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My customer's schematic uses a 0 ohm resistor for connecting analog(AGND) and digital (DGND). I think it is for testing, current measurement , fuse, jumper, test point etc but is there another reason?

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    \$\begingroup\$ The 0R resistor is not needed but is a cheap way of forcing your PCB layout package to keep your analogue and digital grounds separate and only joined together at a single point. Without it you would need to pay more attention to how 0V is routed. \$\endgroup\$ Dec 12, 2014 at 9:37

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In a mixed signal system it's recommended to join AGND and DGND close to the mixed-signal IC, often the one performing the A-D function. The reason for this is to reduce differential noise in the mixed-signal IC. See the diagram below. The objective of minimising noise coupling from B to A is aided by joining the GNDs on a low-impedance ground plane. In a system with both an AGND plane and a DGND plane it is common to connect both the planes at this point with a link to minimise differential voltages. Sometimes this link is a wire with a ferrite bead, sometimes it's a zero-ohm resistor or even just a narrow PCB trace.

See a good description here.

As for the Zero Ohm resistor. Think of it as a link that is easily placed by a pick and place machine during manufacture and also easily removable for experimentation on prototypes, if required. Because it's a physical component, the ground plane connection is easily identifiable in PCB design tools which makes it easier for the designer and layout engineers to communicate intent.

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    \$\begingroup\$ This technique is not used just for joining analog and digital circuits, but often enough within analog circuits as well. "Star ground" seems the most common name for it; googling it will reveal several (other) good sources. \$\endgroup\$ Dec 12, 2014 at 10:00
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    \$\begingroup\$ If cheese cartoons are your preferred way of learning concepts, then Maxim's app note 4345 "Well Grounded, Digital Is Analog" has no competition. Sorry, I couldn't help myself after seeing that one... \$\endgroup\$ Dec 12, 2014 at 10:43
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    \$\begingroup\$ To further add. Having a physical resistor instead of just a small trace allows designers to experiment. Some people bring out voodoo black magic when it comes to grounding. Things like capacitors or inductors get put into the connection. Having those pads means they can be replaced or probed just incase a designer isn't feeling confident in their grounding topology. \$\endgroup\$
    – lm317
    Dec 12, 2014 at 15:00
  • \$\begingroup\$ Just in case someone want to read the article "Well Grounded, Digital Is Analog" linked by @Fizz: it is now here: analog.com/en/technical-articles/… \$\endgroup\$
    – deralbert
    Nov 7, 2023 at 23:21

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