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Old 20th June 2008, 12:18 PM   #91
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Perhaps I did, but you are also evading mine

I'm not sure I know the motivation - I'm tempted to say because the the analog has the more demanding requirements than digital, so a supply which is adequate for the analog section will more than meet the needs for digital section.

Still seems like you are swimming against the tide regarding the dac grounding however:

http://www.analog.com/analog_root/st...ndingADCs.html
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Old 20th June 2008, 05:29 PM   #92
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Analog Devices recommands splt analog and digital grounds that are joined inder the the DAC chip via an SMD ferrite bead.

Their excellent AD1955 eval board takes this approach.

-David
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Old 20th June 2008, 09:56 PM   #93
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Quote:
Originally posted by spzzzzkt
Perhaps I did, but you are also evading mine

I'm not sure I know the motivation - I'm tempted to say because the the analog has the more demanding requirements than digital, so a supply which is adequate for the analog section will more than meet the needs for digital section.

Still seems like you are swimming against the tide regarding the dac grounding however:

http://www.analog.com/analog_root/st...ndingADCs.html
Tidewave? They all look more like Calculus 101 to me!

I think probably you know not much about chip designs hence have no idea about what I was talking about.

In most mixed signal chips, the digital ground actually has far more serious current bounce problem than the analog. Take the PCM63 as an example. The voltage swing on the digital ground is so server such that the ground is practically floating, a big no no for a high performance DAC. It's crucial to maintain a common reference point for the analog domain and certain circuit in the digital domain.

If you connect the grounds through external pins. First of all, you have to deal with the impedance of the I/O pads. The thin metal wires inside the chip have voltage drop as well. Eventually this reference loop is still way too long. A solution is to build a voltage reference inside the chip then refer both grounds to it. You need to build some sort of bridge for the digital ground such that the device will maintain the ground reference yet stop the digital noise/voltage swing running into the analog ground. Looks like BB engineers were so confident with their solution such that they felt separate power supplies are not required.

Those bridging devices are built in silicon. They are not active until you apply power to PCM63 so just picking up a PCM63 chip then measure the pins will not tell you anything.

Sure, it is still important to connect both ground pins together to reduce the capacitive load among the planes to improve the current dumping

If you look at D1V3's layout. Since we know both digital and analog planes have the common reference inside the chip, instead of being paranoid about connecting AGND and DGND pins immediately, we have to see how to provide the dumping paths for those ground currents. Since SMD beads are out of questions here, we could not join the digital and analog grounds under the chip through beads. Beads are critical here since we need to prevent the HF digital noises running into the analog plane. Direct connection between both grounds will not get you much benefit, instead, will just bring you more troubles. Our solution is to put the joins right beside the chip. And the connections are not just one, instead, we use two beads in parallel. We feel this is the best compromise.

We still use separate analog and digital power supplies mainly to get a better rejection rate.

As you can see, connecting both ground pins immediately together is just a basic concept. It's far more important to see what kind of ground currents you are dealing with and find out a good solution to address the challenge.

Analog power supply is very different from digital power supply. You cant say one is more demanding than the other, BTW. So what's your next guess why the datasheet would say separate power supplies cant gain you much?

Oh, probably you do not know D1V3 is a joined effort between Spencer and me?
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Old 20th June 2008, 10:01 PM   #94
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Quote:
Originally posted by dw8083
Analog Devices recommands splt analog and digital grounds that are joined inder the the DAC chip via an SMD ferrite bead.

Their excellent AD1955 eval board takes this approach.

-David
Recommendation is recommendation, right?

Sure, if SMD parts are an option, it's a good idea to connect both ground right under the chip. However, you will have to keep in mind that SMD parts have very low current rating. AD1955 can surely generate lots of nasty ground current.. eventually you may have to have analog/digital joins somewhere else... then you are just creating another ground loop. Not a very pretty thing.

Anyway, it's just an eval board, isnt it? Dont expect it solves all the problems for you.

AD1955 is an excellent chip. The output current source is simply the best in the industry, very easy to live with. Hell, ADI chips all have better current outputs than BBs.
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Old 20th June 2008, 11:05 PM   #95
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Quote:
Originally posted by finneybear


Oh, probably you do not know D1V3 is a joined effort between Spencer and me?
I read diyhifi.org and have seen your comments regarding your joint project there, so I gathered that Spencer had called in the reinforcements in a couple of threads.

thanks for the explanation. I'd spent some time reading up on things on the web, in particular a walt kester tutorial that covered much the same ground:

http://www.analog.com/en/content/0,2...255F65,00.html

Obviously you disagree with most of the references I've found on grounding strategies, but I guess that it the privilege of those who know enough to break the rules
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Old 21st June 2008, 12:02 AM   #96
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Quote:
Originally posted by spzzzzkt


http://www.analog.com/en/content/0,2...255F65,00.html

Yes, yes, the ADI application note will put you more on the right direction, or I'd say, the real world scenario. The handling of ground planes is a very nasty nasty problem.

