[sdiy] Analog & Digital Ground

[Colin Hinz]

On Wed, 12 Apr 2006, Seb Francis wrote:

> Well, I've just laid out the digital part of the PCB, and I've left
> spaces for ferrite beads on both the digital power connection and
> ground.  So when it's up and running I will get my scope out and do some
> *real life testing* of the difference with and without the ground bead!

Bear in mind that high-speed digital circuits will exhibit flakiness when
confronted with an unstable ground. A big lump of inductance in the
ground path may qualify as an 'unstable ground', depending on the switching
speed of the ICs. How are you going to test for this, in the digital domain?

Ferrite beads on grounds may be OK for specialized applications such as
a separate ground pin for a PLL circuit within a big digital IC. But for
powering all of your digital stuff.....my experience suggests that this
will cause you grief.

Remember, a microcontroller is not made of 4000-series metal-gate CMOS....

- Colin Hinz
  Toronto, Canada