How do I make a seperate analog and digital ground?

[Henk Boonsma]

I'm trying to design a circuit which has both analog and digital power
supplies but I'm not sure how to make seperate analog and digital grounds. I
assume just making seperate ground planes isn't enough because in the CAE
schematic that would merely look like two seperate ground symbols connected
to the same net. I think I've seen some schematics where an inductor is
placed between the analog and digital ground nets. How does this work?

TIA

 

[Rob Gaddi]

Henk Boonsma wrote:

I'm trying to design a circuit which has both analog and digital power
supplies but I'm not sure how to make seperate analog and digital grounds. I
assume just making seperate ground planes isn't enough because in the CAE
schematic that would merely look like two seperate ground symbols connected
to the same net. I think I've seen some schematics where an inductor is
placed between the analog and digital ground nets. How does this work?

TIA

Don't know what program you're using. In OrCad I've found that doing
plane splits is a royal pain. My solution has been to use different
symbols and different names for the two planes, and then at just one
point on the schematic to connect the two with either a zero ohm
resistor or a ferrite bead, either of which fits the same roomy 0805 pads.

 

[Joerg]

Hello Henk,

... How does this work?

In my experience it often doesn't.

While it can be done in pretty much any CAD system I found that split
grounds are a real hassle when circuits become large and complex
structures. Noise coupling becomes nearly unpredictable, EMI
certification turns into a nightmare. You connect the grounds at one
spot and get noise. You connect them at another and get another kind of
noise. When you connect them at multiple places pandemonium starts, hair
turns gray, the aspirin consumption goes up. I could go on. Bottomline I
have never designed a split ground system in my 20+ years of circuit
design and probably won't in the next 20 years. By then I am hoping to
retire.

The lone exemption is isolation for safety or code reasons. But then the
grounds are usually never connected anywhere and the barrier needs to
withstand a few thousand volts. I did design a few of those.

Regards, Joerg

http://www.analogconsultants.com

 

[Rene Tschaggelar]

Henk Boonsma wrote:

I'm trying to design a circuit which has both analog and digital power
supplies but I'm not sure how to make seperate analog and digital grounds. I
assume just making seperate ground planes isn't enough because in the CAE
schematic that would merely look like two seperate ground symbols connected
to the same net. I think I've seen some schematics where an inductor is
placed between the analog and digital ground nets. How does this work?

The schematic capture software are not handling this well.
Either you choose different symbols for analog and digital GND
and short them at one point leading to a single identifyable
error but is technically autoroutable, or
you assign them the same symbol, and do the routing yourself
and don't get any errors at all.
The digital GND is a copper pour on all layers, while the analog
GND is star-connected.
Now there usually are signals crossing from one to the other
domain. There you have to consider what current is flowing
in repect to what and where the return current flows.

Rene
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net

 

[Ken Smith]

In article <4XDEd.21$hi6.1793@news.uswest.net>,
Larry Brasfield <donotspam_larry_brasfield@hotmail.com> wrote:
[...]

I have often told people, (and been paid for doing so), that
[.. never split ground planes ..]

William Shatner has acted and been paid for doing so. :>

(Yes I know that was a cheap shot but I just couldn't resist)

[...]

If you do not see why this is true, I suggest you procure
and study "Noise Reduction Techniques in Electronic
Systems" by Henry W. Ott. Or hire someone who has
and can explain it better than I can here.

I have a copy of said fine work.

Quoting near the bottom of page 58: "The seperate ground system (parallel
connection) shown in Fig. 3-7 is the most desirable at low frequencies"

This means star ground or split ground plain. He later suggests that "low
frequency" mean less than 10MHz. Less than 10MHz includes most audio and
switching supply work.

The book does not actually talk about PCB layout. Neither does Ralph
Morrison's "Grounding and Shielding techniques in instrumentation", which
is a little light on the math but a bit better on practical issues than
the Ott book.



--
--
kensmith@rahul.net forging knowledge

 

[Ken Smith]

In article <rTEEd.10134$5R.2621@newssvr21.news.prodigy.com>,
Joerg <notthisjoergsch@removethispacbell.net> wrote:
[...]

Regarding harm: The most horrifying scenario I have seen was where the
single point connection jumper between AGND and DGND began to glow. The
lab smelled like a camp fire. Then, kapoof, the switcher decided it had
enough of this.

