Joining analog and digital ground

Just MHO:

Generally, splitting grounds that way is not a good idea. If your nickname "Hammy" indicates that you operate ham radio gear then it's even less of a good idea because it can spew EMI and also get upset by high field strengths.

Since you have such easy access because of only two layers, why not at least provide plenty of dummy bridges that you can later solder shut or use 0603 size zero-ohm resistors? Then you can simply try it out without risk.

I still remember the faces at a client when I soldered a split shut (all the way, after rattling their nerves with a Dremel) under a bunch of Analog Devices ADCs despite staunch reminders in its datasheet to split. Priceless. Tons of noise they had fought for weeks vanished into thin air.

On a sunny day (Wed, 21 Oct 2009 15:28:48 -0400) it happened Hammy wrote in :

No, because then you have different voltage drops between those at the IC point, so introduce noise between analog and digital ground on the AD9834.

I dont even own a CB.

Joerg you never watched Hammy Hamster when you where a kid :).

Thanks for the advice thats what I'll do.

Thats two votes for joinning on the DDS so I'm going to join them on the DDS board.

Thanks

Having one ground simplifies life.

John

Heres a picture of me going on a sunday drive with the little lady.

If you are not sure of this, first have a look up the manufacturers data sheet and app notes for the recommended layout. If there isn't one, email or call application engineering. They can be very helpfull.

We did some work using some ti high speed serdes (internal pll clocks) links a year or so ago and ended up using power islands around the device, with separate decoupling cap trees (1nF, 10nF and 100nF) on both analogue and digital Vcc and Gnd. We haven't had time to find out if that was overkill by removing some of the parts, but the product passed emc tests just fine...

Regards,

Chris

Nope, didn't. I grew up in Europe. But I did watch Archie Bunker :-)

Ah yes, in a souped-up VW no less :-)

But wait until the electoral complaints commission has thoroughly validated all the ballots ...

I've read everything I could find including there evaluation board. The layout image is poor quality.

It does look as though they are leaving the grounds separate on the board. There is one component (fig 8) across the isolation between the grounds maybe R4 or C15 looking at the component side? I cant make out any other connection between the grounds.

The board is on page 10 of the pdf. If you can see a connection between the grounds you have better eyes then me.

I'm going to do as Joerg suggested leave it unconnected and then use jumpers to see what's better.

Figure 5 shows a short but the layout isn't readable. Sometimes they'll let you have the Gerbers, much better.

But you'd have to provide one position every 1/2" or so. Else connecting at just 2-3 available spots can create loops and make it worse, not reslting in the performance a full common ground plane would afford.

On a sunny day (Wed, 21 Oct 2009 13:14:54 -0700) it happened Joerg wrote in :

LOL, he in not in Afganistan :-)

Florida? ;-)

Cheers! Rich

Every board I have designed at work that has A/D or D/A converters (or both) ALWAYS gets the analog ground and digital ground connected underneath (or very close) to those ICs. Other than making sure there is a common potential point close to the mixed signal ICs, I also arrange the ground planes such that all digital return currents don't get anywhere near the analog areas of the board, even if I have to segregate ground planes with 10 mil cut- outs (in the plane copper) that serpentine over the board.

Tom P.

I've never seen the split ground work well either.

(not directed at you specifically)

AFAIK the digital ground pin of an ADC does not go to digital ground - it goes to analog ground!

It is there to conduct the "digital" currents *within the chip package*, otherwise these would flow down the same bond wire as the chip analog ground and disturb ADC operation.

See e.g.

I don't think you want digital ground at all at the ADC itself (but it can be a good idea to locally buffer the ADC outputs with a buffer referenced to DGND).

My ADC's are on Analog ground if that was directed to me.

Fig.4 seems to be what they are doing on the evaluation board. Separate ground planes joined only at the power supply.They use shottkys to keep potential differences between the planes to below

It's a tad confusing in the next example "GROUNDING AND DECOUPLING MIXED-SIGNAL ICs WITH LOW DIGITAL CURRENTS". They say that the IC DGND should be joined short trace to the AGND. This to me means one ground plane? Then they show a buffer referenced to DGND?

This is a quote from the AD9834 data sheet.

"If the AD9834 is in a system where multiple devices require AGND to DGND connections, the connection should be made at one point only, establishing a star ground point as close as possible to the AD9834."

For the time being I dont have multiple devices but I will.

Oh well looks like the old trial and error method applies. I was hopeing to get this right the first time.

Yes, I would say so, around the ADC anyway.

" the AGND and DGND pins should be joined together externally to the analog ground plane with minimum lead lengths."

That is what I was saying below, it is for the case where the ADC output has to drive long (high capacitance) traces or wires. In this case there is a current pulse each time that capacitance charges, so a buffer helps isolate the ADC and referencing the buffer to DGND returns the current there.

Never used a DDS but I can't see why they would be that different....