William Chops" Westfield" wrote:
> A 0.1uF cap has long been the standard for power supply bypass
> capacitors of most ICs.  Occasionally you'd see smaller values
> (0.01uF) in higher frequency designs.

Yes, back in the pleistocene.  I've been using 1uF SMD ceramics as the
standard bypass caps for a bunch of years now.  Look at the datasheet of one
of these and compare it to a 100nF cap.  The high end frequency responses
aren't all that different, and definitely better than the 100nF leaded caps
of the dark ages.

Up to PIC-level digital circuits, just use a 1uF SMD cap and be done with
it.  If you've got something special RF or otherwise high frequency going
on, then you still have to look at the datasheets of specific model caps.
You should still put the 1uF near the chip at the common power and ground
feed points, but then possibly use smaller values with higher frequency
capability for local decoupling within the private power/ground of the
specific chip.

In one design I was envolved with a few years ago, we used specific 100pF
caps on each of the power pins of a RF chip because those had the lowest
impedence at the RF frequency that we could find.  In that case we also
isolated each power lead with 10 ohm resistors, and there were larger caps
at the common feed point with a series inductor and even larger caps on the
other side of the inductor.  Sometimes it helps to think of the power and
ground feeds to high frequency circuits as a tree structure, with lower and
high frequency caps needed at the leaves.


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