[SI-LIST] Re: Sacrificial ground: Is it useful?

  • From: Geoff Stokes <gstokes@xxxxxxxxx>
  • To: "'si-list@xxxxxxxxxxxxx'" <si-list@xxxxxxxxxxxxx>
  • Date: Mon, 27 Oct 2003 08:38:03 -0000

Hi John,

Some years ago, equipment in rack assemblies (sonar for example) used to
have a "frame continuity" network.  This network was isolated from the rest
of the system except that it was joined at one point to the power supply
ground.  All the power returns of each electronic assembly where separately
returned to a star-point ground terminal near the power supply unit.  At
that time, there was very little digital processing - it was slow - and high
frequencies greater than 1MHz or so where not really of any concern.  The
idea was that a ground loop would be set up if the units were grounded at
more than one point.  Any such ground loop would be susceptible to inductive
mains frequency coupling, and with lots of unscreened analog circuits in the
system, this could easily be demonstrated if a unit was wrongly wired!

I think maybe that kind of thinking has now changed and been replaced by a
requirement for tight bonding between well-shielded units.  Where necessary,
isolated power supplies and differential signal interconnects or optical
coupling are used for sensitive situations.  Could it be that for lower
frequencies, the Frame Continuity principle is still useful?  However it
seems to me that the decoupling caps you mention around the edges of the
boards could possibly present a problem, coupling noise in at mid to low
frequencies.

Cheers
Geoff
> -----Original Message-----
> From: John Coupland [mailto:coupswork@xxxxxxxxxxxxx]
> Sent: 24 October 2003 17:59
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Sacrificial ground: Is it useful?
> 
> 
> Hi gurus,
> We've traditionally used a "sacrificial ground" (a 
> trace around the edge of PCBs) which is connected to 
> chassis ground (when the board is installed). We also 
> distribute some caps (like 0.1uF) from this trace to 
> signal ground around the edge of the PCB.
> 1. Is this trace used to help protect the active 
> circuitry when one picks up the board by the edges 
> (i.e. when ESD handling procedures are not being 
> followed) OR is it beneficial for some other purpose?
> If beneficial:
> 2. Are the caps beneficial or counter-productive?
> 3. Should it be placed on all layers?
> 4. How wide should it be?
> 5. How near the edge it should be?
> 6. How much spacing should there be from it to active 
> traces? To power/ground planes?
> Thanks,
> John
> PS, I didn't see anything about it in the archives.
> 
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