Our idea is pretty much the same as Fig.10. Using beads has a lot of drawbacks. The capacitive load, that is, the voltage Vn created between the noises in diginal and ground planes, is another headache. Both will hurt the LSB resolution of the DAC yet we figured, PCM63 was not famed for resolution anyway. Nowhere it can compete with modern delta sigma gens such as PCM1794, CS43122, etc, so we took the compromise. We just wanted to put out a design which was fun and flexible. It would give out the best possible performance for DIYers. Beyond that, probably it will be a board nobody can handle manually.

The soul of D1V3 is the I/V stage by Mr.Pass. This I/V and PCM63 simply have the syngery. The impedance curve, the current rating, the bandwith, simply have the best match. You cant help but just call Pass a genius! Anyway, this simple I/V with no NFB can simply bring out the best of PCM63. The strength is in the image, the sound stage, the advantage of a good R2R DAC. It also has a very smooth and relaxed sound.

We tried to mate this I/V with PCM1798... well, just didn't work. The bottom is a bit thin. Now I see why Charlie had to use transistors in his Ayre machines.
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Old 21st June 2008, 12:17 AM   #97
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Saw this in the article you posted the link:

"Most ADC, DAC, and other mixed-signal device data sheets discuss grounding relative to a single PCB, usually the manufacturer's own evaluation board. This has been a source of confusion when trying to apply these principles to multicard or multi-ADC/DAC systems. The recommendation is usually to split the PCB ground plane into an analog plane and a digital plane. It is then further recommended that the AGND and DGND pins of a converter be tied together and that the analog ground plane and digital ground planes be connected at that same point as shown in Figure 8. This essentially creates the system "star" ground at the mixed-signal device.

All noisy digital currents flow through the digital power supply to the digital ground plane and back to the digital supply; they are isolated from the sensitive analog portion of the board. The system star ground occurs where the analog and digital ground planes are joined together at the mixed signal device. While this approach will generally work in a simple system with a single PCB and single ADC/DAC, it is not usually optimum for multicard mixed-signal systems. In systems having several ADCs or DACs on different PCBs (or on the same PCB, for that matter), the analog and digital ground planes become connected at several points, creating the possibility of ground loops and making a single-point "star" ground system impossible. For these reasons, this grounding approach is not recommended for multicard systems, and the approach previously discussed should be used for mixed signal ICs with low digital currents. "

This is exactly the scenario we encountered on D1V3 which had 4 PCM63s on the same board.
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Old 21st June 2008, 01:59 AM   #98
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Quote:
Separate power supplies for analog and digital circuits are also highly desirable, even if the voltages are the same. The analog supply should be used to power the converter. If the converter has a pin designated as a digital supply pin (VD), it should either be powered from a separate analog supply, or filtered as shown in the diagram. All converter power pins should be decoupled to the analog ground plane, and all logic circuit power pins should be decoupled to the digital ground plane as shown in Figure 6.
So it seems that Kester is suggesting the DAC(s) should placed over the analog ground plane and powered from the analog supply. I suppose that would be a fairly clean way to partition the board and powering the DAC VD regs from the same 15V supply used for the DAC VA regs doesn't seem like a real issue. The resistors on DATA, LE, CLK could the bridge the split in the ground planes in a similar manner to Figure 6.
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Old 21st June 2008, 02:24 AM   #99
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Quote:
Originally posted by spzzzzkt


So it seems that Kester is suggesting the DAC(s) should placed over the analog ground plane and powered from the analog supply. I suppose that would be a fairly clean way to partition the board and powering the DAC VD regs from the same 15V supply used for the DAC VA regs doesn't seem like a real issue. The resistors on DATA, LE, CLK could the bridge the split in the ground planes in a similar manner to Figure 6.
From the board level, this is a clean arrangement yet the DAC chips may not like it. PCM63, based what BB engineers stated, actually loves this setup!

Still, the decoupling cap on the digital power rail is critical. It's hard to find an ideal cap for this purpose.

The bottom line is since we decided to go for separate analog and digital power, this was not an option.
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Old 21st June 2008, 04:17 AM   #100
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Using the D1V3 to feed an unbalanced amp means you lose the potential for common noise rejection.

Would the +- 18V supply on the board have the headroom to power this circuit to do a balanced to unbalanced conversion ?

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