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How do I make a seperate analog and digital ground?

Discussion in 'Electronic Design' started by Henk Boonsma, Jan 10, 2005.

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  1. Henk Boonsma

    Henk Boonsma Guest

    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
     
  2. Rob Gaddi

    Rob Gaddi Guest

    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.
     
  3. Joerg

    Joerg Guest

    Hello Henk,
    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
     
  4. Nico Coesel

    Nico Coesel Guest

    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.
     

  5. 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.
     
  6. Tim Wescott

    Tim Wescott Guest

    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.
     
  7. Joerg

    Joerg Guest

    Hi Larry,
    That makes two of us ;-)
    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
     
  8. Ken Smith

    Ken Smith Guest

    [.. 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)

    [...]
    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.
     
  9. Ken Smith

    Ken Smith Guest

    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.
     
  10. 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
     
  11. 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
     
  12. I read in sci.electronics.design that Rene Tschaggelar <>
    It might not even include a 50 Hz lamp dimmer! The harmonics of the 50
    Hz typically extend beyond 10 MHz.
     
  13. Ken Smith

    Ken Smith Guest

    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.

    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.
     
  14. Ken Smith

    Ken Smith Guest

    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.
     
  15. John Larkin

    John Larkin Guest


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

    John
     
  16. Ken Smith

    Ken Smith Guest

    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.
     
  17. Ken Smith

    Ken Smith Guest

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

    Fred Bartoli Guest

    Pfff...
    What about 16 millions gain at 100kHz ?
     
  19.  
  20. 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:
    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!
     
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