> > Ceramic discs are fine for decoupling.
>
>Hi,
>
>In general, which capacitor types are best for decoupling? Or doesn't
>it matter that much?
Well, I'm going to assume that you asking about decoupling
high-impedance IC's such as op-amps. Decoupling caps for tube
cathodes requires much larger values.
The short answer is that it doesn't matter that much. Decoupling
caps should be physically small, inexpensive, and bipolar. Their
precise value doesn't matter much, so temperature stability isn't an
issue.
Decoupling capacitors perform two functions.
Firstly, the word "decouple" means to isolate or separate. As such,
they act as filters to keep noise out of the circuit.
Secondly, since PCB traces have some resistance, and are therefore
subject to Ohm's Law, a sudden demand for current will result in a
drop in voltage at that point. So decoupling caps also act as little
power reservoirs.
Ceramic monolithic capacitors are the best choice for decoupling.
While they are available from a few picofarads up to as much a
microfarad, generally, you want 100nF. In the United States, this is
often written as ".1uF". While that might be bad math, such notation has
the connotation of referring to a ceramic bypass cap. The caps
themselves are often labeled "104". If you are using a particularly
noisy chip, such as a timer or logic IC that can cause switching
noise as it changes states, you might want a larger value. Although,
with linear devices, such as op-amps, you are usually trying to keep
the noise out of the chip, not the other way around. If you are
using a particularly prissy op-amp, or a fast op-amp to drive a large
load, or if the rails are particularly long or noisy, you might want
to use two bypass caps, with one a hundred times larger than the
other, in parallel. In this case, the larger cap would be an electrolytic.
Ceramic caps are rated by their temperature coefficient. COG or NP0
capacitors are the most temperature stable, but are also the largest
monolithic ceramic caps. X7R capacitors have a greater temperature
coefficient, but are a bit smaller in size. Z5U capacitors have the
worst temperature stability, but are the smallest in size. That's
what I use for power supply bypassing.
The most important aspect of ceramic caps is that they have a low
series inductance. This is also why physical size is so important --
the smaller the cap, the smaller the leads and traces connecting it.
The inductance of a capacitor raises its impedance, thereby limiting
the frequencies it can filter. This is why bypass caps are placed as
close as possible to the power terminals. This limits the series
inductance, so it can filter high-frequency noise.
To understand different capacitor values, here's a cute web page with pictures:
http://www.kpsec.freeuk.com/components/capac.htm
This page isn't as cute, but lists all the dielectric codes:
http://xtronics.com/kits/ccode.htm