Dual grounds?

[Magnus Danielson]

>>>>> "MM" == Mark McLemore <mgm at eden.com> writes:

 MM> Hi folks,

Hi Mark!

 MM> I'm about to put together the wiring harness/power supply for a homebrew
 MM> modular and have some thoughts and questions about grounding that I'd
 MM> like to get advice on before I start soldering.  I'm building with Paia
 MM> FracRack panels and interconnecting signals via 4mm banana jacks.  The
 MM> FracRacks are mounted in an oak cabinet to eliminate ground loops, and
 MM> I've been thinking of using a "star" grounding system where the ground
 MM> to each module is tied directly to a single point near the power supply.

Star grounding, that should help.

 MM> I've seen schematics for old Moog modulars where TWO grounds are being
 MM> used: a reference ground and a "dirty" ground -- the latter appears
 MM> to be intended to eliminate noise from switch triggers.  I'm likewise
 MM> considering the use of two grounds, especially since I'm using standard
 MM> 4-pin disk-drive-style molex connectors and have an extra conductor:

A similar approach is to us a analog and a digital ground.

Taking a reference ground is a good thing.

I would not use the disk drive type of power contact. My experience
say that these have bad long life properties. The metal will slowly
bend itself open.

 MM>              V+                   +------+
 MM>               +-------------------|      |---  +15 V
 MM>               |    G              |      |
 MM>          +--------+N   +----------|      |---  Reference Ground
 MM>          | PWR    |D  /           |      |
 MM>          | SUPPLY |--*------------|      |---  Earth Ground
 MM>          +--------+  |            |      |
 MM>               |      |            |      |
 MM>               +----- | -----------|      |---  -15 V
 MM>              V-      |            +------+
 MM>                      |            Module      -> To module
 MM>                   +--+--+         Connector
 MM>                  /  /  /

 MM>               Earth Ground

Looks OK.

 MM> The idea here is to attach Earth Ground to the FracRack chassis and PCB
 MM> "surround-ground" trace, but use Reference Ground for all pots, switches,
 MM> LED's and board electronics.  The single point of ground (shown by
 MM> the asterisk, above) ties together both grounds on the back panel of
 MM> the system along with earth ground from the AC plug and the ground
 MM> connections from 1/4" jacks on the back panel.  I intend to connect
 MM> the rear 1/4" jacks to front panels using shielded cable, but with the
 MM> shield connected only at the 1/4" jack end so as to avoid a ground loop.

I would move LEDs, switches and similar switchy/transient over to the
dirty ground.

I would use the protection ground of the powersupply for chassi and
keep the dirty/clean earth at some suitable resistive impedance from
chassi ground. Some would say 10 ohm, I would go higher but not above
100k, especially for a project of this size.

Also, for some cursuits it may be a good thing to have clean and dirty
powersupplies. Op-amps pulling transients migth be better of on the
dirty supplies as the reference should be on the clean one.

At an extreme box I have seen had more or less every step of the
analog path it's own power stabilisation. It could be something to
consider elsewhere too... :)

 MM> Since I'm not using shielded patch cables, I want to get all the noise
 MM> immunity that I can get.  I think I remember someone on this list
 MM> suggesting this method several years ago.  My questions are:

 MM>     1. Does this work or does it create more problems?  Does it prevent
 MM>        ground loop hum and RF interference any better than a single
 MM>        ground?

 MM>     2. Would it be better to just use the extra wire as another
 MM>        conductor for the same ground and tie the chassis and signal
 MM>        grounds together within each module?

By using star wireing you have a lot more of area for the ground
currents.

The reference/clean ground can be helpfull, if properly applied.

 MM>     3. If I use the dual-ground method, which ground do I attach
 MM>        bypass capacitors to?

If you use the dirty ground for transient device, put bypasses ther,
since these will act as local sources of extra charge. This makes
better sense in a dirty/clean power line configuration thougth.

Bypass caps near the power contact for the clean earth migth be
useful. However, I keep thinking that such caps could also leak noise
from the more dirty powerlines and thereby destroy these
properties... this is a matter of size of caps and their RF properties.

 MM>     4. I used to have a problem with VCO's locking due to intermodulation
 MM>        over the power supply lines, does the dual grounding system
 MM>        help this problem at all?

It can, but you could also get similar problems from the powerlines.

Try to isolate transients and less transient signals.

 MM>     5. Does anyone see a pitfall or disaster waiting to happen by using
 MM>        the above approach?

 MM> I'd appreciate any advice or comments from those of you with insight
 MM> into the black magic of effective grounding.

It's black magic alright ;)

Cheers,
Magnus