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]

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

 

[Nico Coesel]

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 reason for splitting ground planes is to have the current from the
digital devices return to the digital supply and have the current from
the analog devices return to the analog supply.

In most cases I draw a circuit diagram of the ground net. Then I
determine through which legs large currents are flowing. These legs
should not carry any current to sensitive devices. Picture each leg as
a tiny resistor, so each leg has a voltage drop across it.

Placing an inductor between ground planes doesn't sound like a good
idea because the signals it rejects are superimposed on the power
supply rails. Also, ICs which are connected to both ground now have 2
ground levels. If these are too far apart, you may run into trouble.
In general, ground should be rock solid.

 

[Larry Brasfield]

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?

I have often told people, (and been paid for doing so), that
split ground systems fall into 2 categories: those in which the
split does nothing; and those where the split does harm.

By the time you arrange that return currents do not have to
flow across your split, you have done most of the work
needed to avoid shared impedance coupling. The rest of
the job is keeping field coupling below tolerable levels.
Once that is done, the split will have no effect in ordinary
circuits at ordinary frequencies. The exception is for DC
where small potentials are significant.

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.

 

[Tim Wescott]

I have often told people, (and been paid for doing so), that
split ground systems fall into 2 categories: those in which the
split does nothing; and those where the split does harm.

By the time you arrange that return currents do not have to
flow across your split, you have done most of the work
needed to avoid shared impedance coupling. The rest of
the job is keeping field coupling below tolerable levels.
Once that is done, the split will have no effect in ordinary
circuits at ordinary frequencies. The exception is for DC
where small potentials are significant.

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.

The description I like on how to use split grounds (on a PC board) is thus:

Split your ground planes, except for one "bridge" segment. RELIGIOUSLY
pay attention to ground return paths for ALL signals, keeping them over
the correct plane. ANY signals that gave to cross from one domain to
the other goes over the bridge.

Then when you're done make the ground plane one continuous pour.

 

[Joerg]

Hi Larry,

I have often told people, (and been paid for doing so), ....

That makes two of us ;-)

... that split ground systems fall into 2 categories: those in which the
split does nothing; and those where the split does harm.

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.

Regards, Joerg

 

[Ken Smith]

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.

 

[Ken Smith]

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.

 

[Rene Tschaggelar]

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

 

[Rene Tschaggelar]

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

 

[John Woodgate]

I read in sci.electronics.design that Rene Tschaggelar <[email protected]>

No, it does not include switch mode supplies.

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

 

[Ken Smith]

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.

 

[Ken Smith]

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.

 

[John Larkin]

I have often told people, (and been paid for doing so), that
split ground systems fall into 2 categories: those in which the
split does nothing; and those where the split does harm.

By the time you arrange that return currents do not have to
flow across your split, you have done most of the work
needed to avoid shared impedance coupling. The rest of
the job is keeping field coupling below tolerable levels.
Once that is done, the split will have no effect in ordinary
circuits at ordinary frequencies. The exception is for DC
where small potentials are significant.

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.

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

John

 

[Ken Smith]

Henk Boonsma wrote:
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.

Actually on a mixed PCB, the digital stuff usually forces you to have more
layers than the analog needs. This allows you to set aside one layer
under the analog stuff as the ground for it. This leaves the digital
ground in place to conduct the currents past and an independant ground
over it to act as the reference for the analog signals. This works well
at frequencies below 1MHz.

In very dense boards where the number of layers is very high, you can set
aside three extra layers of power/ground. This lets you have low impedance
power lines and grounds within the analog section. There can be as much
trouble from AC coming in on the power as on the ground. The decoupling
caps run the AC current to the ground near the op-amp. This is exactly
where you don't want an AC current injected. Small value decoupling
resistors on the power lines coming into the section and low ESR caps to
ground are helpful here.

 

[Ken Smith]

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"

 

[Fred Bartoli]

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

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

 

[Rene Tschaggelar]

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.

I'm not to sure about the robustness of SigmaDelta ADCs.
The point is the comparator inside the ADC. It doesn't have
a common mode rejection that extends too far into the MHz.

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.

These 100mVpp you measure at 600kHz are 100mV pulses of 1.5ns, and they
can make your ADC measure a constant 100mV offset on top of your signal.
As soon as you need more than say 8bit at 5V, your readings are to be
influenced by the mentioned noise. This is especially ugly when you have
one of those 20 to 24bit ADC that could produce many more bits.
I had a application where with a lot of ferrites, we were able to reduce
these spikes down to 5 mV or so, and the ADC were just 5mV off.
However some were tracking the signal minus 5mV while others had the
signal plus 5mV. This is ugly and rather hard to communicate.
It was the management that insisted on having switchers there ...

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

Sometimes marketing overrules.

Rene

 

[John Woodgate]

I read in sci.electronics.design that Ken Smith
about 'How do I make a seperate analog and digital ground?', on Tue, 11
Jan 2005:

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!