[SI-LIST] Re: Analog & Digital Grounds
- From: Scott McMorrow <scott@xxxxxxxxxxxxx>
- To: Gene Glick <gglick@xxxxxxxxxxxx>
- Date: Wed, 14 Oct 2009 15:06:10 -0400
Gene
Below 5 MHz all the rules about unifying analog and digital ground fly
out the window. Below this point you have to be concerned about
magnetic field penetration through planes. Yes all that low frequency
"stuff" in a digital signal is important. Via-less design of the audio
signal section is especially important to meeting your noise-floor
requirements. Isolation of analog and digital can be performed with
separate grounds, lots of space, and multiple shield planes.
Have fun.
Scott
Gene Glick wrote:
> steve weir wrote:
>
>> Mysterious impedances in the reference are an invitation to nasty
>> problems. That said, suppose you have a 24bit A/D converter, 5V FS.
>> Each LSB is a fraction of a uV. It will take only a small amount of
>> induced or DC current to develop multiple LSBs of noise in the common.
>> In cases like this, the required lateral separation of digital traces to
>> prevent excessive induced EMFs could easily be prohibitive. Proper
>> moating and fencing would allow practical packaging while meeting the
>> noise requirements.
>>
>> Steve.
>>
>>
> Great thread, btw :)
>
> There's a lot of simultaneous constraints:
> 1. EMI
> 2. Signal integrity for high-speed
> 3. Signal fidelity for low frequency , like audio.
> 4. Cross talk (aggressor stuff,trace-to-trace, and shared common return
> currents)
>
> I find it difficult to manage all of this stuff at the same time. Yep,
> totally agree that the A/D, especially high resolution, are very
> susceptible to ground currents. I suppose D/A can suffer if the
> reference voltage is unstable.
>
> Personally, I'm working on a multi-port analog design, supposedly
> high-end audio stuff, where crosstalk really needs to be -100 dB, THD
> 0.001% for example. Keeping the signal fidelity good such that they
> don't mix on the return planes is troubling me. It is not possible to
> make completely isolated return lines resulting in some possible shared
> common (ground) currents. The low frequency audio is probably going to
> see the plane as a low impedance plane and take the shortest physical
> path. Unlike high frequency signals that return directly underneath the
> trace on the ground plane. That mode allows good routing to keep the
> analog and digital signals separate.
>
> Just curious about this - suppose there's a clock, square wave with 1 ns
> edges, running around 25 or 50 MHz (or higher). The edges are certainly
> high enough frequency that the return current follows the path of least
> inductance - probably directly underneath the signal trace. What about
> the high and low state times? They are definitely DC, right? So
> wouldn't the return current flow follow the shortest path, or lowest
> resistance? If that's true, then couldn't the "DC-ish portion" return
> flow right through some analog sections, regardless of how well the
> signal trace is run?
>
> If there's any merit to that last paragraph, then fences and moats may
> be required. I agree with Steve about moats making new problems. Is it
> ever ok to put a slot moat in the middle of a board? I have been taught
> that's a no-no due to possible eddy currents forming around it leading
> to emi. Then again, it makes sense from an analog perspective. So the
> solution may also be a new problem.
>
> gene
>
>
>
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