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For my project, I need to have BNC connectors on the test metal shield. So what I did so far is drilling holes, and mounting BNC connectors on it:

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I used some hot shring on the part which passes through the hole. Those washers are normal fibre ones. I don't have something better at my arsenal now. This will not be a good insulator, because I measured the resistance from outer bnc connection to the metal shield and it happend to be around 1.4 GOhm:

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The problem is, the signal LO (the outer connector of coax cables) should be totally isolated from the metal shield. The best thing I can think of is some sort of teflon washer (shoulder washer) that can pass through the hole of the metal shield and then I can insert the BNC connector through it. Unfortunately I was unlucky to find what I need. What I have found is made from nylon and I think it can not provide a resistance as good as teflon, also it has the problem of absorbing water (humidity) :

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Please let me know what you think, and would be nice if you know a place that I can order the required stuff!

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  • \$\begingroup\$ The application is automated measuring of current leakage and target is sub 10pA so I have to make sure there will be no ground loops between metal shield/switch matrix and picoammeter. 1.4 GOhm is sufficent? \$\endgroup\$ – Sean87 Jan 12 '12 at 14:25
  • \$\begingroup\$ Reality check - if someone uses his sweaty fingers to connect the cable, and he touches you BNC metal and the case metal, won't he establish a conductive path that will provide much more trouble than your washer? I am no expert, but when I read 1.4 GOhm I get pictures of dead-bug mounted components. \$\endgroup\$ – Wouter van Ooijen Jan 12 '12 at 15:37
  • \$\begingroup\$ I know about reality and contamination and other stuff, thats why I am planning to clean wash the setup before each use, that should take care of that as much as possible. I found a BNC connector with PTFE as outer insulator that means ~1 Tera insulation resistance, much higher than 1.4 Gig I achived with these washers. Wouldn't it be a better choice? Also can you please explain about dead-bug mounted component? P.S I am a big fan of voti! \$\endgroup\$ – Sean87 Jan 12 '12 at 16:10
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How about using insulated BNC sockets?

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(This one available here in the UK)

Then you shouldn't have to worry about insulating them yourself.

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