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Reference image

while going through the reference designs, I came across the capacitor between grounds. Is it for RF noise protection? Is ground for U4 and other part of the circuit is different?

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  • \$\begingroup\$ As drawn, that capacitor connects to the same GND on both sides. Ergo, it is shorted and does nothing. My guess is that perhaps they needed some shape on the PCB to solder a wire to or this is simply an error. \$\endgroup\$ Jan 25 '17 at 21:45
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    \$\begingroup\$ You removed from that schematic the part where it says those caps are not mounted... I think its basically a visual identification for different models, as you can see on the pcb that thing is soldered directly onto the gnd layer \$\endgroup\$
    – PlasmaHH
    Jan 25 '17 at 21:50
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    \$\begingroup\$ I once did this as an experiment to eliminate an occasional glitch in an A/D converter. The converter called for analog GND and digital GND to be tied at one point at the converter. The system had them tied offboard back at the power-supply. The experiment worked, but the rework was not allowed as it was part of aviation equipment that had already undergone governmental testing. \$\endgroup\$
    – Tut
    Jan 25 '17 at 21:53
  • \$\begingroup\$ @PlasmaHH This particular capacitor, c18, is connected on the main board. Others were not mounted, so I removed them from schematic. \$\endgroup\$ Jan 25 '17 at 21:55
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    \$\begingroup\$ @HarshSingh: if you don't know why something is in the schematic, leave it in. This is a crucial information, as it says that the others are not mounted. It says that becase -- as I mentioned already -- they are functioning as visual identification for the different models, as is hinted by the "_Differentiation" heading for that schematic section. \$\endgroup\$
    – PlasmaHH
    Jan 25 '17 at 21:57
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Its a NL\switchable part that is used for identification only (almost definitely). It appears that they use a capacitor to indicate what kind of board it is visually. On other boards, if the grounds were different on either side I'd say it was a ground connection but the ground is the same so it serves no purpose electrically. enter image description here

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    \$\begingroup\$ Ah the joys of components being dirt cheap in bulk. \$\endgroup\$ Jan 25 '17 at 22:08

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