I'll see your wimpy little jumper wire fire and raise you about 6 feet of
ribbon cable + about 150K of electronics. It seems that somewhere there
is a designer who thinks that fuses should go in the (-) power line on DC
equipment but who also thinks that the (-) power should be connected to
signal ground.

--
--
kensmith@rahul.net forging knowledge

 

[Rene Tschaggelar]

Ken Smith wrote:

I have a copy of said fine work.

Quoting near the bottom of page 58: "The seperate ground system (parallel
connection) shown in Fig. 3-7 is the most desirable at low frequencies"

This means star ground or split ground plain. He later suggests that "low
frequency" mean less than 10MHz. Less than 10MHz includes most audio and
switching supply work.

No, it does not include switch mode supplies.
I recently had a 600kHz switcher for a LED from 1.2V to 3.5V
or so at 20mA. The switching spike repeating every 1.5us or so
had harmonics extending far beyond 600MHz as it was just 1.5ns
wide. These are exactly those that make the ADCs measure
whatever that is not there.
And if your design doesn't cope with these 600MHz up,
the spike remains there. Filtering the 600kHz amounts to
nothing, there is nothing there, just the rep rate.

Rene
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net

 

[Fred Bartoli]

"Ken Smith" <kensmith@green.rahul.net> a écrit dans le message de
news:cs0r3v$l2p$5@blue.rahul.net...

In article <h9r7u0lgg1bukdv8fp5qivdua5cv0r46c2@4ax.com>,
John Larkin <john@spamless.usa> wrote:
[...]
Right. Except in the most exotic situations, splitting planes is
insane.

Would you call an amplifier with a gain of about 1 million at 4KHz "most
exotic"

Pfff...
What about 16 millions gain at 100kHz ?


--
Thanks,
Fred.

 

[Ken Smith]

In article <41e3ac07$1@news1.ethz.ch>, Rene Tschaggelar <none@none.net> wrote:

Ken Smith wrote:

I have a copy of said fine work.

Quoting near the bottom of page 58: "The seperate ground system (parallel
connection) shown in Fig. 3-7 is the most desirable at low frequencies"

This means star ground or split ground plain. He later suggests that "low
frequency" mean less than 10MHz. Less than 10MHz includes most audio and
switching supply work.

No, it does not include switch mode supplies.
I recently had a 600kHz switcher for a LED from 1.2V to 3.5V
or so at 20mA. The switching spike repeating every 1.5us or so
had harmonics extending far beyond 600MHz as it was just 1.5ns
wide. These are exactly those that make the ADCs measure
whatever that is not there.

Any variation on the input of a SAR ADC will cause it to misread. Delta
mod, ADCs are much less sensitive to it. All the noise components from
the switcher matter at the ADC.

And if your design doesn't cope with these 600MHz up,
the spike remains there. Filtering the 600kHz amounts to
nothing, there is nothing there, just the rep rate.

If your design couples 100mV of 600KHz into the ADC the 600MHz junk won't
matter much. You are right about the filter having to cope with the high
frequencies but it also has to cope with the low frequencies. The source
is the right place to try to stop such things.

No boiler plate rule can substitute for good design. The design of the
grounding and power routing is an important part of the design.


--
--
kensmith@rahul.net forging knowledge

 

[John Woodgate]

I read in sci.electronics.design that Ken Smith
<kensmith@green.rahul.net> wrote (in <cs0q8c$l2p$3@blue.rahul.net&gt
about 'How do I make a seperate analog and digital ground?', on Tue, 11
Jan 2005:

In article <SpTBM3B4J74BFwu7@jmwa.demon.co.uk>,
John Woodgate <noone@yuk.yuk> wrote:
[...]
It might not even include a 50 Hz lamp dimmer! The harmonics of the 50
Hz typically extend beyond 10 MHz.

Move to North America and you won't have to deal with 50Hz lamp dimmers :

But seriously: I'd expect there to be a whole bunch more 100KHz than
10MHz in the lamp dimmer's output. You have to keep it all bottled up if
there is a sensitive circuit nearby.

The spectrum does fall with increasing frequ3ncy up to about 1 MHz, and

then stays flat to around 10 MHz, then drops again quite steeply. At
least, on those I've measured.

60 Hz dimmers would be lower impedance all round, so may be expected to
have a wider bandwidth!
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[John Woodgate]

I read in sci.electronics.design that Ken Smith
<kensmith@green.rahul.net> wrote (in <cs0r3v$l2p$5@blue.rahul.net&gt
about 'How do I make a seperate analog and digital ground?', on Tue, 11
Jan 2005:

In article <h9r7u0lgg1bukdv8fp5qivdua5cv0r46c2@4ax.com>,
John Larkin <john@spamless.usa> wrote:
[...]
Right. Except in the most exotic situations, splitting planes is
insane.

