Having found some more information, I thought I'd revive this issue.
See the attached figure showing capacitor impedance vs. frequency.
Consider the power regulation issue again:
If the supply is, say, 5 V, and the ripple is to be kept less than 1% = 50
mV, and the package is going to pull 100 mA, then an impedance of 50 /100 =
0.5 ohm can be tolerated. If only a 10 mA load, then this increases to 5 ohms.
A 1 uF tantalum has a flat impedance curve below 1 ohm from 100 kHz up to
10+ MHz.
A 0.1 uF monolithic ceramic has sharpness (Q) in its impedance curve, so
its impedance is below 1 ohm only between 2 and 10 MHz. It acts as a notch
filter at about 1 MHz.
A 1000 pF poly film cap has < 1 ohm impedance only from 50 MHz to 1 GHz,
and acts as a notch filter at about 100 MHz.
If a PIC circuit operates < 20 MHz, why would you prefer a smaller ceramic
over the 1 uF tantalum, when it's got a better bandwidth on low impedance?
Particularly since at 20 MHz the tantalum is probably lower impedance than
the smaller capacitance ceramic?
If op-amps with frequencies below 1 MHz are driven by the supply, higher
capacitance is a more important factor in impedance than type is.
So I ask again: Why use a small capacitor to bypass power on these types of
circuits? Why isn't the 1 uF tantalum a good solution, albeit a few cents more?
Tantalums have their uses, but in general they aren't nice components. They have very little, if any overload resistance. A small voltage surge exceeding the working voltage will likely cause the device to go short circuit, or at least reduce it's life. If you have any requirements to build a circuit with very high MTBF's then avoid tants.
Electroylitic caps are also large. When board space is important, and you simply don't need a 1uF cap, then why use it?
It still seems to me you only need a very large capacitor at the input of
the regulator, with 1-100 uF at the output, and 1 uF tantalums at the chip
level to control switching noise.
Reasonably large caps on the input are important, especialy if the regulator is a fair distance from the PSU reservoir caps. Without this cap, the regulator can turn into a pretty good oscillator!
As I said before, I'm not an EE, so perhaps I'm missing something in the
spec sheets here. << File: capacitor impedance vs freq.gif >> << File: ATT427318.txt >>
A device running at 20MHz (usually) produces harmonics that extend far beyond the base frequency. At 100MHz a tantalum will be as usefull as a chocolate fireguard. I'm guesing you based your figures on leaded components? SMD ceramic caps have a tiny inductance and good for far more than 10MHz
Regards
Mike Rigby-Jones