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This buck converter uses 2 separate grounds. AGND and PGND. Why do we require 2 separate grounds?

I have seen in another schematic which mentions something like track fuse which ties the board ground and the input supply ground.

What is track fuse as it is mentioned below?

enter image description here

Are both (tying AGND & PGND and tying GROUND-GND) the same/same purpose or different?

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3 Answers 3

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Ground is only a definition of 0V But shared currents may conduct crosstalk from the trace resistance and inductance. Thus isolating power return currents from analog signal currents is paramount in any high SNR design. If the power currents go directly to the power source and the signal ground does as well but without noise added then they can share a common 0V ground but distinct paths.

A fuse link allows a designer to troubleshoot parasitic noise by inserting a small resistance and measuring voltage to detect bi-directional currents on a 50 Ohm shielded spectrum analyzer or Ac coupled 50 Ohm terminated scope to determine the next course of action. Meanwhile probe errors from ground loop resonance with probe cable capacitance adds false noise pickup about typically 20 to 50 MHz on 10:1 probes and lower for 1:1 probes (worse)

Multiple ground paths and parasitic leakage C and L mutual coupling can pose challenges that can be overcome by a layout change,shield,orientation or low ESL Litz wire or STP ground terminations or feedthru caps , CM chokes or other means.

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GND is a generic term that can refer to many different things. Simply put, it is a common point.

A track fuse is a fuse made from a pcb track. For all intents and purposes it is a fuse.

AGND is analog ground - this is usually a low current path that does not have significant current flowing as compared with PGND which is the power ground. This is the high current return path. Due to resistance and current, you can expect a voltage drop. This means you need to consider this when you tie AGND to it.

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  • \$\begingroup\$ Thank you for the answer. But why is there a need to separate GND (common point) into AGND and PGND? Because, anyway in the end, we need to connect AGND and PGND to a common point, right? I'm confused \$\endgroup\$
    – user220456
    Commented May 13, 2021 at 10:58
  • \$\begingroup\$ And a basic question. I've seen fuses that are placed at the start of the supply. Near to the +ve terminal of the battery. But the above track fuse is placed at the end. By end, I mean the GND which is the return path or the -ve terminal. So, the placement of the fuse (near the +ve or near the return path) doesn't make any difference? I think it should not make any difference. Just to double check, I'm asking this \$\endgroup\$
    – user220456
    Commented May 13, 2021 at 11:01
  • \$\begingroup\$ All grounds are not equal. Nevertheless,TI gives you guidance on what to do with the AGND pin,so just follow it. Every wire is an inductor and a resistor. Two wires side by side is a capacitor and maybe a transformer as well. With experience you'll be able to estimate if these parasitic parameters are going to affect your circuit based on current, frequency and voltage. Apply basic theory to solve. For the pcb fuses, without specific context it can be hard to judge what the design intent is. I can only guess that due to the unpredictability of a pcb fuse, have two of them to be sure,to be sure \$\endgroup\$
    – Kartman
    Commented May 13, 2021 at 12:27
  • \$\begingroup\$ Ground = Ground only if there is NO CURRENT flowing. Because copper is not zero ohms, any current flowing will cause a (usually small) voltage drop. (by ohms law V=IR... If "I" and "R" are non-zero, then V is non-zero) If you're using a track fuse, this is probably a relatively high-current thing, so there is probably measurable voltage losses there (I'm talking like millivolts, but not 'zero') \$\endgroup\$
    – Kyle B
    Commented May 13, 2021 at 13:53
  • \$\begingroup\$ You use separate AGND, PGND, etc when you want to control where these ground currents are flowing. For example, if you have a motor in your project, the current flowing to & from this motor will have alot of high-frequency noise. If you allow this very noisy current to flow on the same copper ground trace as a very sensitive analog sensor, the sensors accuracy will be greatly affected (because it won't have a solid zero-volt ground reference... It has something that's jumping around) This is called 'ground bounce'. "Usually" you'll connect all the grounds where power enters the board. \$\endgroup\$
    – Kyle B
    Commented May 13, 2021 at 13:56
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I think you may be missing the original intent of the designer. If you have two ground regimes and you need to connect them at one point only AND, you want to avoid the two ground regimes making unwanted connections when you create a circuit board design, the standard method is to insert a zero ohm link. This means that the PCB creation software will regard the two grounds as separate because it allows two different net names. Eventually when placing the final piece of the jigsaw (the single connection between the two), you use the zero ohm link for that purpose.

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