PCB layout for ADC

Use lots of ground pins.

Ground plane for sure. Power runs can be fat traces or planes. If planes, you might put analog planes on the opposite side of the ground plane from digital power.

One inner-layer ground plane should be enough. Surface fill-in pours are mostly cosmetic.

Better to use surfmount ceramics, close to the adc power pins, 0.33 uF maybe. The ADC ref bypass cap should be local, too.

Since this ADC has a single-ended input, ground noise is going to be an issue. If the board is quiet before each ADC shot, that will help a lot.

HoJo's book is half good stuff and half nonsense.

John

I agree. What do you think of Tom Van Doren's material though?

Haven't seen his stuff. Got any references?

John

Hello John,

16 bit converters of this caliber should be treated like "high speed designs".

Hmm, I tend to recommend it to clients who ran into termination issues and things like that. What didn't you like about the book?

--
Regards, Joerg

http://www.analogconsultants.com

Make cluster of gnd/Vcc pins adjecent to eachother?

Maybe this doesn't apply. But inductor between digital and analog power supply Vcc seems to help (when they draw power from same "outlet").

Small decoupling capacitors near consumer(s) proberly doesn't hurt.

Maybe shielding could help?

Linear Technology have an interesting application note AN71 entitled "The care and feeding of high performance ADCs: Get all the bits you paid for" which addresses the issues you mentioned. Google for: linear technology an71

Well, there's an average of about one factual error or insanely fatheaded remark per page, starting on page 1, where we learn that few electrical parameters remain constant over a span of 20 decades in frequency, provided that we use a log scale.

Did you know that the risetime of a type N connector is 4 picoseconds, and a foot of RG-8 is 200 fs?

His ideas about bypassing, and his obsession with return currents, are just silly. Can anybody make sense of page 273? Page 261 is insane, "An interesting consequence..." and "At the point...". And the 5 rules starting on that page are plainly crazy... nobody does this; well, #4 isn't so bad, assuming multiple ground planes, which is a whole nother debate.

P279, we learn the startling fact that "Combining TTL and ECL in one system without considering the system design consequences is not a good idea." Oh, please. Bullet 4 below is crap.

Really, there are too many to list. But if you give me a random page number, we can find a few nearby absurdities.

John

Just make sure the analog and digital ground currents don't get mixed (this is not trivial in many cases) and the analog and digital suppy are well seperated (using independant supplies or LC filters). Using pours is usefull for spreading heat and/or creating a (tiny) capacitor together with the ground plane.

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That's a good pointer and a nice little appnote - short and to the point. I had a question about their recommendation that you can benefit from using both an analog and a digital ground plane on two separate layers.

Wouldn't the capacitive coupling stuff things up?

Also re someone's suggestion of using an inductor to couple the two planes. Is this a good idea in the layout LT show, and what inductance might you be looking for?

Hello John,

Well, at least our mains supply remained nearly constant after the last governor was kicked out :-)

Maybe he assumes that everybody has a defense-style budget and everything is rigid coax.

"Liz has designed...". Yeah, I always wondered who Liz is. This way of calculating the bypass cap budget is indeed a bit odd. I just do that with charge calcs.

Rule #4 would also apply to a common ground. I have seen many boards where the number of ground vias was so paltry that I had to drill and jumper to demonstrate to a client how to make it work (initially they didn't believe it).

Agree. Splitting power planes can at times be as bad an idea as splitting grounds.

I guess you are right. But we can't judge the book from the position of a seasoned engineer. To us those kinds of books often appear to talk the obvious. Until the next consulting trip comes, you look at someone's design and realize that it ain't quite so.

Most readers, at least the ones to whom I recommended the book, don't have our experience and tend to make rather blatant mistakes. Split planes, bypass caps out at sea, non-terminated clock lines and so on. For them it should still be a good read unless there is a better book. I haven't come across one. But then again the EE section of our local Border's store has now shrunk to less than 3ft of shelf length. Sigh...

--
Regards, Joerg

http://www.analogconsultants.com

With some intelligent editing, it could have been very good.

