inversely Proportional to Frequency. Now, lets say we have chosen

our capacitor value to give us a specific reactance value at a given

frequency. And this frequency is the lowest we have to deal with.

In this case, as frequency goes up, reactance goes down, and the

impedance gets lower. In bypassing, this means that as frequency

goes up, the capacitor acts as more of a short than at lower

frequencies.

So, the statement

"This suggests that lower capacitance a priori works better at

higher speeds."

is somewhat correct. What it really means is that you need less

capacitance at higher frequencies to get the same value of reactance

that you get with a larger capacitor at lower frequencies.

The statement:

My understanding of the specifications suggests the opposite: for

any make of capacitor, higher capacitance always works better at

any frequency.

also is somewhat true. From a capacitive reactance point of view,

the higher the frequency, the less the capacitive reactance of any

capacitor. Therefore the above statement is true. But, there are

other losses that are also frequency dependent. The most obvious is

INDUCTANCE. This is directly proportional to frequency, and opposes

a change in current. So, at some point, what you gain in lower

capacitive reactance will be offset some by an increase in Inductive

reactance. Therefore, you have the potential for any degree of

inefficiency. So in this respect, the above statement is not true.

The statement:

What might be true is that certain types of capacitors are only

available in small denominations, and these types have lower ESR

and ESI so they work better at higher frequencies.

Is basically true. ESR and ESI have the most effect on Q, which

translates to effeciency. Of course this means overall effeciency,

not just at a given frequency or range of frequencies. It reduces

the amount of heat generated inside tha cap which means that less

energy is consumed by the cap and transformed into heat, which

indicates higher effeciency.

Just my 2 cents worth. Sorry for butting in uninvited.

Regards,

Jim

On Mon, 05 June 2000, "Robert A. LaBudde" wrote:

{Quote hidden}

jimspam.....jpes.com

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