Analog and digital GND

Further reading in an application note reveals that the ADC is supposed be mounter on the AGND with its AGND and DGND pins on the AGND. Go ahead, there are quite extensive application notes at the various manufacturers websites.

Rene

Hi,

I am sorry if I wasn't clear. You are absolutely right, the datasheets specify clearly how the pins on the ADC should be connected. But it doesn't say anything about the decoupling capacitances. So my questiod was really about them and not really about the ADC itself. Should the decoupling capacitances be connected to AGND or GND?? Or maybe is it implicitly saying that the decoupling capacitances should be connected to the same ground as the ADC, but that is what I can't find out.

Thank you!

"Frida" a écrit dans le message de news: snipped-for-privacy@localhost.talkaboutelectronicequipment.com...

Use a unique, large, full ground plane, and connect all pins that should be connected to all xGND to this ground plane. This is by far the safer alternative for 99,999% of the designs. Dual-ground plane architectures are VERY difficult to design, and are the root cause of serious headackes when doing EMC testing... Except of isolated / high voltage designs of course.

Anybody disagree ? (this is a subject discussed quite often so I expect thousands of answers ranging from "stupid answer" to "of course"....)

Friendly, Robert

To GND. The underlying principle is that there should be no currents flowing in the AGND circuit. Currents have spikes, and cause IR drops, and generally create havoc for analog inputs. Decoupling capacitors shunt current in order to maintain voltages.

The answers will be far more likely to be correct if the question is asked in sci.electronics.design where it is on-topic. While it is true that some EEs are also interested in embedded systems programming, many are not and thus don't read comp.arch.embedded.

Hello Robert, Hello Frida,

Amen. Meaning I agree and thus I guess I'll fall into the category "of course". In my 20 years on the beat I have never seen a split ground system work well. A few of them kind of worked but then they didn't pass EMI until we performed a common ground relayout.

One way to think about it is that a typical system has more than one connection to the world. So while there may be just that single ground connection under the ADC there are signals coming in from the analog side to be converted and then other signals going to displays, keyboards etc. on the digital side. All these will also carry ground leads. This causes lots of not well controlled loops with each of them having to be broken by ferrites or other means, something that rarely works well. This is just one of the reasons why a split ground architecture can lead to sleepless nights.

On my designs I always start out with a nice full ground plane and make sure that any currents on that plane won't cause more transients at the converter than can be tolerated. It has never been a problem, as long as the placement of components is suitable.

Cordialement & med bästa hälsningar, Joerg

It's there - between the lines: if the only ground connected to the chip is AGND, the by-pass capacitors have to be connected to it.

A different story is that the incoming power supply should be decoupled by a choke to avoid polluting the analog ground with the crud on the supply line via the caps.

"Joerg" wrote

Depends on what is meant by 'split ground'.

I regularly use a star ground that is then grounded to the 'system' ground at the center of the star. This creates a 'ground plane', though a pretty cruddy one as the lines going to the star are not gridded. Often more than one star.

And I have had boards that were split horizontally - left half of the board analog and right half digital. 5v and Dground layers for the digital side and +/- V and Aground layers on the analog side. One connection between the two grounds made at the power-supply reference point.

For process control instrumentation it is not uncommon to have one ground per process input/output. The back panel of process control cabinets often have a drilled copper bar for _the_ signal ground (used to be labeled 'Mecca Ground'). Often sensors are grounded at the source.

I haven't had any problems with the exception of clients who insist on 2-layer boards for high speed designs. The problem has been EMI susceptibility.

YMWV.

Hi Nicholas,

EMI is usually the first problem I find with approaches that didn't use a single gound plane. It was more prevalent than performance issues (although there were tons of these, too). A split or otherwise separated plane makes for a nice big dipole antenna that radiates and receives. Any RF currents flowing across the node where they are connected are at the same time imposed onto that unwanted dipole antenna. Inevitably any system will have some physical connections to the rest to the world. Then these dipoles become huge loop antennas and it all gets even worse. Just my experience, and I have seen it over and over again.

Until now I or rather my clients haven't gotten any mileage out of separate ground approaches, except the miles for the repeat trip to the EMC lab ... ;-)

Then again, many EMC labs are located in rather pristine areas of the country so everything has an upside.

Regards, Joerg

"Joerg" a écrit dans le message de news: dleKd.7658$ snipped-for-privacy@newssvr14.news.prodigy.com...

Fully in agreement with Joerg. In fact in you use a "split" ground plane with a unique connection under an ADC chip, then in order to avoid any EMI-receiving/emitting coil you have only one solution : All signal lines going from one side to the other must be routed EXACTLY ABOVE the connexion point between the two grounds ! If not you have a marvelous current loop...

Cheers, Robert

Hello Robert,

Yes, and when the product is now behind schedule the only way to fix these loops is with the 'Aspirin method'. Lots of #43 material toroids and other cores. The worst case I ever saw required about two pounds (!) of ferrite to make it perform and pass EMC. Later after a redesign with a common ground structure that dropped to zero pounds, with even better EMC margins. The only sad person was the sales guy of the ferrite supplier.

Regards, Joerg

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techonline has a good analog lecture on A/D grounding you may want to play

The VCC supply is for the digital circuitry of the ADC chip. It will therefore be noisy, so the decoupling capacitor should be connected to digital groud.

The suggestion of having only one xGND plane will obviously work, and is a good solutions for many applicaitons if you have a very solid ground plane and are not critically concerened with ADC accuracy. However, if you need to measure very small signals or get ultimate accuracy, then you need to have separate AGND and GND planes that are connected at only at a single point.

regards, Johnny.

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I think the folks here (as well as the a/d manufacturers) are suggesting a single ground plane is the best solution (lowest noise) not a comprimise solution for the majority of cases when using commerical A/D's. I think the reason is that an A/D is already an integrated mixed signal device and you have to deal with the complications and limitations of the grounding system thats brings. Connecting its GND to a seperate GND plane(assumed noiser) can internally couple noise into the analog portion of the a/d via stray capacitance. For the rest of digital circuitry on the board the idea is to physically seperate them from the analog devices on the board so that the returns of the noisy digital chips don't have to cross over analog returns to get back to the power supply return.

Obviously there are exceptions to this rule especially for strange shaped PCB's or motherboard/daughterboard designs, or discete optically isolated A/D's but for large single pcb systems using integrated A/D's or microcontrollers, its a pretty reliable option.