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November 18th 03, 01:55 PM

In article ,
(Dave Platt) wrote:

As you know, coupling capacitors are used to minimize ripple in
batteries/power supplies.

"Coupling" is usually used as a term for capacitors which are actually
in the signal path - they're used to pass signal and block DC.

Power supply caps are usually referred to as "filter" or "bypass" caps.

I have also seen circuits that use smaller value capacitors. What's
the point of .01uF and .001uF coupling capacitors? Is the stray
inductance THAT much lower? I have 1uF 50V monolithic ceramic
capacitors (JIM-PAK MD1), and they're actually smaller than my .1uF
50V ceramic disc capacitors (DC.1). Why doesn't everyone just use 1uF
ceramic capacitors for power supply coupling? At 1.8MHz, capacitive
reactance of a 1uF capacitor is only .0884 ohms.

What's the point of relatively low value electrolytic capacitors?
Their ESR values are usually on the order of 10-100 ohms. At 60Hz, a
1uF capacitor has 2650 ohms of capacitive reactance. A 10uF capacitor
has a capacitive reactance of 265 ohms, and a 100uF capacitor has a
capacitive reactance of 26.5 ohms. Given this, shouldn't all
electrolytic capacitors be at least 100uF?

Not necessarily. There are several other issues to take a look at.

For one thing - examine the _actual_ ESR/impedance of electrolytic
caps over a wide frequency range. You will observe that these caps
tend to behave in a capacitive fashion only up to a certain
(self-resonant) frequency... above that point, they behave much more
like inductors. This is due to the inductive reactance of the
spiral-wound foils (or metallizations on the plastic films).

The higher the capacitance, the more "turns" the foil/film takes
inside the can, and the higher the inductive reactance. This can lead
to the larger caps having a lower self-resonant frequency, and a
reduction in circuit stability at high frequencies.

Is there any point to a
1uF electrolytic capacitor when there are 1uF ceramic capacitors that
have the advantages of lower ESR and nonpolarity?

Yup.

Circuit stability can actually suffer if you try to do all of your
power supply bypassing using low-ESR, high-Q capacitors. These
capacitors can interact with the inductance of the wiring (or PC-board
traces) to create high-Q resonant circuits. If your circuit's
operating behavior tends to excite one of these resonances, you can
get some very severe "ringing" on the power supply lines.

An easy, and common way to prevent this from being a problem is to
"swamp" the high-Q resonances, by including substantial amounts of
lower-Q (higher-ESR) "bulk" capacitance on the power supply rails.
A rule of thumb I've read is to include about 10 uF of aluminum
electrolytic, or 1-2 uF of tantalum, for every half-dozen TTL ICs or
op amps, and to have at least one such "bulk" cap every 4" or so on
the board.

The interactions between wire and trace inductance, component lead
inductance, capacitance, ESR, etc. are many and subtle.

Don't forget "ground bounce." That is one reason why each IC had a 0.1
uF capacitor adjacent to it. The signal going into the IC might look
clean, but if a large number of outputs switched simultaneously, the
ground level in the IC would change and if a signal was at a threshold
level, it might pass through it twice, giving a false extra pulse.

What devices would have all of the outputs go high or low at the same
time? A ripple counter at one time or another would have all outputs go
either high or low at the same time.

Al

--
There's never enough time to do it right the first time.......

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