Voltage measurement leads using coaxial cable - how to shield?

Thread Starter

Jake-S

Joined Dec 2, 2020
3
Hello all,

I am setting up some measurement leads for some accelerometers which are IEPE (Integrated Electronics Piezo Electric) transducers.

These are with co-axial cables & BNC connectors, as commonly used with accelerometers. (central conductor = +ve, outer conductors = -ve)

What I dont understand is how best to shield the cables?

Below is the datalogger connector (I am not using the BNC optional adaptors - im making my own), showing the cable shield connection, but as the coaxial uses the shield wire as a conductor/transducer ground, it therefore is passing a current.

Should I simply common the transducer ground/outer conducter with the datalogger shield connector? Or have I misunderstood something?

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Thanks in advance.
 

DickCappels

Joined Aug 21, 2008
6,788
If the shield is grounded then the cable is shielded. Otherwise, pass several turns of the cable through the center of a toroid core, or do both!
 

Thread Starter

Jake-S

Joined Dec 2, 2020
3
I have just discovered that the accelerometer bodies are not isolated from the -ve supply, and I dont expect users to put an isolation barrier between the magnetic bases of the accelerometer bodies and the metallic object being measured.

If I were to also earth the coax shield - then wouldnt I run the risk of a ground loop?

Since the (~4mA) transducer supply is galvanically isolated from the mains supply feeding the datalogger, ive become alittle confused over the potential for ground looping :/

Thanks
 

crutschow

Joined Mar 14, 2008
25,991
Yes, that can generate a ground loop if the data logger input is connected to earth ground.
So don't connect the coax shield to earth ground, only to circuit common.
The coax shield is already connected to earth ground by the transducer.
You could have differential inputs at the data logger if needed to break the loop.
 
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