Joerg wrote:
Phil Hobbs wrote:
Joerg wrote:
snip
If you have a power plane over a ground plane you don't need 3-4
caps. One or two at the most will be fine because the planes take
care of everything 100MHz. So, if something is this critical I
always make sure there is a power plane.


But that's not necessarily a big help at lower frequency, and can even
be harmful sometimes.

I had a weird bug once in a circuit that had three (nominally
identical) high gain phase sensitive detectors. Two of them were
well-behaved, but the third had an obscenely large offset at the output.

It turned out to be *one pad* sitting over a noisy power plane that
was bouncing up and down by ~50 mV at the signal frequency of about
100 kHz. 100 femtofarads or thereabouts was all it took. I
eventually patched it by putting 10 ohms+100 uF decoupling on the
switching element that was making the power bounce.


50mV is a whole lot of noise at 100kHz. Were you guys a bit skimpy on
bypass caps? I have a (huge) laser + photodiode board in layout right
now and it's got oodles of 0.1uF, plus 47uF caps sprinkled about.


I was the culprit. That plane was a 12V supply from a wall wart, and
was just running a bit of insensitive stuff like RS-232 comms and the
chopped LED light source that was causing the ripple. The low noise
supply was derived from it using a capacitance multiplier, which caused
absolutely no issues (~90-100 dB rejection, pretty amazing for a
MMBT3904). All those 47-uf caps would have cost a lot...the idea was to
have a head tracker that was supposed to cost $10 in quantity, minus the
wall wart and serial cable. Getting rid of the noise at the source was
cheaper than fixing it afterwards, but a small layout change would have
worked just as well.

The thing was, I thought I had it right at the time--the power plane was
split, half being the noisy-12V and half the quiet-11.3V from the cap
multiplier. All the analog stuff was routed on top of the ground plane
or over the quiet supply plane, except for that one lonely 0.7 mm square
pad.

Cheers,

Phil Hobbs