[SI-LIST] Re: Need technical ammunition for switching to single ground plane !

Good answers.


> [Original Message]
> From: Jon Keeble <jkeeble@xxxxxxxxxxxx>
> To: <si-list@xxxxxxxxxxxxx>
> Date: 9/23/2007 1:23:32 PM
> Subject: [SI-LIST] Re: Need technical ammunition for switching to single
ground plane !
>
> Hi Gilles
>
> To elaborate on Lee's theme;
>
> The kinds of problems that emerge in mixed-signal audio circuits aspiring
> to120dB numbers include both in-band and out-of band energy.
>
> The source of the energy can be
> - inside the box, including
>  - power supplies
>  - routine noise in analog circuits
>  - jitter in converter clocks
>  - low level almost-invisible instability in gain elements
>  - vibration in piezo-electric elements such as ceramic capacitors
>  - inadequate analog conversion of differential to single-ended signals
>  - inadequate removal of out-of-band noise in D2A converters
>  - non-linear effects in general, including demodulation of out-of-band
> energy
>  - cross talk (e.g. inband energy coupled across circuits)
> -outside the box, including
>  - power supply events (HVAC switching)
>  - ESD
>  - current flowing through chassis and signal reference conductors
>    - e.g. non-medical-rated SMPS grounds return upto 2mA
>    - e.g. mains frequency potential energy differences across cables
>  - capacitance-change cable noise
>  - hi energy radio circuits (mobile phones and mobile stage microphones)
> -connector effects, including
>  - intermittent contacts
>  - oxides that rectify RF
>
> and so on. How does the split/solid plane alternative play for each of
> these?
>
> Coming to terms with these issues and their measurement generally requires
> consideration of several frequency bands (inband, out of band, HF).
Optimal
> solutions depend on type of signal and where gain setting elements are
> placed in a system (differential, single ended, magnitude) and making
> decisions about how much out-of band energy is permissable (your circuit
> might work fine but require filters to remove OOB switching energy from
the
> input of your analog analyser to achieve a good figure).
>
> 120dB might sound difficult, but remember we can hear well into the noise
> floor. Evaluate your circuit with FFT averaging so that the FFT 'floor' is
> lower than -150dB; a good circuit will have energy spikes at least 15dB
> below the 120dB analog noise floor..
>
> Single-ended signals are a hassle in audio systems; signal reference is at
> different potientials in different boxes (to a greater or lesser degree)
and
> this appears in the signal and wrecks 'system' behaviour, even if the
boxes
> measure ok in the lab. Reducing the difference using low resistance cable
> braid may or may not be effective (depending on the impedance driving the
> difference) and may result in significant current flow which passes
through
> planes. Split planes isn't going to help this, as there still has to be
some
> DC connection between the two sides of the split; a solid plane will have
> the lowest DC resistance and will minimise the voltage drop across the
> board.
> Circuit techniques can be used to manage this effect, such as building
> quasi-differential inputs and outputs, but have their own limitations.
>
> There is really only one 'maybe' that I can think of for not using solid
> grounds. The SI pros will tell you that HF currents return 'mostly under'
a
> conductor coupled to a plane; this is true of high frequency energy (note
to
> self; should quantify this). However, current with DC to audio-and-above
> frequency components spreads out and introduces small and maybe relevant
> voltages into ground planes; if this energy is relevant (magnitude,
> spectrally) some means might be taken to contain it or reduce the effect.
> You could test by building the same board with different weight ground
> planes (1oz, 2oz, 4oz).
>
> My advice is to put  lots of design energy into making sure the power
> supplies behave well (turn on and off nicely without outputs thumping,
have
> very low energy standby state); address immunity; and make the box behave
> well in a system. It's one thing to build a 120dB box, it's another to get
> the levels required to take advantage of it. The sales guys might want a
> better number, but the box on the stage or in the studio is the one that
> doesn't make noises when the radio mike gets turned on or the HVAC
switches.
> Sold planes rule!
>
> Jon Keeble
>
>
>
>
> > The proponents of this practice need to answer these questions for you.
> >
> > 1. Is there a problem that needs to be solved?
> > 2. What, exactly is the problem?
> > 3. Does the proposed solution solve the problem?
> > 4. Does the proposed solution not create any new problems?
> > 5. Is there no better solution?
> >
> > In almost all cases, things stop at 1, becasue the proponents of this
> > practice are solving an imaginary problem.
> >
> > Lee
> >
> >
> >
> >> [Original Message]
> >> From: Gilles Aminot <gilles.aminot@xxxxxxxxxxxxxxx>
> >> To: <si-list@xxxxxxxxxxxxx>
> >> Date: 9/21/2007 2:39:55 PM
> >> Subject: [SI-LIST] Need technical ammunition for switching to single
> >>
> > ground plane !
> >
> >> Hi Everyone,
> >>
> >> I'm working on a new design and would like to eliminate split ground
> >> planes. In the past we've always split the analog ground plane (for
> >> analog audio circuits and codecs) and joined it at one location
> >> (sometimes shorting or through an inductor). I've read lot's of
postings
> >> on this group explaining good reasons not to split ground planes, but
> >> the only real supporting documentation I have seen is the following
> >> article:
> >> http://www.analog.com/analog_root/static/pdf/raq/RAQ_groundingADCs.pdf
> >>
<http://www.analog.com/analog_root/static/pdf/raq/RAQ_groundingADCs.pdf>
> >> and it does not provide much depth..
> >>
> >> I have been meeting some resistance to eliminating the split ground
> >> planes and I am looking for technical articles showing the advantages
of
> >> using a single ground plane in designs with mixed high speed digital
and
> >> sensitve analog circuits. I keep bumping my head against datasheets or
> >> articles that say analog & digital grounds should be separated (such
as:
> >> http://www.ultracad.com/articles/planesplits.pdf), which makes it
> >> difficult to convince my peers that changing our approach is the way to
> >> go. Can anyone share or point me to articles which may help me plead my
> >> case. I would also appreciate if anyone could share some real life
> >> experiences of going from split to single ground plane.
> >>
> >> Kindest Regards,
> >> Gilles Aminot, P.Eng
> >> Hardware Design Engineer
> >>
> >> Librestream Technologies Inc
> >> Unit 200 - 55 Rothwell Rd
> >> Winnipeg MB. Canada
> >> R3P-2M5
> >> PH: 204-487-0612 Ext 218
> >> FX: 204-487-0914
> >>
> >>
> >>
>
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