[SI-LIST] Question about split gnd planes - a history

Folks:

The concept of AGND and GND (digital ground) comes from (1) audio
design, (2) high gain amplifier design, and (3) emissions control. In
all cases, the object is to prevent circulating currents carrying
undesirable spectral energy from affecting the desired circuit
operation or conditions of operation.  With regard to (1), in some
cases, the paths are separated to avoid feedback or instabilities and
in other cases to keep the noise from the circulating currents flowing
through an impedance and presenting noise voltages at the high gain
inputs. Also, the separation may be used to eliminate 50/60 Hz hum
coupled from power supply.

In System Ground design, one has to have as a design objective (1)
either a high impedance return path (single point grounding with
separate AGND and DGND planes connected at only one point, or (2) a
very low impedance return path achieved by using lots of AGND to DGND
connections, ala Lee's Analog Devices reference. BTW, I also have some
Analog Devices data showing isolated power and ground reference designs
using motes.

The story here is that intermediate values of impedance in the return
path will result in either undesired signals coupled in the forward
direction or EMI emissions or both.

A solution ofen used to couple split planes that is almost always
suggested is to connect the planes with an isolating capacitor.  It may
come as a surprise to most of you that "apple" has a patent on that
solution. I know it surprised me.

Please remember those who have served and those who do serve now this
weekend and have a safe and happy holiday. 

Note: NESA's new address and phone number below!

ed
=============
Dr. Edward P. Sayre, P. E.
North East Systems Associates, Inc.
1 South Avenue, 3rd Floor
Natick, MA  01760
[T] 508-652-0088
[F] 508-652-0083


> -------- Original Message --------
> Subject: [SPAM] [SI-LIST] Re: Question about split gnd planes
> From: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
> Date: Fri, May 26, 2006 9:31 am
> To: "Ayan Bhattacharyya" <ayan_bhattacharyya@xxxxxxxxxxxx>,
> weirsi@xxxxxxxxxx, "Manickavelu M." <manickavelu_m@xxxxxxxxxxxx>,
> istvan.novak@xxxxxxxxxxx, "Ed Troy" <etroy@xxxxxxxxxxxxxxx>,
> si-list@xxxxxxxxxxxxx
> 
> I'm not sure where the notion came from that there should be an AGNDand a
> DGND in a PCB.  It's not true.  I've attached a short piece written by the
> technical specialist from Analog Devices stating just  this.
> 
> > [Original Message]
> > From: Ayan Bhattacharyya <ayan_bhattacharyya@xxxxxxxxxxxx>
> > To: <weirsi@xxxxxxxxxx>; Manickavelu M. <manickavelu_m@xxxxxxxxxxxx>;
> <istvan.novak@xxxxxxxxxxx>; Ed Troy <etroy@xxxxxxxxxxxxxxx>;
> <si-list@xxxxxxxxxxxxx>
> > Date: 5/26/2006 3:03:02 PM
> > Subject: [SI-LIST] Re: Question about split gnd planes
> >
> > Hi,
> >  As per I know no tool will allow you to connect the AGND and DGND
> > directly....u can connect them through CAPs also...
> > Else in same ground plane u can maintain less noise interference by making
> > bottle-necks in the layout for the different noise sources.
> > "Istvan Novak"'s approach is a good one...ground guard signals also help a
> > lot ...specially for clock signals...around crystals.
> > Regards
> > Ayan Bhattacharyya.
> >  
> >  
> > -----Original Message-----
> > From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On
> > Behalf Of steve weir
> > Sent: Friday, May 26, 2006 4:47 PM
> > To: Manickavelu M.; istvan.novak@xxxxxxxxxxx; Ed Troy;
> si-list@xxxxxxxxxxxxx
> > Subject: [SI-LIST] Re: Question about split gnd planes
> >
> > Manix, no.  There are many misconceptions out there about the myth of 
> > ground and its relation to noise isolation.  Those myths get 
> > propagated into misguided applications of moats and such.  Think in 
> > fields and the misconceptions go away.
> >
> > Steve.
> > At 09:50 PM 5/25/2006, Manickavelu M. wrote:
> > >Istan,
> > >Is it not that the analog and digital grounds planes can not be connected
> > >together anywhere but only under the chip that sources the analog
> signals?
> > >Also that while coupling these two planes we should not use direct Cu
> plane
> > >connection but couple them via inductors?
> > >
> > >Manix,
> > >MindTree.
> > >
> > >-----Original Message-----
> > >From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On
> > >Behalf Of istvan.novak@xxxxxxxxxxx
> > >Sent: Friday, May 26, 2006 6:26 AM
> > >To: Ed Troy; si-list@xxxxxxxxxxxxx
> > >Subject: [SI-LIST] Re: Question about split gnd planes
> > >
> > >Ed,
> > >
> > >Splitting more than one ground plane in the stackup requires a lot of
> > >consideration, and mostly it is not necessary.  Isolating a sensitive
> > circuit
> > >(e.g., analog input, low-jitter oscillator) may be a good idea, but
> instead
> > >of cutting a large solid ground plane, you may want to try first to put
> the
> > >circuit to be isolated on a grounded patch on a 'non-ground' layer.  You
> can
> > >make ground surface patches under and around your circuit to be
> isolated, or
> > >you can put the patch on a signal layer.
> > >
> > >Regards,
> > >
> > >Istvan Novak
> > >SUN Microsystems
> > >
> > >
> > >From: Ed Troy <etroy@xxxxxxxxxxxxxxx>
> > >Date: Thu May 25 15:38:33 CDT 2006
> > >To: si-list@xxxxxxxxxxxxx
> > >Subject: [SI-LIST] Question about split gnd planes
> > >
> > >If you have a circuit board that requires a split gnd  plane over a
> > >small section of the board, and you have several ground planes,
> > >should only one have the split (the one nearest the side containing
> > >the components that require analog ground)  while the rest of the
> > >ground planes are continuous, or should the split section be on all
> > >ground layers? I would think that you should only have it on one
> > >layer. Also, if it should only be on one layer, I would imagine it
> > >would be best to connect it to the digital ground with one, and only
> > >one, via. Is that generally correct? What are some good references
> > >for layer stackups, etc? I know I saw one, once, but can't remember
> where.
> > >
> > >Ed
> > >
> > >
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> -- File: AnalogDigitalGround.jpg
> -- Desc: AnalogDigitalGround.jpg
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