shorting of analog and digital grounds

Con't forget capacitor, and that the inductor might be implemented as a ferrite bead.

Lay out pads that can accept at least one type of any of these. Label it Z1.

thanks for ur respose can u elaborate ur ans.i am unable to come to conclusion

To put it simply: Lay out pads for a 1206 part.

Judging where to put the part or parts (electrically and physically) is another issue.

Worry more about 'where' you are going to join the grounds, than 'how'. The diode would only apply, if there was a possibility of a potential difference. If so, how would the rest of the connections cope?. Inductor, or resistor, would be 'normal' methods. However you could use either method, and end up with problems. If (for instance), you have analog signals 'crossing' to an ADC, and you joined the grounds to a point in the digital curcuit, where there are high currents flowing (perhaps near to a switch mode regulator), you would never get good results. So start instead, by asking yourself, what signals need to cross between the two areas?. What filtering ineeded on these signals?. Which components they go to?. What other potential ground routes exist (ground loops must always be considered)?. How is the supply got to the analog part?.

Best Wishes

It can be any value resistor really in theory. If well designed there should be no net current flowing there. In practice I might use 100 ohms or even simply track the two together at a suitable Kelivin connection.

Graham

Use a single, common ground plane and you won't have this dilemma.

Splitting grounds rarely makes sense.

John

Unless dealing with high-quality video, or high-quality audio, or noisy analog loads, or noisy digital loads.

Hello Richard,

I'll second John's comment. He works near a hill that spews lots of EMI from antenna towers. My dose of EMI reality comes from huge freight planes calling in their approach right above the lab.

Even then. I have never seen split grounds work well and this is after about 20 years in medical ultrasound. Those machines always have to see and hear the grass growing just a tad better then the competition. Plus they had to perform in the vicinity of strong RF emitters which can become next to impossible with split ground planes.

Regards, Joerg

It certainly does in audio.

Graham

It does allow you to avoid thinking about a few things. But magnetic field pickup and emi issues are still there in full force. And al least some ground loops will persist. So you may as well save a plane and do it right.

Besides, audio isn't "electronics" as far as I can tell.

John

Having digtial ground currents flowing in the audio ground is a non-starter. It's audible ! No matter how good your ground plane it, it still has resistance.

Is that some kind of elitist snobbery ?

Graham

John is still miffed about being called on his defiance of the Laws of Thermodynamics ;-)

...Jim Thompson

No, it's an opinion that the things that apparently matter in audio are subjective and non-quantifiable.

John

No, I'm a great fan of the Three Laws. I just can't get anybody else to respect them. I've spent enough years in steam engine rooms to appreciate how easy it is to accumulate entropy.

We'll just have to wait until gasoline is $12 a gallon and electricity is 40 cents a kwh, then thermo will get a little more respect.

Do you really believe that a resistive electric water heater is 100% energy efficient?

John

The transfer to air is the hard part. A small percentage goes out as some useless wavelength stuff... so maybe 98% goes to heat.

You really should do an exact calculation for the heat-pump case. Figuring in compressor inefficiency, and TWO transfers to air, ought to make it a real wiener.

Nice in theory, but crap in practice.

Want to analyse something useful... analyse an evaporative cooler... water changing states is MARVELOUS ;-)

...Jim Thompson

I don't subscribe to 'audiophoolery'.

I can however clearly hear 'digital birdies' when digital ground currents get into the analogue path.

For those only operating with ~ 12 bits of resolution it may not matter much however.

Graham

I do NMR gradient amps with 20 bit DACs, a couple ppm wideband noise, and switching regs on the same ground plane as the low-level stuff. All you have to do is keep the power loops fairly local and don't span low-level loops all over the place. Splitting planes just gives you two planes at different potentials, and, unless the circuit is very simple, too many parts have to span both planes. Splitting causes about as many problems as it cures.

I'll occasionally nestle a low-level circuit in the middle of a c-shaped cutout of the ground plane, so plane currents don't make microvolt drops within the sensitive area. You can get similar results by creating a square of ground surrounded by a break in the plane, with just some small connecting tabs left; the resistivity of the tabs is enough to greatly reduce the current through the sensitive region, but the thing will still be emi tight.

But external magnetic fields will still create potentials across a plane, so it's best to keep loops small and handle delicate stuff against one ground via, or differentially. Fans are killers, both from mag fields and (for dc fans) nasty pulse currents.

John

You'll note I'm sure from my comments that having 2 planes at different potentials is *exactly* what I want to avoid. The whole point of the 'zerohm link' or even low-R between AGND and DGND is to ensure both planes are equipotential ( where it counts ). Clealry they can't be equipotential everywhere due to copper resistance.

'Power loops' can't be that simply defined to be honest.

That sounds not entirely unlike what I have done.

I *always* keep loops small ! I recall some colleagues thinking I had 'done magic' when re-tracking a pcb to avoid 50Hz pickup. They simply didn't 'get it'.

Graham

You'll note I'm sure from my comments that having 2 planes at different potentials is *exactly* what I want to avoid. The whole point of the 'zerohm link' or even low-R between AGND and DGND is to ensure both planes are equipotential ( where it counts ). Clealry they can't be equipotential everywhere due to copper resistance.

'Power loops' can't be that simply defined to be honest.

That sounds not entirely unlike what I have done.

I *always* keep loops small ! I recall some colleagues thinking I had 'done magic' when re-tracking a pcb to avoid 50Hz pickup. They simply didn't 'get it'.

Graham