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I've read at several places (1,2) that ICs such a DAC have both analog and digital ground pins, both should be tied to AGND at the chip. However, the specific part I'm planning to use (MAX5104) does not claim so in the datasheet, showing instead that the DGND pin beconnecto to the boards digital ground plane. Why exactly is that?

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In general, your analog signals should stay in the analog section of the board and your digital signals should stay in the digital section of the board. You should have separate ground planes for each. However, these ground planes must connect to provide a consistent reference point to both sections.

Ideally, the analog sections and the digital sections should connect at a single point. The "star" connection mentioned in your links and by MoJo. Though sometimes this is not possible. But basically what you want is a small "bridge" to connect the two sections. Any signals that must cross between the sections should be routed over this bridge.

The reason for doing this is to prevent or minimize any noise from switching signals on the digital section from disturbing or corrupting any analog signals that you are trying to generate or measure. The split prevents digital currents from flowing into the analog ground plane. While the "bridge" connects the two ground planes so the ground reference is at a consistent level in both sections.

A good reference for why and how to ground mixed signal systems is Henry Ott's Grounding of Mixed Signal PCBs. Some diagrams from that page will help to explain some best practices:

Signal traces crossing a split ground plane Separate analog & digital gnd plane regions

As for the power supplies, they can come from the same source. Usually the bypass capacitors specified in the datasheets are sufficient to decouple the power between the sections. However, if the noise on the board is particularly bad, sometimes additional filter circuitry is necessary. For instance the one seen here:

Power supply filter circuit

Or, you could have a separate regulator to provide clean, stable power to any of your analog devices. This will most likely provide the best accuracy but at extra expense, board area, and circuit complexity.

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  • \$\begingroup\$ What about power supplies? I have, for instance, 5V switching regulator powering a DAC (and other A/D ICs like opamps) and Shift Registers/H-Bridge ICs. Do I need two separate 5V supplies or can it be shared, as long as the rails and the ground planes are properly separated? \$\endgroup\$ – joaocandre Jul 9 '14 at 14:13
  • \$\begingroup\$ @joaocandre Updated to address your comment. \$\endgroup\$ – embedded.kyle Jul 9 '14 at 14:26
  • \$\begingroup\$ Ott's guide is a very interesting read. However what if I was lookinng to create a separate power ground? \$\endgroup\$ – joaocandre Jul 10 '14 at 9:44
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    \$\begingroup\$ @joaocandre The same principle applies. You can have as many sections as you want but they all need to connect through "bridges" or "stars". For example, look at the diagram above labeled "Seperate Analog & Digital Gnd. Plan Regions". Instead of "Analog 3" you could just as easily place your power supply there and call it "Power 1". That would keep the noise from your switching regulator confined to that section so it wouldn't interfere with your digital or analog sections. But the digital and analog sections still have to have the same reference point, hence the narrow connection. \$\endgroup\$ – embedded.kyle Jul 10 '14 at 12:55
  • \$\begingroup\$ My issue is with ICs I'd be placing on the 'border'. I have ICs that require Analog and Power ground, and ICs that require Analog and Digital grounds. Thus I would somehow need to create those narrow connections between all of them, whcih I means I won't get a single connection out of each plane. \$\endgroup\$ – joaocandre Jul 10 '14 at 14:29
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The grounds need to be connected together, the question is where do you connect them. If you use the same ground plane for both the IC will work, but you will get digital switching noise on the analogue signals. The most common solution is to use a separate ground plane for analogue and digital, with them connected together at a "star" point very close to the power supply connector. That way any noise from the digital side will be isolated from the analogue side as current flows back to the power supply connector, not past the analogue ground pins.

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  • \$\begingroup\$ I am aware of that, the issue is where do I connect them. MAX504's datasheet suggests is to be at the IC, in addition to a connection at the power supply, but MAX5104 does not provide any information about that... \$\endgroup\$ – joaocandre Jul 9 '14 at 14:01

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