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I have a system were both the signal generated and the sensing equipment is sharing the same ground.

Which one of these shield terminations scenarios is correct? (if any). I searched the internet for about 2 hours and could not find a clear definite answer.enter image description here

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  • \$\begingroup\$ Answer will vary depending on whether those ground on either side of those cables is common or not. But in reality, none of them are great. \$\endgroup\$ – Trevor_G Aug 29 '17 at 15:40
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    \$\begingroup\$ Are you concerned with noise contamination, or are you concerned with signal integrity regarding reflections, rise/fall time? \$\endgroup\$ – glen_geek Aug 29 '17 at 15:49
  • \$\begingroup\$ The 2 grounds are common, which is sort of the problem. I'm trying to eliminate noise and not have a ground loop through the COAX. \$\endgroup\$ – Tony Aug 29 '17 at 16:15
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    \$\begingroup\$ What kind of frequencies does the sensor output? DC or 2.21 gigohertz? \$\endgroup\$ – peufeu Aug 29 '17 at 16:28
  • \$\begingroup\$ ~100kHz range. No more than 1MHz \$\endgroup\$ – Tony Aug 29 '17 at 17:15
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None are correct if you want accuracy and the cable is long. You have to consider the fact the although both grounds are supposedly connected there may be ground currents (due to other equipment) that will create milli-volt differences along the ground and this adds to the signal received. This means you get signal+stuff feeding into your sensing circuit.

You also have to consider air-borne noise and to properly cancel this you need impedance matching at the sending end and a differential amplifier at the receiving end. The noise that "hits" the shielded cable may induce currents in it and these currents will also induce currents in the central "hot" wire. If those currents do not terminate to ground in the same way you get a difference voltage and a noise voltage at the receiver.

Of course, if the cable is short and you have the "system" in a "clean" environment then you won't see much noise or interference.

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  • \$\begingroup\$ I know that grounding the system at both ends is not good... What I want to know is how to terminate the COAX so that it still provides shielding without causing ground loops. \$\endgroup\$ – Tony Aug 30 '17 at 12:09
  • \$\begingroup\$ Try the scenario where one end is coupled to GND via a capacitor, maybe 10 nF but you can and should expect problems if the last paragraph of my answer isn't met. \$\endgroup\$ – Andy aka Aug 30 '17 at 14:51
  • \$\begingroup\$ Thanks! As far impedence matching 95% of the time the signal will have low output impendence and the sensor will have high input impendence. \$\endgroup\$ – Tony Aug 30 '17 at 15:57

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