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My question is very hard to put in words therefore sorry for using six photos along with the details.

For investigating a noise issue when using a sensor with a CAT6 cable for single ended signals, I use a 9V battery as a floating source with 100 Ohm output impedance as below.

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Since the receiving ADC daq system is also floating, I use a scope isolated from earth.

Below is how I first hook up the source output to the CAT6 cable:

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Now at the receiving end (where the end of CAT6 and scope probe meet) I do wiring this way(CT6 shield wire is not connected to anywhere):

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And here is what I see as noise in AC coupled view:

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But if I wire the scope probe this way(cable shield is connected to ground of the scope):

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The noise reduces significantly as below:

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My question is:

1) Why is the noise attenuated?

2) Is it okay to wire the shield of CAT6 to GND of the sensors in receiving end as in my second way to mitigate noise? Is it okay if both sides are not earth grounded?

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  • \$\begingroup\$ Your 9V resistor-battery is no grounded, so the only thing you see is 50 Hz + overtones capacitively couples into your scope. When you are using your shield like that, it is doing it's job and the ground clip of your scope is connected to actual ground. Variations of this has bern asked before. \$\endgroup\$ – winny Oct 19 '17 at 9:08
  • \$\begingroup\$ If both side's DC GND is earth grounded dont you think there will be ground loops? Thats why the battery I kept floating \$\endgroup\$ – GNZ Oct 19 '17 at 9:11
  • \$\begingroup\$ Possible duplicate of electronics.stackexchange.com/questions/262021/… \$\endgroup\$ – winny Oct 19 '17 at 9:12
  • \$\begingroup\$ Not both, but one of them. Otherwise your contraption is floating. \$\endgroup\$ – winny Oct 19 '17 at 9:14
  • \$\begingroup\$ Ok but scope is earthed and battery is not what is wrong? This question is about shield being grounded to earth or not \$\endgroup\$ – GNZ Oct 19 '17 at 9:15

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