Is insertion loss bad for decoupling caps?

[pop11]

When I look at various capacitor data sheets, they often have a insertion loss vs freq plot (in dB). If I want to find the best IC power supply decoupling cap, shall I look for the one with the minimum insertion loss?

If so, it is usually the smallest value (e.g. a few pF) has the lowest insertion loss at the highest freq, but this is not enough for poweer decoupling purpose, right?

so ,I am at a loss with this "insertion loss" for the cap thing. Please help ....

[sreten]

insertion loss is not an issue for power decoupling capacitors,

sreten.

[pop11]

Really? I didn't know that.

I thought that there should be as little loss as possible for the power decoupling cap, so the noise will flow through the cap much "easier".

I guess I am missing something here right? Please help. Thanks.

[Jocko Homo]

The purpose of a decoupling cap (on a power supply) is to provide a return path to ground at the lowest possible value, over as wide a range as possible.

Of course.....it depends on capacitance and frequency, so no one cap works in all situations.

Someitmes, it is desirable for a cap to have a slightly higher than normal series resistance. Some regulaotors have peaking problems if the series resistance is zero ohms.

Insetion loss is used to describe the characteristics of a L-C filter, in terms of the signal that is absorbed by the filter.

Two different cases.

Jocko

[sreten]

Originally posted by pop11
Really? I didn't know that.

I thought that there should be as little loss as possible for the power decoupling cap, so the noise will flow through the cap much "easier".

I guess I am missing something here right? Please help. Thanks.

A cheap capacitor with 10% insertion loss but 100 x the value of a capacitor
with 1% loss is far better for basic power supply decoupling duties.

If high frequency loss is an issue add higher quality bypass capacitors.

sreten.

[HDTVman]

Originally posted by pop11
When I look at various capacitor data sheets, they often have a insertion loss vs freq plot (in dB). If I want to find the best IC power supply decoupling cap, shall I look for the one with the minimum insertion loss?

If so, it is usually the smallest value (e.g. a few pF) has the lowest insertion loss at the highest freq, but this is not enough for poweer decoupling purpose, right?

so ,I am at a loss with this "insertion loss" for the cap thing. Please help ....

Insertion loss refers to the amount the signal is attenuated in a RF circuit when the cap is used as a series pass element. This is generally stated in "dB insertion loss @ X mHz". Most of the time this will be spec-ed at 50 or 75 ohms.

Not an issue for PS bypass use.

If for some reason known only to you, you think this is a big deal choose a cap with lowest loss at the frequency where the electrolitics you are bypassing impedance starts to rise.

Later BZ