Here's a copy of a message of mine from a previous thread that I do not want to hijack or interfere with. So I'm starting this new one:

The circuit in page 9 in the attached datasheet in particular was a headache to understand a few years ago when I tried to replicate it, trying to build my own high quality ADC. And truth being told, I do not fully understand it yet. I mean, what's the purpose of the 10 ohm resistors (R2, R8 and R9) connecting the analog and digital ground planes on the board? Wouldn't it be best to simply connect the analog and digital grounds to a single point as close as possible to the power supply, as is customary? And why the three power planes under the chip? Is it really that extremely susceptible to noise?

On the other hand, it wasn't until much later that I understood that soldering temp is extremely important when dealing with ADC chips, and that most of my woes stemmed from my unknowingly damaging them during that process, and that those resistors were just a nuisance compared to that.

But most high quality ADC's come in SMT packages. That one in particular has a DIP equivalent, but I couldn't find a single one of them for sale on any of the major available suppliers... It was exactly that experience that taught me that datasheets should be very carefully and thoroughly read before taking any sort of action.

As you already know, I can very well defend myself when dealing with MCUs and digital circuits... but I'm a half-baked amateur when it comes to analog.

https://forum.allaboutcircuits.com/attachments/hi7190-evaluation-board-an9505-pdf.139811/

 

The circuit in page 9 in the attached datasheet in particular was a headache to understand a few years ago when I tried to replicate it, trying to build my own high quality ADC. And truth being told, I do not fully understand it yet. I mean, what's the purpose of the 10 ohm resistors (R2, R8 and R9) connecting the analog and digital ground planes on the board?

Beats heck out of me. I could see them connecting AGND and DGND on the board with a jumper that the user could remove/snip if he preferred to connect the two grounds off-board, back at the power supply; but three 10Ω resistors in parallel? I don't get it.

Over the years I've noticed that evaluation board designers sometimes do some seemingly boneheaded things; all I can think of is that they're trying to design the board to do everything for everybody. I can't find anything in the HI7190 data sheet that would mandate this odd "three parallel resistors" gimmick.

Wouldn't it be best to simply connect the analog and digital grounds to a single point as close as possible to the power supply, as is customary?

Yup. And the app note does say, "The power supplies are provided to the board via the edge connector located at the top of the board. It is recommended that twisted pair wires be used to connect the power supplies to the connector and that analog and digital grounds be tied together back at the power supplies" which makes perfect sense.

And why the three power planes under the chip? Is it really that extremely susceptible to noise?

Yes, Δ-Σ converters usually are quite susceptible to noise, and the mfgrs. usually suggest putting a ground plane (or ground and analog power planes) directly under the chip. I haven't often seen them put a DVDD plane under the chip though, because digital power is often rather noisy.

 

I'm dealing with some ADC issues now. I was handed a project with a 16-bit ADC that is giving unstable readings and I need to sort it out. Variances of .01v make a very noticeable difference in the output, and the board as-is has a couple hundred mV of high frequency (~500kHz) noise. This one is going to be a challenge.

 

I'm dealing with some ADC issues now. I was handed a project with a 16-bit ADC that is giving unstable readings and I need to sort it out. Variances of .01v make a very noticeable difference in the output, and the board as-is has a couple hundred mV of high frequency (~500kHz) noise. This one is going to be a challenge.

Funny you should have this sort of problem with a 16-bit ADC. I've been using the AD7680 for years, and I've never had a problem with it. Of course, it all depends on the circuitry surrounding your project too.

 

I mean, what's the purpose of the 10 ohm resistors (R2, R8 and R9) connecting the analog and digital ground planes on the board? Wouldn't it be best to simply connect the analog and digital grounds to a single point as close as possible to the power supply, as is customary?

I have seen ferrite beads connecting analog and digital grounds together. The idea is to have a DC ground connection but keep the digital ground noise coupling to the analog ground. The ferrite beads also slow the rise times of the digital signals going into the A/D, I think.

I suspect that the 10 ohm resistors are a version of the ferrite bead scheme.

 

Funny, I never seem to have trouble with ground noise in high-res A/D circuits. But I've been doing it a long time and I always do it the same way.

My secret is that I choose a node on the board that I consider a "reference" analog ground. All analog signals (and shields and planes) are referred to that junction -- and I ensure the absence of any ground loops or high currents (especially A/C currents) to/from that node.

That's pretty much it. And it always works.

 

Oh, and I read this book once about 25 years ago. Hadn't had noise problems since.

 

Oh, and I read this book once about 25 years ago. Hadn't had noise problems since.

View attachment 139866

Thanks for the tip! ... just bought used at Amazon for $18.00 dlls, delivery included ... many thanks!

 

Oh, and I read this book once about 25 years ago. Hadn't had noise problems since.

It was not uncommon for analog engineers I worked with to ask: "What does Ott say?"

 

I'm dealing with some ADC issues now. I was handed a project with a 16-bit ADC that is giving unstable readings and I need to sort it out. Variances of .01v make a very noticeable difference in the output, and the board as-is has a couple hundred mV of high frequency (~500kHz) noise.

You could have an analog-digital ground separation problem.

I had a board with a similar problem and it turned out the board designer had connected the ADC analog and digital ground pins to the wrong ground planes.
Correcting that solved the problem.
Those two planes were tied together at one spot on the board near the converter. Don't remember if there was a ferrite bead or not in that connection but it's a good idea since it attenuates high frequency ground currents between the two planes but keeps the DC levels the same.

Is that 500kHz, digital clock noise?

What do you mean, variances of .01V make a noticeable difference in the output?
Aren't they supposed to?

 

Oh, and I read this book once about 25 years ago. Hadn't had noise problems since.

Allright, bought it on Amazon, so I hope it is as good as you describe

 

Analog Devices also has some good application notes on grounding, shielding, guarding, decoupling and so forth:

AN-202 An IC Amplifier User’s Guide to Decoupling, Grounding, and Making Things Go Right for a Change
AN-214 Ground Rules for High-Speed Circuits
AN-342 Analog Signal-Handling for High Speed and Accuracy
AN-345 Grounding for Low- and High-Frequency Circuits
AN-347 Shielding and Guarding
MT-031 Grounding Data Converters and Solving the Mystery of "AGND" and "DGND"
MT-101 Decoupling Techniques

All good stuff...

 

Hello,

@OBW0549 , I copied your post to the following thread for reference, as it contains very good stuff:
Decoupling or Bypass Capacitors, Why?

Bertus