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Originally Posted by Francis_Vaughan
The option might be available to create an external clock - the 2702 will accept one. Then build a low jitter clock - for instance Elso Kwak's designs. They are not large. Once that was done one could think about a different DC-DC converter - one that can take an external sync, and then run the converter at a divided down DAC clock rate, avoiding the possibility of heterodyned image products appearing. And so it goes.
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Honestly, I think it is overkill for this project. This will push the price up while giving not so much value. Also the board will become too big. The target enclosure is the smallest Hammond. And an amp should fit in it too!
Why don’t we keep this idea for a high(er)-end DAC design? The size and the price will not be a problem then! A lot of people are asking for this too.
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Originally Posted by Francis_Vaughan
I think where I was trying to get to with the layout ideas is to look at the IC not as a monolithic thing, but as a system of connected units. The designers give you this ability since they bring out all the power pins. So if you refer to the block diagram in the data sheet it becomes clear what the goals are. Treat each of these units separately, give each its own well bypassed power so they can't interfere with one another, but also be cogniscent of the currents that must flow between the units, and thus the consequent return currents. Return currents are trivially dealt with with the solid ground-plane. It is hard to do better. So we worry about power. The digital power is in two places, clock and main digital logic. Each should be separately fed, and in this case this really means adding an appropriate ferrite bead isolator before each of the bypass capacitors. Thus split the pad C18 and C19 attach to, and feed each of the splits with an SMD ferrite bead.
Something similar might be appropriate for the analog side, although the lower frequencies involved might suggest some R in series with a bead as well, and perhaps more C.
You are building a mini LRC power supply filter for each unit - one designed to keep the crud floating about the board out of each unit. Since the units each have different functions we can design the filter to match the unit.
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Despite my complex relationships with beads and inductors, I think this is a good idea. Definitely it is worth a try. If things go terribly wrong and the beads cause problems, we can easily fall back by jumpering the beads and corresponding resistors.
I updated the schematic and added new components as per your post. I plan to use Wurth 1206 package ferrite beads. Anything I missed there?
Schematic v3
Francis, I leave the joy of calculating the values to you if you don’t mind.
In fact, I have no preference.
