PCB Layout (for ADC and DAC)

ADCs and DACs always have the issue of the digital circuit messing with the analog workings. Internally, the chip has a path from the digital logic ground to the analog ground. You don't want any current to flow in this path. To prevent this, the digital ground must have a very low impedance path. You want the digital ground plane to provide all of the return currents for the digital lines so it needs to be under them.

Making a layer for a ground makes it work as a continuous sheet of low impedance ground. When you combine it with a signal layer, you have to plow traces through it. This increases the impedance.

And ...

At RF frequencies slots and gaps in planes look like little tuned circuits and antennas. If you circuit can drive an RF current into the structure, it will radiate.

Use your single sided PCB with the traces making the top side. Scatter largish holes (0.05 or more) for the ground connection. Take an unprocessed sheet of copper clad and place it under your PCB and match drill smaller holes (0.035) in it for the ground.

Solder long lengths of wire into the holes of the copper clad. Don't use too much solder on the top surface. Bend the bottom side of the wires over and not the top.

Feed the wires through the holes in the PCB and slide the PCB along the wires down onto the copperclad. Bend over the wires and solder and snip.

Beware of increasing the impedance of the digital ground paths.

Filter the heck out of your supplies. An RF bead, inductor or resistor and a couple of 47u capacitors can make quite a difference.

Consider this bit of ASCII art:

---+------[ZT]-------+-------[ZT]--------+-----[ZT]-------+--------- ! ! ! ! --/\\/--+---A --/\\/--+---B --/\\/--+---C ! ! ! ! --- --- --- --- --- --- C1 --- ---C3 ! ! ! ! GND GND GND GND

Notice how if the reistors were zero ohms, some of an AC current applied to B would flow through C1 and C3 into the local grounds at those locations. It doesn't take much resistance to reduce this.

Splitting planes is almost always a bad idea.

If you are restricted to etching single-sided boards, consider using double-side copperclad and leaving the back side unetched, a solid copper ground plane. Drill topside ground pads or, better, islands, and solder via wires through to the ground plane. This will give you far better signal integrity than a 1-sided, split-ground layout is likely able to do.

The big performance cut in pc boards is between boards with true ground planes, and boards without.

John

What I like to do is use double sided PCB material, but dedicate one side as a ground plane. This side does not need to be etched, so it is just like making a single-sided board from the processing point of view. All ground signals go to a via (and all vias are to ground). Then, for these "ground" vias just drill them and use tinned copper wire (or resistor leads etc) to make the connection. This works very well for surface mount boards, and is much easier than a full double-sided layout (no alignment issues). Since there is no ground routing on the "tracking" side (the component side) a single sided layout is not too difficult. You could use the same artwork for production boards too. (Actually I now only make my own boards for quick prototypes of tricky bits of the circuit).

I haven't tried this technique, but it sounds very useful. For through hole boards, I would use one of those spur or brad point bits (that cut a circle around a central point, made for drilling wood) to clear the ground plane from a little circle around the lead hole where components are passed through the plane to connect to traces on the other side. Do you think that is practical?

The biggest problem might be finding bits smaller than 1/8".

I'd probably have to grind my own low rake versions to cut copper without diving through the board.

I found a 2mm version, but look at the price!

On Jul 22, 12:29 pm, John Popelish wrote: > John Popelish wrote: > > John Devereux wrote: >

Not to pick on anybody, particularly John Popelish, but but when I think of folks spending several hundred bucks on computers and cell phones and $100+/month on cell service, internet and cable TV, I'm always a bit taken aback when there is a comment on a 1 time expense like a carbide drill bit being expensive.

As for the technique, I've seen low quantity broadcast gear with one side ground planes manually drilled for clearance.

GG

price!

That link is broken, can't see the price. I got a few 1.2mm and 1mm steel bits if you want them.

Carbide bits are easy to break, especially for these sizes.

$10 for a carbide bit might not be so bad, but that one:

Yes, in fact that is precisely what I used to do. You can buy special bits from Farnell IIRC. (It has been so long since I made a through hole board that I had genuinely forgotten all about this)!

(replying to own post)

Thinking about this, it might be better to have the components on the groundplane side, and the tracks on the "solder" side. Then you use a normal ~3mm bit to countersink the area around non-ground pins. The ground pins can then be soldered on the component/ground plane side. Can be tricky when the pins are under the device.

It's much easier with surface mount.

Thanks for all the info guys.

I'm curious as to just how bad not having a ground plane is. Surely the circuit will still function properly? How much noise will be introduced? I'm willing to sacrifice quality for ease to make at this point. (since if the only issue with the layout is going to be noise once I get everything working through a prototype I can add a ground plane to the layout and then send off and get it fabricated)

I think here there won't be any issues masking noise from other sources but that might be a problem. (I think at this point I'm just trying to get the system to work as a whole)

I assume that the issue is with the analog domain or is there a reverse issue with digital? i.e., can the digital be affected by the analog domain also when not using these special precautions such as a ground plane? (so that the circuit could actually not function properly rather than just having noisy conversions)

I think that I will end up doing the 2 sided boards though. I'm sure its much easier to route/layout ;) Since there are vias that I can buy that look like a snap to use and it will just involve a drilling a few holes.

(mainly just curious about the consequences of not using a ground plane. I know its not as good but I don't think any mentions just how bad. (remember its for prototyping so I can sacrifice some things))

Thanks, Jon

Oh, not sure if I mentioned this but its for audio. I'm not doing really high speed stuff and my sample rate will actually be 96khz or max 192khz but definitely nothing past 100Mhz.

I suppose though that it is actually in the mhz range since these are oversampling so I guess I do have to look at it as high speed?

The important number is the gain * bandwidth value. Even with only audio signals, having a gain of a million and no ground plane is asking for it.

It depends.

BTW if you open a consumer electronics device, you will see single layer PCBs. Works fine for them.

It depends on the quality of the layout and on what exactly are you trying to accomplish.

I'm

Good plan.

Don't worry upfront. As the matter of fact, quite complicated systems can be built using just two layer PCBs without a solid ground plane. Of course, the layout of the power and ground requires careful attention.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

No, this question comes up often, follow the manufactures advice, there is "noisy" digital and "quiet" digital, this is quiet digital (noisy digital would be a DSP or high speed parallel buffers), see link for an explanation of the ADC/DAC digital vs analog ground pin issue

Sure, for a cheap radio, or other low density designs where they can get away with a handful of jumpers. When was the last time you saw a single layer computer motherboard? Most of the computer monitors I've had apart have double sided PC boards. The highest layer count I've worked on in industrial designs were 16 layer. They were in a DSP based diversity telemetry receiver system.

Following some links I see

and figure 8 shows exactly what I was talking about with straddling!