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I have been designing a PCB which includes an SD card connector. As I'm not a PCB designer expert, I was looking for a reference design when I came across with the circuit bellow, extracted from Texas Instruments

enter image description here

My question is about the purpose of the capacitor (C206, 0.1uF) tied between GND and MicroSD_earth (Chassis Ground). Other reference designs seldom includes that capacitor, and usually the chassis ground pins are tied directly to GND of the PCB or left floating. Whats the correct way to go and why? I'm really confused.

Hypothesis:

1) Tie SD connector metallic shield pins directly to the ground (PCB GND);

2) Tie them to the ground with a capacitor, like depicted in the figure shown above;

3) Left shielding pins floating.

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  • \$\begingroup\$ The cap probably has nothing to do with the MicroSD. They probably want the earth (chassis) ground surrounding the card for EMC reasons. One can only guess they needed DC separation for some reason. The MMC card will work just as well if C206 is open or short; the rest has to do with your EMC and chassis ground strategies. \$\endgroup\$ – Cristobol Polychronopolis Jun 20 '18 at 20:21
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They most likely have a ground for the micro SD card case and signals, and one for the rest of the circuit. It is common to tie shields to chassis ground to shunt ESD away from the rest of the circuit.

Its a stitching capacitor its to ground the plains to provide a return current path across a slot or ground divide. If you run a trace across a ground divide then the return current has to go around the slot creating more inductance. The capacitor lets the high frequency portion of the signal jump the gap.

enter image description here

Unless you have a very large noise source, in my experience multiple grounds is a bad idea. Get Electromagnetic compatibility engineering by Henry Ott

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