Yeah. Our nearby Borders has a huge, room-size "computer" section that has not a single book on PC hardware. You can buy the (something like)

6-volume, 10k page shrink-wrapped ".NET Foundation" set for $450.

John

Hello John,

Agree, the writing style needs some polishing.

I noticed that .net heaviness. Sad. I think they are pricing themselves out of some markets. Win's AoE book was a whopping $90 there. Ouch. I had purchased the 2nd edition just when it had come out at Tower's for $49.95. You can still buy it around that so I don't know what their marketeers are thinking.

--
Regards, Joerg

http://www.analogconsultants.com

Maybe talked so much the wires were energized ;)

What happends in the splitted powerplane setup ..?

Less books => Less competition => More business ? :)

Software is nice, but if it can't interface to reality it's hardly going to make a real difference..

Pretty sure that the Feds arresting Kenny Boy & his band of thieves had a bigger impact on the situation.

The same stuff that happens with split ground planes, for example that your assessment of what behaves digital and what behaves analog isn't corroborated by the actual circuit. AD converters can be particularly nasty here. If you at least have one common ground plane and bypass well things might still be ok though.

True, currently I have no idea who is going to fix serious noise issues

50 years from now.

That's what all the hotshot dot-com "web portal designers" realized when their paychecks stopped coming.

--
Regards, Joerg

http://www.analogconsultants.com

Hello Jeff,

It had an impact but the main mistake in those days was that utilities were prevented from signing long term contracts. That was IMHO rather stupid. Every properly run enterprise hedges against all kinds of fluctuations. Energy, exchange rates, raw materials, you name it. Yet politicians appear to have taken it upon themselves to think that wasn't necessary.

Luckily this is now behind us.

--
Regards, Joerg

http://www.analogconsultants.com

Your point is valid, but you're talking about making rules for folks who didn't feel bound by rules. Pretty sure the rolling blackouts were completely artificial. http://66.102.9.104/search?q=cache:ZV9hu2nZK5IJ:

*-*-*-smartest-*-in-the-room+artificial-shortages

1 pin for each would be the way to go, but if you are designing a plug-in daughter board then you have to be careful where that "ground" pin goes on the main board. If you have mixed digital and analog grounds then you have to be extra careful and keep them seperate. Without full details of your system it's hard to say for sure.

Forget planes for the rails unles you have clearly delineated analog and digital sections. Use one plain for GND as you already have, run the power rails as thick traces on the other inner layer, and then copper pour around the power rails on that inner layer also. Remember to stitch the grounds together too.

Generally a good idea, although it may or may not actually do anything for you, but hey it looks good though ;-) If you need seperate analog and digital grounds then it's probably best left off, unless you know the specifics and you want to copper pour around your analog section only for example.

Don't worry about it. High frequency bypass caps should be sufficient.

The key to A/D board layouts is keeping your analog and digital grounds and sections as physically seperate as possible. Really high end 24bit sigma-delta converters for example will have one side of the chip package with all the analog pins and the other side with all the digital pins to aid in such layouts. Dave :)

You should read carefully all the reference designs provided by the ADC manufacturer. I guess your company has signed a NDA with that company so, you should have all informations. There isn't a single rule for all ADC about separating analogic and digital ground, even this is a common issue. Some ADC manufacturers of fast ADCs, requires a clock which usually is treated as analogic clock. (see fast ADC from Maxim). This clock is the biggest perturbator on the board.

Ground plane or fragmented ground plane on the routing layer is usually must. Multiple vias will connect the ground planes on all layers.

Worry as well. 10uF + 100nF +10nF +1nF is one example where the capacitors datasheet must be readed carefully. Choose a combination with an equivalent lowest ESR on the whole frequency range, covering at last the third harmonic of the biggest perturbator (ie sampling clock).

Is not clear even in practice. One trick is to design 0 resistor between various ground planes in various positions on the board and just check the effect. Noise simulation is usually useless.

greetings, Vasile Surducan