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A datasheet suggests separate grounding for analog and digital portions of circuitry (http://www.ti.com/lit/ds/symlink/am26c32.pdf#page=12).

Is this to say that the IC's outputs should have a separate ground to the IC's power supply (VDD)?

If so, how is this possible when all components operate from the same supply?

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  • \$\begingroup\$ It is "good PCB layout practices." Also, this question seems to be a duplicate: electronics.stackexchange.com/questions/128637/… (See the comments for even more duplicates.) \$\endgroup\$ – adamaero Jul 23 '18 at 13:25
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    \$\begingroup\$ It used to be good practice. This is a slow device and separate planes probably would be ok. The device itself is ancient (1990). I answered this for someone using modern (fast) logic. electronics.stackexchange.com/questions/185306/… \$\endgroup\$ – Peter Smith Jul 23 '18 at 13:28
  • \$\begingroup\$ I thought 52Mbps was quite fast? So all inputs/output on the IC can share the same ground, and I shouldn't run into problems? \$\endgroup\$ – 19172281 Jul 23 '18 at 13:49
  • \$\begingroup\$ @B4039 The output transition time is listed as 4 ns typical, which is about 24 inches on most flavours of FR-4. Provided the grounds do not have more than about 2 inches of loop, there should be little effect. \$\endgroup\$ – Peter Smith Jul 23 '18 at 14:16
  • \$\begingroup\$ Would having the digital signal reach ground first make any difference? \$\endgroup\$ – 19172281 Jul 23 '18 at 14:30
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Separating analog and digital ground planes is good practice and helps reduce noise. Its not true that all components must operate from the same supply. Infact, you can have separate power supplies for the digital and analog circuitry. They may be supplied from the same source originally, but filtered as not to interfere with each other.

Here is an example of this: https://m.eet.com/media/1049897/C0130-Figure3.gif

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  • \$\begingroup\$ Separate planes can induce noise due to ground loops; it all depends on edge rates. \$\endgroup\$ – Peter Smith Jul 23 '18 at 13:29
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    \$\begingroup\$ I don’t think the OP was stating that they MUST operate from the same supply. I think the question was asking about how to handle it when they do operate from the same supply. \$\endgroup\$ – Blair Fonville Jul 23 '18 at 13:30
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    \$\begingroup\$ EMI consultants like it when people split ground planes. When the boards fail emissions testing, it is more business for them! \$\endgroup\$ – mkeith Jul 24 '18 at 7:51
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Is this to say that the IC's outputs should have a separate ground to the IC's power supply (VDD)? If so, how is this possible when all components operate from the same supply?

I suppose your referring to this:

Separate grounding for analog and digital portions of circuitry is one of the simplest and most-effective methods of noise suppression. One or more layers on multilayer PCBs are usually devoted to ground planes. A ground plane helps distribute heat and reduces EMI noise pickup. Make sure to physically separate digital and analog grounds, paying attention to the flow of the ground current.

The current will return back to the source (usually the ground pin of the cable tied to the power supply ground). In the DC world this usually means the shortest path (assuming resistance of the ground plane is equal and the plane is continuous). In the AC world this means the lowest impedance path, which usually means the return current will follow the trace that is carrying the AC signal because of mutual inductance.

Regardless, if a return current crosses through an analog subsection, it will show up as a (usually small) voltage. This voltage offset will cause noise in an analog subsection.

The best thing (in my experience) is to find where the return current is going and orient the analog subsection so currents do not cross through it. This allows you to maintain a simple grounding system, with a continuous ground plane.

Another way to do it is with slots in the board:

enter image description here

Source: Staying well grounded

A problem with slots is they turn the PCB into a dipole antenna, and if the design needs to be FCC compliant, it may fail.

Another way is to isolate the grounds with digital isolation, and or a star ground. Both of which I wouldn't recommend.

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    \$\begingroup\$ The other idea would be to place the precision analog circuitry (if possible) somewhere else, rather than in between the main power input and the load. \$\endgroup\$ – mkeith Jul 24 '18 at 5:20
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    \$\begingroup\$ Thats option 1) Quote: "The best thing (in my experience) is to find where the return current is going and orient the analog subsection so currents do not cross through it" \$\endgroup\$ – Voltage Spike Jul 24 '18 at 6:31

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