Re: Grounding
- From: "Kevin White" <kevinjwhite@xxxxxxxxxxx>
- Date: 8 Aug 2006 10:49:51 -0700
John Larkin wrote:
On 8 Aug 2006 02:01:17 -0700, "CeeRox"
<christer.persson@xxxxxxxxxxxxxxxxxxx> wrote:
Hi!
Here a question that problably will have equal answers as there are
engineers but I'll give it a shoot:
Where is the best place to splitt digital and analog ground?
Don't do it. Use a single solid ground plane and handle any low-level
problems locally.
Splitting planes implies an impedance between them, hence high
relative AC potentials between "grounds". That will generally cause a
lot more trouble than it cures.
John
I agree, I have never found an advantage in splitting the grounds. I
try to keep the ground impedance as low as possible.
I do carefully manage the current flow from the digital and power
sections to minimize noisy current flow in traces used for analog
signals. Usually this means shaping the ground planes or cutting it in
various places - I keep the connection between these sections as broad
as possible though - definitely not a narrow trace or zero ohm
resistor.
Other techniques are to put resistors in series with the digital
signals driving A/Ds, so reduce the current flow in the input
capacitance of the A/D. Often it is useful to buffer the A/D outputs
with digital buffers so that the A/D does not drive significant
capacitative load and thus put high currents in its ground system.
I like to put lots of copper everywhere to keep as low an impedance
gorund as possible - for boards I do myself using Eagle CAD the polygon
capability works well for this. Admittedly this can increase
capacitance to ground which may be undesirable.
Be careful to avoid the digital signals passing over the ground region
for the analog signals. Analog Device has some useful app notes on
this subject.
Which approach is used depends upon the signal frequency. At low
frequencies the resistance of the ground system is the dominant effect
and using star wiring back to the power source may be the most
appropriate - at video frequencies and above (includes most digital
systems) the inductance is a major problem so putting lots of paths in
parallel and avoiding capacitative or inductive coupling between
sections will probably work better.
A design I am working on currently uses 17 LTC1403A 14-Bit 2.5MHz A/Ds
in an optical measurement system with bandwidth of around 1MHz.. I
have surrounded the pre-amp sections with the A/Ds such that the
control signals (isolated with resistors at the driving ends) pass
around the periphery of the analog section. I have flooded the analog
section with copper to reduce the ground impedance.
These A/Ds are pretty nice to work with - they have differential inputs
and serial outputs - reduces the number of digital lines causing noise,
and a pretty small MSOP-10 package.
In initial measurements the noise level seems to be pretty much as
expected.
cheers
kevin
.
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- From: CeeRox
- Re: Grounding
- From: John Larkin
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