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DAC PCB design guidelines help!
DAC PCB design guidelines help!
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Old 24th March 2021, 08:30 AM   #1
manueljenkin is offline manueljenkin  India
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Join Date: Feb 2020
Default DAC PCB design guidelines help!

I would like to know if there are useful guides available on PCB design for DACs (probably something oriented towards medical grade devices)? How would I go about getting libraries for estimated stability and other parameters if I were to work using free tools like KiCAD, LTspice, qucs and gnu octave.

I have had formal education in certain branches of electronics if that helps and have also taken considerable time to learn designs online (few nice places like allaboutcircuits.com, and some earlier op-amp schematics and design choices). I am open minded about design choices, don't quite subscribe to sine sweep objectivism.

I started going through amb gamma 3, which is a diy dac. I am able to comprehend the design (data flow path, and components like clock circuits, switches etc) and I could also understand the analog circuit in their beta 22 amp but I couldnít get to understand the PCB part or how the design process is carried out for the pcb and choices in the design. I am currently looking into PCB design for raspberry Pi compute (there are resources on the same) and have been able to learn a little with respect to digital pathways, but for random analog signal like music in not able to gather useful information.

I havenít done anything like this before but I have tried spice tools with basic circuit simulations and fairly comfortable with MATLAB (have implemented few low pass filters in it) and certain stuff about design rule checks. I am trying to get used to KiCAD and am fairly decent at scripting.

I have fair (beginner level) understanding of available components for clocks, buffers, flip flops, switches etc (got by looking at teardowns of many amplifier designs and then exploring their design choices) but would again love to see more known good choices for each. Few parts that have currently got my attention are - si86xx series of isolators, el2001 buffer amplifier, quite a few opamps (analog devices ad8397, ti alm2402F), some transistors from vishay, sowter/Jensen transformers (if I were to make the output transformer coupled). Haven't thought a lot about attenuator for the dac yet since it is likely I'll let either xxhighend or the amplifier following it do that task. Will need to choose the right components as I explore making the design.

I want to try and build one based on ti 11001a (itís a new unit) or any other r2r chip dac like ad1865. I believe it would be a little more complicated than delta Sigma due to the transition nature of r2r which could cause additional instability and ringing, but thatís the challenge I want to face and learn. And it would be NOS, with interpolation being handled by pc source (likely xxhighend+arc prediction) so atleast one complexity being taken away. Digital input interface would be custom as well (haven't confirmed but likely usb since I need to use xxhighend).

Of the existing diy dac designs, dddac1794 is something that has got me interested a lot (except the waveio board), because of its high bandwidth analog section. I also found some chips from audio gd for diy builds but couldn't find any schematics on the same.

I don't expect to get a masterpiece in first shot, rather want to really grow myself as an analog designer over time, and this is something I treat as a learning process.

Last edited by manueljenkin; 24th March 2021 at 08:39 AM.
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Old 24th March 2021, 10:16 AM   #2
MarcelvdG is offline MarcelvdG  Netherlands
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Join Date: Mar 2003
Location: Haarlem, the Netherlands
I have no experience with the DAC chips you mention, but here are some very generic remarks about DAC PCB design:

A DAC multiplies its digital input signal with its voltage or current reference and the output signal gets phase modulated by jitter of the clock. When data-dependent signals end up on the reference or clock, the multiplication with the digital signal causes distortion products to be produced. Any other interference ending up on the reference or clock causes intermodulation products to be produced, except when the interference is at an exact multiple of the sample rate.

All in all, you have to ensure that the reference and the clock remain clean. That is, try to minimize crosstalk from the digital part to the reference and clock. As the digital signals have harmonics far into the megahertz range, you have to apply RF techniques like ground planes and minimized loop areas.

When the current that the DAC draws from the voltage reference is data-dependent, which it usually is to some extent, reference decoupling capacitors have to be placed right next to the DAC to minimize data-dependent voltage variations due to trace inductance.

DAC chip manufacturers usually write application notes that include PCB design recommendations, so it would be a good idea to read those.

Last edited by MarcelvdG; 24th March 2021 at 10:18 AM.
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Old 24th March 2021, 10:55 AM   #3
manueljenkin is offline manueljenkin  India
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Thank you very much. I will expore further on these topics. I donot have much rf background (other than basic Maxwell's laws and a beginner level knowledge in electromagnetic waves), any particular books/blogs you'd recommend? I came across a few articles and videos on ground plane design earlier but since I'm not familiar with it yet, I only studied the very basics (how the return current causes voltage fluctuations and something about type of metal wires for some kind of rf suppression).

Last edited by manueljenkin; 24th March 2021 at 11:08 AM.
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Old 24th March 2021, 11:29 AM   #4
MarcelvdG is offline MarcelvdG  Netherlands
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I rather like J. J. Goedbloed, Electromagnetic compatibility, but if you look for something specific to DACs, DAC application notes may be a better start.
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Old 24th March 2021, 11:37 AM   #5
manueljenkin is offline manueljenkin  India
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Thank you. I'll start going through the AN for the 11001A (and few other dacs).
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Old 24th March 2021, 11:51 AM   #6
Mark Tillotson is offline Mark Tillotson
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Location: Cambridge UK
Many datasheets give layout and decoupling recommendations for mixed-signal chips like ADCs and DACs, follow these as accurately as you can and keep digital and analog sections of the PCB separate. You need to ensure the analog ground isn't polluted with digital currents, and that star-grounding is used in the analog side to avoid loops that pick up interference. The datasheet performance figures are dependent on suitable layout and decoupling, and often on the reference voltage source quality (if the chip uses a reference).
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Old 24th March 2021, 12:12 PM   #7
BesPav is offline BesPav  Russian Federation
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Also mostly all of those:
Technical Articles/Talks
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Old 24th March 2021, 12:39 PM   #8
manueljenkin is offline manueljenkin  India
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Join Date: Feb 2020
Quote:
Originally Posted by Mark Tillotson View Post
Many datasheets give layout and decoupling recommendations for mixed-signal chips like ADCs and DACs, follow these as accurately as you can and keep digital and analog sections of the PCB separate. You need to ensure the analog ground isn't polluted with digital currents, and that star-grounding is used in the analog side to avoid loops that pick up interference. The datasheet performance figures are dependent on suitable layout and decoupling, and often on the reference voltage source quality (if the chip uses a reference).
Thank you very much. By keeping digital and analog sections separate, does it mean galvanic/optical isolation or something else more profound? Si86xx was recommended by few people (in another electronics server) for its speed while still having good isolation.
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Old 24th March 2021, 12:39 PM   #9
manueljenkin is offline manueljenkin  India
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Quote:
Originally Posted by BesPav View Post
Also mostly all of those:
Technical Articles/Talks
This is a fantastic list. Thank you very much. Going through the PDFs now.
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Old 24th March 2021, 06:38 PM   #10
Markw4 is offline Markw4  United States
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ESS has some advice on dac board design in the following document: http://www.esstech.com/index.php/dow...e/view/69/267/

Also, one can download the ES9038PRO evaluation board design files, so one can study how they laid out the reference dac board.
https://ismosys.com/wp-content/uploa...CH-Ver-1.3.zip
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