adding ceramics across power pins

[Jon Slaughter]

Why does one have to add ceramic's across power pins along with larger caps
such as tantalum. e.g., 1uF tant an 0.1uF cer?

In theory they just add to 1.1uF and it shouldn't make any difference. It
seems that one should then also add 0.001uF and so on?

Why don't they make special "power" caps that combine tantalum and ceramics
in one package just for this purpose then?

Thanks,
Jon

 

[Leon]

Why does one have to add ceramic's across power pins along with larger caps
such as tantalum. e.g., 1uF tant an 0.1uF cer?

In theory they just add to 1.1uF and it shouldn't make any difference. It
seems that one should then also add 0.001uF and so on?

Why don't they make special "power" caps that combine tantalum and ceramics
in one package just for this purpose then?

Thanks,
Jon

http://www.google.co.uk/search?hl=en&q=decoupling+capacitor&btnG=Google+Search&meta=

Leon

 

[Jon Slaughter]

http://www.google.co.uk/search?hl=en&q=decoupling+capacitor&btnG=Google+Search&meta=

That doesn't answer my question. Why does the theory fail in practice? Why
doesn't a capacitance add?

I know that caps have a non-capacitive impedence but surely its no that bad?

What I'd like to see is the frequency response of a tantalum cap with and
without a ceramic to see how it actually works... it's nice to know that it
should be done but I want to actually know how useful it is(so far for all
my projects I have gotten away with just tant's... of course I don't do
anything about 40mhz)

 

[MooseFET]

Why does one have to add ceramic's across power pins along with larger caps
such as tantalum. e.g., 1uF tant an 0.1uF cer?

In theory they just add to 1.1uF and it shouldn't make any difference. It
seems that one should then also add 0.001uF and so on?

All real components have some amount of inductance. The best you can
do on inductance is limited by the mechanical size of the part. To
have a very low impedance at very high frequencies, you need the
inductance to be low and hence want a mechanically small part.

At lower frequencies, you need a lot of capacitance to make the
impedance low. This favors a large mechanical size.

It is hard to make a component that is both large and small at the
same time.

 

[CCD]

Why does one have to add ceramic's across power pins along with larger caps
such as tantalum. e.g., 1uF tant an 0.1uF cer?

In theory they just add to 1.1uF and it shouldn't make any difference. It
seems that one should then also add 0.001uF and so on?

Why don't they make special "power" caps that combine tantalum and ceramics
in one package just for this purpose then?

Thanks,
Jon

Hi,
the ceramic provides quick bypass against high frequency transients/
spikes, while the tantalum provides quite a steady reference by
storing extra energy to supply back in case of little longer
transients!!
Thanks.
-CCD
http://dharmanitech.blogspot.com

 

[[email protected]]

Why does one have to add ceramic's across power pins along with larger caps
such as tantalum. e.g., 1uF tant an 0.1uF cer?

In theory they just add to 1.1uF and it shouldn't make any difference. It
seems that one should then also add 0.001uF and so on?

In my - moderately humble - opinion, it's all about self-resonant
frequencies. Tantalum's have quite high equivalent series resistances
(ESR), and are perfectly useless at high frequencies, but the ESR is
high enough to damp the much sharper self-resonance of a 100nF ceramic
disk.

http://www.kemet.com/kemet/web/home...CD004EBC04/$file/TechTopics Vol4No5 Sep94.PDF

A 10nF "microswave" capacitor can look like a capacitor up to even
higher frequencies.

 

[[email protected]]

On multilayer boards with power and ground planes, and HC-type logic,
one or even 0 caps per board will usually work. Opamps circuits are
usually happy with a ceramic cap here and there.

Faster stuff, or crosstalk-sensitive stuff, may need a few more caps.

We use a scattering of 0.33 uF 0603 ceramic caps on most things.
Tantalums tend to explode, and aluminum 'lytics crap out at low
temperature.

Most boards have too many bypasses, often absurdly too many.

What are the functions of bypass capacitors?

To prevent local instability
To reduce coupling via supply lines between different parts of the
circuitry
To reduce noise
To absorb static discharges
Small ceramic caps are good at that. Using them might save your
circuitry from transient spikes on the supply line, and when fitted on
input circuitry will enable your design to (survive) pass EMC static
discharge tests.

John