[SI-LIST] Re: Need technical ammunition for switching to single ground plane !
- From: "Jon Keeble" <jkeeble@xxxxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Sun, 23 Sep 2007 10:24:42 +1000
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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