R
rickman
- Jan 1, 1970
- 0
I build picosecond stuff that works. Do you?
That is exactly the sort of stuff that makes you sound like a hack. "I
use the XYZ rule of bypassing and all my boards work". Yeah, but what
does that prove? Mostly it means using way more caps than needed.
I often include SMA connectors on PCB layouts, so I can TDR/TDT the power planes
on bare boards and measure the plane noise on operating boards.
A plane/pour system is a good HF cap all by itself. Adding a few more ceramic
bypasses here and there makes it better. It's pretty much that simple. Of
course, if you expect gross low-frequency current steps, you need enough bulk
capacitance to handle that until the power supply can respond.
Most systems are grossly over-bypassed. And the classic "use lots of different
value caps" papers were mostly authored by people who sell caps.
I won't argue about the "over-bypassed" boards. But if you use the
methods that are promoted by the rational engineers who *don't* sell
caps, you can actually use a lot fewer bypass caps than if you just use
one value.
I do mixed-signal stuff, uPs and FPGAs and fast ADCs and picosecond delay
generators. I don't use a lot of caps and I've never used too few.
Here's a signal conditioner and a 250 MHz 12-bit ADC inches away from a big FPGA
and a bunch of switching power supplies and line drivers. It has a few 330 nF
bypass caps here and there. It worked first time. There are no parts on the
bottom side.
https://dl.dropbox.com/u/53724080/Circuits/ESM/ESM_PCB.jpg
Absolutely!
One piece of bad info is that all systems need to be treated
Another is that high speed systems need a lot of different bypass caps. On a
multilayer board, bypassing is easy.
That's not bad info, it is useful if you actually engineer your PDS. Or
you can just scatter around a few caps and hope it works like the others
you have done seem to.
But assuming that the info on how to design the PDS for
HoJo's Black Magic book *is* silly.
I have never studied "HoJo's" book. I took a class with Lee Ritchey and
was very impressed with his knowledge, but more importantly his
techniques of understanding the theory, analyzing it in simulation, and
then proving it all correct by building the hardware. He showed that
caps on good ground/power planes don't need to be as close as possible
to the pins of the chips, again, by using all three methods.
Rick