Would you call an amplifier with a gain of about 1 million at 4KHz "most
exotic"

Depends on the bandwidth. 120 dB and 3 Hz bandwidth is not all that

exotic. 120 dB and DC to 4 kHz is exotic.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[Rene Tschaggelar]

Ken Smith wrote:

In article <h9r7u0lgg1bukdv8fp5qivdua5cv0r46c2@4ax.com>,
John Larkin <john@spamless.usa> wrote:
[...]

Right. Except in the most exotic situations, splitting planes is
insane.


Would you call an amplifier with a gain of about 1 million at 4KHz "most
exotic"

Sure.

Rene

 

[Ken Smith]

In article <SpTBM3B4J74BFwu7@jmwa.demon.co.uk>,
John Woodgate <noone@yuk.yuk> wrote:
[...]

It might not even include a 50 Hz lamp dimmer! The harmonics of the 50
Hz typically extend beyond 10 MHz.

Move to North America and you won't have to deal with 50Hz lamp dimmers :>

But seriously: I'd expect there to be a whole bunch more 100KHz than
10MHz in the lamp dimmer's output. You have to keep it all bottled up if
there is a sensitive circuit nearby.

--
--
kensmith@rahul.net forging knowledge

 

[John Larkin]

On Tue, 11 Jan 2005 15:24:47 +0000 (UTC), kensmith@green.rahul.net
(Ken Smith) wrote:

In article <h9r7u0lgg1bukdv8fp5qivdua5cv0r46c2@4ax.com>,
John Larkin <john@spamless.usa> wrote:
[...]
Right. Except in the most exotic situations, splitting planes is
insane.

Would you call an amplifier with a gain of about 1 million at 4KHz "most
exotic"

Probably not; an old tube shortwave receiver had a million times that
GBW on a single metal chassis. Exotic would be, maybe, nanovolts at
DC. Well, maybe not, since I have done nanovolts at DC (along with a
power supply, a uP, and a 10 amp PWM) on a single ground plane.

John





>--

 

[Ken Smith]

In article <h9r7u0lgg1bukdv8fp5qivdua5cv0r46c2@4ax.com>,
John Larkin <john@spamless.usa> wrote:
[...]

Right. Except in the most exotic situations, splitting planes is
insane.

Would you call an amplifier with a gain of about 1 million at 4KHz "most
exotic"

--
--
kensmith@rahul.net forging knowledge

 

[Larry Brasfield]

"Ken Smith" <kensmith@green.rahul.net> wrote in message news:cs0118$mrm$1@blue.rahul.net...

In article <4XDEd.21$hi6.1793@news.uswest.net>,
Larry Brasfield <donotspam_larry_brasfield@hotmail.com> wrote:
[...]

I have often told people, (and been paid for doing so), that
[.. never split ground planes ..]

William Shatner has acted and been paid for doing so. :

(Yes I know that was a cheap shot but I just couldn't resist)

Yes. Mr. Shatner deserves better.

If you do not see why this is true, I suggest you procure
and study "Noise Reduction Techniques in Electronic
Systems" by Henry W. Ott. Or hire someone who has
and can explain it better than I can here.

I have a copy of said fine work.

Quoting near the bottom of page 58: "The seperate ground system (parallel
connection) shown in Fig. 3-7 is the most desirable at low frequencies"

This means star ground or split ground plain. He later suggests that "low
frequency" mean less than 10MHz. Less than 10MHz includes most audio and
switching supply work.

He says "Normally, at frequencies below one megahertz a single
point ground is preferable; above 10 MHz, a multipoint ground
system is best."

If you look at his diagrams of grounding topologies, you will see no
signal connections between the separately grounded circuits. If you
look a few pages back (start of section "SIGNAL GROUNDS"),
you will see the comment "In the following discussion of grounding
techniques, two key points should be kept in mind: 1. All conductors
have a finite impedance, generally consisting of both resistance and
inductance. At 11 KHz, a straight length of 22-gauge wire one inch
above a ground plane has more inductive reactance than resistance."

