[sdiy] (de)coupling capacitors

[harrybissell]

Hi Theo (and thanks for the plug... your check is in the mail :^)

There is a major difference between "coupling" and "decoupling"

Decoupling is usually used to reduce unwanted coupling on power supply
terminals. The important factor is to have low impedance at all
frequencies.
For this reason, the usual practice is to use parallel caps that have
different
impedances at different frequencies.

example... for low frequencies you need a LOT of capacitance, so a
tantalum or
electrolytic is the usual choice.  The internal "self inductance" will
make these caps
ineffective at high freqs, so they are paralleled with a small ceramic
cap.

Variations of capacitance with temperature etc, and variations with
voltage do not
mean much in this application. You usually use way more than you need.

My twin-separated-at-birth (Jim Patchell) and I both use a variation of
this scheme...
a .1uF cap, at every IC power pin, on every supply voltage.  This
results in a huge total
capacitance... sometimes as many as 200 decoupling caps !!!   We also
route the power
busses and decoupling caps FIRST, before any signal traces... as if the
decoupling was the
single most important thing to do (it probably is).  Keeping the "loop"
formed by the decoupling
caps and the traces small is key to them working properly.  Added
inductance from the traces
can result in having the decoupling caps act as if you had installed a
resistor in series with them...
not a good thing.

I go so far as to run power busses to the cap first, then BACK to the IC
pin. Any noise MUST
hit that cap first, before hitting the pin. In my day job I play with
decoupling thousands of amps...
its really a must.

Coupling is another story...

Here you are trying to pass AC signals to another stage, and block DC.
Changes in capacitance
with voltage, temperature, and nonlinearities can contribute to
distortion.  That said... for a 10uF
cap in a synth circuit I would use a Non-polar electrolytic.  You can
use a polar electroltyic if you
are sure of the polarity of the circuit.  Some recommend two series
polar lytics, back to back, with
a bias resistor to the negative rail at the center point.  That seems
silly to me, since one NP Electrolytic
does the same thing.

In high RIPPLE CURRENT applications, like a speaker crossover... I would
NEVER use an electrolytic.
I'd use a film capacitor, paralleling many if it had to.

Theo made the best point...can you scale the impedances to get the cap
smaller, and then use a film
capacitor ? If not, I'd go with NP Lytics.

H^) harry

Theo wrote:

> IMO rubbish.As a buffer cap for power supply decoupling a elco will do
> fine.Only in High-end equipment aimed at gold-eared bimbos MKS are
> sometimes used as power buffer, marketing claim could be that this
> gives more depth to the silence.Obviously nonsense, only manually
> rolled paper/silver foil caps filled with 5 times filtered hand
> pressed chestnut oil are acceptable. Think for signal coupling in a
> synth a elco would do, that is if there are no polarity issues.As you
> mention 10u is rather large. Usually the large caps are needed to
> prevent low frequency loss when input "resistance" of the next stage
> is low.Maybe you can redesign for use of a 1u MKS?Often the cap and
> input "resistance" form a RC HP filter.The cutoff would be f =
> 1/(2piRC)So if you need 15hz to pass you need a C = 1/(2pi*R*10)
> capacitor.In this case if it needs to be 10u then go for the elco. In
> general for signal coupling a plastic cap like MKS or MKP type would
> be the quality choice.Silvermica would be highend for mic amps and
> critical stuff.Paper/oil the pick for pp who need expensive stuff to
> show other pp they have very good ears.But commercial HIFI equipment
> often uses ceramics or even elcos.So take your pick. Our Happy Hairy
> Harry wrote a cap guide that is on line at multiple sites.Don't have a
> link at hand, but sure others will mention it. HTHTheo
>
>      ----- Original Message -----
>      From: M.A. Koot
>      To: synth-diy at dropmix.xs4all.nl
>      Sent: Sunday, July 18, 2004 3:34 PM
>      Subject: [sdiy] (de)coupling capacitors
>       Hi all, This subject has probably allready been discussed,
>      but I'm affraid my synth-DIY database isn't large enough to
>      find that may-have-past discussion.The situation is that I'm
>      building a filtercircuit which has a coupling and decoupling
>      capacitor of 10uF. This is rather large, and since I believe
>      these capacitors have to be rather good, I'm questioning
>      myself what kind of capacitor would be best.In fact, what
>      capacitor would be overal best for coupling/decoupling
>      functions? Styroflex maby? Since this particular one is that
>      large, I can use a electrostatic one (bad, isn't it?), or an
>      expensive MKS one. Could anyone maby tell me if it's worth
>      the price to buy a MKS, or better, does one hear actually a
>      difference using MKS instead of electrostatic, or is that
>      just rubbish? Cheers,Michiel
>