It seems to me that any digital IC had better make the positive rail an
effective AC return, otherwise digital switching noise (which is
responsible for as much if not more current than the oscillator itself AND
has more harmonics,since the oscillator is a good sine approximation,as
opposed to the square,fast edges of the digital switching) will radiate
very well.

ALSO, without external components, I think Vss looks the same to AC as Vdd.
Internally on the chip, they are pretty symetrical (only asymetry for a
pure CMOS digital IC should the the difference between PMOS and NMOS,which
is pretty small). It is the fact that we always guard the integrity of
ground very well by using ground planes,star points,and short runs,etc.
that makes GND usually look different to AC than the positive supply. IF
you were to use an equally large power plane, etc. OR have bypass caps
located very close to the IC, I think Vss and Vdd look almost identical to AC.


Sean


At 11:52 AM 6/12/99 -0500, you wrote:
>> Whoops, I didn't check the specs before I blurted that out. I mean the
>> positive supply pin. Its also an AC ground, which is all the osc caps care
>> about.
>
>
>The positive supply pin (vcc) is NOT an AC ground, unless external
>components make it so. If you've got space for the externals, then you can
>just as well return the osc caps to the chip ground (vss). Returning them to
>VSS through any component (which will have a finite, non-zero impedance)
>will always be worse than returning them to VSS directly.
>
|
| Sean Breheny
| Amateur Radio Callsign: KA3YXM
| Electrical Engineering Student
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