Now, think for a moment about what happens if those separate
circuits pass single-ended signals between separately grounded
subsets. The return currents have to travel through the star or
the equivalent split plane join point. Those currents give rise to
voltage drops along the ground conductors, and for frequencies
over 10-100 KHz, depending on what level of interference is
significant, B-field coupling becomes significant, unless the return
currents have been made to closely follow the signal currents.

People who are enamored with split ground systems generally
do not consider B-field coupling, and rarely understand where
return currents actually flow when a shared ground plane is
used. Typically, they will invoke return current paths distant
from the nominal signal path, failing to appreciate how, at
higher frequencies, the return currents concentrate beneath
the signal traces, unless blocked by splits and such.

I believe Mr. Ott's advice, with respect to what frequencies
the split ground schemes can be used, ceases to apply when
signals spanning a wide dynamic range are involved.

The book does not actually talk about PCB layout. Neither does Ralph
Morrison's "Grounding and Shielding techniques in instrumentation", which
is a little light on the math but a bit better on practical issues than
the Ott book.

Morrison's work is mostly good for low frequency circuits.
Many of the techniques he suggests begin to be counter-
productive above the ~10 KHz transition frequency where
conductor reactance exceeds resistance.

One of my burned-in memories involves a system that was
put together by a "holy ground point" advocate. It had a
magnetically deflected CRT display system, enough digital
stuff to store and display captured ECG signals, and a high
bandwidth (200 Hz!) ECG acquisition module. Routed to
the holy point were hundreds of black wires. It took some
time to figure out that the reason it blew its fuses whenever
turned on was because of extraneous coupling that arose
solely because of that misguided grounding scheme. The
deflection power amplifier would give its all replicating a
fast signal that seemed to come from nowhere, at least until
the actual whole circuit was considered rather than what
the holy point advocate had idealized. For some strange
reason, those signals would just not content themselves to
stay *in* the wires.

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.

 

[John Woodgate]

I read in sci.electronics.design that Ken Smith
<kensmith@green.rahul.net> wrote (in <cs13et$r18$4@blue.rahul.net&gt
about 'How do I make a seperate analog and digital ground?', on Tue, 11
Jan 2005:

In article <2mKLHyAFf$4BFwqC@jmwa.demon.co.uk>,
John Woodgate <noone@yuk.yuk> wrote:
I read in sci.electronics.design that Ken Smith
kensmith@green.rahul.net> wrote (in <cs0r3v$l2p$5@blue.rahul.net&gt
[...]
Would you call an amplifier with a gain of about 1 million at 4KHz "most
exotic"

Depends on the bandwidth. 120 dB and 3 Hz bandwidth is not all that
exotic. 120 dB and DC to 4 kHz is exotic.

BW = about 200Hz

That's about the square root of exotic, then. (;-)

--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[Ken Smith]

In article <xm9K3nAZd$4BFwr7@jmwa.demon.co.uk>,
John Woodgate <noone@yuk.yuk> wrote:
[...]

60 Hz dimmers would be lower impedance all round, so may be expected to
have a wider bandwidth!

Its the dI/dt in the triac that determines the high frequencies so I'd
expect the spectrum from it to look nearly identical up at the high
frequencies. There is usually some EMI circuitry but I doubt that the
50Hz and 60Hz versions differ because of the mains frequency. The
differences are more likely to be FCC vs CE.

--
--
kensmith@rahul.net forging knowledge

 

[Ken Smith]

In article <2mKLHyAFf$4BFwqC@jmwa.demon.co.uk>,
John Woodgate <noone@yuk.yuk> wrote:

I read in sci.electronics.design that Ken Smith
kensmith@green.rahul.net> wrote (in <cs0r3v$l2p$5@blue.rahul.net&gt
[...]
Would you call an amplifier with a gain of about 1 million at 4KHz "most
exotic"

Depends on the bandwidth. 120 dB and 3 Hz bandwidth is not all that
exotic. 120 dB and DC to 4 kHz is exotic.

BW = about 200Hz

--
--
kensmith@rahul.net forging knowledge

 

[Rich Grise]

On Tue, 11 Jan 2005 07:59:36 +0000, Ken Smith wrote:

In article <4XDEd.21$hi6.1793@news.uswest.net>,
Larry Brasfield <donotspam_larry_brasfield@hotmail.com> wrote:
[...]

I have often told people, (and been paid for doing so), that
[.. never split ground planes ..]

William Shatner has acted and been paid for doing so. :

(Yes I know that was a cheap shot but I just couldn't resist)

Two Words: Denny Crane.

;-p

Cheers!
Rich