which ground to use for the SID ?

[Theo]

Why not give the sid a ground of its own.
That way they can't pollute the AGND and
they won't be polluted by the digital ground.

Just a thought,
Theo

------------------
----- Original Message -----
From: Michael Buchstaller <buchi at takeonetech.de>
To: <synth-diy at node12b53.a2000.nl>
Sent: Friday, January 12, 2001 11:44 AM
Subject: which ground to use for the SID ?


> Hello Gurus,
>
> after completion of my monophonic SID-Synth (Monovoice), i am working on a
> polyphonic version.
>
> I want to have a controller board and 7 Voicecards.
> The bus will be made from a standard 50-pin flat cable (like an internal
> SCSI cable).
>
> The Voicecards will run from an unregulated 20 VDC power supply
> and incorporate 2 78xx voltage regulators to get the +5V for the SID
> and bus glue logic as well as the +9 or +12V for the SID output
> amplifier.
> I had some trouble in the Monovoice with noiose from the digital circuitry
> appearing on the SID´s audio output (8 MHz and 1 MHz clocks showing up
> at a level of about 10 mV -  of course i could filter it out, but i want
to prevent
> this by design)
> I suspect that having separate ground connection for clean (Audio) and
dirty
> (digital) will help. Also excessive bypassing and the separate regulators
for
> every voice card might help.
>
> But, the SID chip does have only one pin to connect it to GND. Shall i
connect
> that to the clean or dirty GND ?
> Personally, i think i should use the dirty GND, because the sound
generation and
> logics registers are all digital and only the output amplifier and filter
are analog.
> Or shall i connect it to the clean ground and risk polluting that one with
the digital
> noise from the 1 MHz oscillator and the switching transients from the MCU
controlling
> the SID ?
>
> Has anybody experience with that ?
>
> Or should i try to isolate the SID´s ground somehow else ?
>
> -Michael Buchstaller