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I have seen a few electrical schematics where there are separate symbols for grounds. "Agnd" and "Gnd"

Can someone help me understand what are the differences between those two and where to use "Agnd" and "Gnd" ?

And it would really help if you could provide an answer relating to the Common Mode Choke component (as I have seen the different grounds in power sections having CMC)

Please help in very basic electrical terms.

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AGND and GND are generally "Analog Ground" and "Everything Else Ground"

The general reason they are separated is that AGND is the reference to sensitive analog circuits (amplifiers, ADCs, DACs, etc...) and GND is the reference for general digital circuits (gates, shift registers, MCUs, CPUs, FPGAs, etc...), which can impart a lot of switching noise onto their grounds.

Now, you want your grounds at the same DC potential so your data converter (just an example) has the same potential analog and digital grounds (analog ground for analog reference, digital ground for communications reference), which is where the CMC (common mode choke) comes in. The CMC has essentially zero resistance at DC (just the DC resistance of the choke), but has a high resistance to AC or frequency components, like switching noise. So by separating your grounds with a CMC you get two grounds at the same DC potential, but you don't get digital switching noise affecting your sensitive analog circuits and/or measurements.

Hope that helped!

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  • \$\begingroup\$ Thank you so much for the immediate help with the answer. I would just request you to help a little more with the CMC understanding. Could you explain a little bit more in this context. perhaps an image having two different grounds with CMC ,like what you said for better understanding. \$\endgroup\$
    – user220456
    Nov 21, 2019 at 18:02
  • \$\begingroup\$ I understand CMC has 4 terminals. Could you please explain how the signals and the grounds are treated at the four terminals during this type of ground connection? \$\endgroup\$
    – user220456
    Nov 21, 2019 at 18:03
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    \$\begingroup\$ A CMC is basically two independent inductors that share a common core, it's very similar to a transformer in this way. To connect a CMC in the way you're describing you would use one of the "two inductors" in the CMC to place between the two grounds and the other to place between two power rails (for example if you had a 3.3V rail that went to your digital circuitry you would run that through the CMC and get a 3.3V analog rail - often called A3V3 or 3V3A). \$\endgroup\$
    – SlashLP97
    Nov 21, 2019 at 18:09
  • \$\begingroup\$ Another use for separate grounds is for isolation. For example, isolated AC/DC converters (flyback, psfb, llc, forwards, etc) have a ground for the AC side and a ground for the DC side. Then there is also the Protective Earth "ground." \$\endgroup\$
    – Stiddily
    Nov 22, 2019 at 15:37

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