From: jkubicky@cco.caltech.edu (Joseph J. Kubicky)
Subject: Re: Noise problems on the PC bus

>A while back I designed a 12bit A/D card to plug into the IBM-PC bus and
>have had a lot of trouble reducing the noise levels which feed through
>from the supply +5V rail and digital ground. I generate +/- 5 and 
>+/- 12 volts on the card with DC/DC converters and use decoupling at 
>each analog IC (16uF tant and .01uF ceramic) and use a ground plane in 
>the analog section of the board.

Many years ago I tried to build a decent A/D circuit with a switching
supply and I had a similar experience.  While good grounding is important,
it may not be enough.  Particularly, be aware that many switching
converters, even those spec'd for low-noise output, still pass any
INPUT noise through (that is, the spec really speaks to the amount
of additional switching noise introduced).  Thus, you're really
at the mercy of the power supplied by the PC's switcher (which is
usually pretty ugly, particularlly on a loaded bus).

Analog Devices suggests a circuit in their 1992 Amplifier Application
Guide for filtering a 5V (digital) supply for use in single-supply analog
applications.  Should work just as well for dual-supply (with appropriate
modifications):

   -------/==\------*-------*------*------  +5 (filtered)
          \__/      |       |      |
         2 turns   ---     ---    ---
          around   /-\     /-\    ---
        Fair-Rite   |       |      |
     #2677006301    |       |      |
      ferrite bead  |       |      |
   -------/==\------*-------*------*------  +5 return (filtered)
          \__/    100uF   10-22uF  0.1uF
                  elec.    tant.   cer.

In the book they show rather dramatic reduction of high-speed
switching transients for loads up to 100mA.  For much higher loads
you'll have to be careful not to saturate the ferrites.