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I am designing a device that output analog voltage in the range of -5 V to +5 V representing a certain measurement (single-ended earth grounded). My plan is to use a regular BNC cable, (earth grounded) connecting to a ADC acquisition device.

The bandwidth of the signal is 0 - 10 kHz

How to avoid a ground loop here if both devices are earth grounded?

What do you think of the following approaches?

  1. Make sure that both devices are connected to the same power outlet
  2. Earth ground the BNC cable only at the ADC acquisition device side
  3. Use twin-ax cable and make the analog voltage differential instead of single ended.
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  • \$\begingroup\$ Is your signal AC? \$\endgroup\$
    – winny
    Mar 8 at 19:39
  • \$\begingroup\$ I would connect the devices with grounds at both ends first. Then, if there is a ground loop problem, you can look at other fixes. Of course, balanced, differential connections are great, they just cost more. \$\endgroup\$
    – user69795
    Mar 9 at 1:57
  • \$\begingroup\$ The band-with of the signal is 0 - 10kHz Agree, will test the ground loop problems and if there any will think about more complex solutions with differential signaling. The point is also to finish and not to overkill the project with every detail \$\endgroup\$
    – Mr No
    Mar 9 at 14:32

2 Answers 2

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  1. This does not avoid a ground loop, it creates a loop but limits the size of loop. However many devices such as signal generators and oscilloscopes ground their BNC outer shell to mains earth so it usually is not a problem. It depends how much current is induced in the loop by the equipment so it may or may not be a problem in your application.

  2. Not very good idea. While it breaks the ground loop, it means that the signal return path is not via BNC shield but via mains earth wiring. It may be even more noisy as power supply filter noise flows in earth wires.

  3. Might work if easy to arrange, but could be more than what you need and solution (1) might be good enough.

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You are dealing with a low frequency signal, i.e. ground loop disturbances do impact the signal as they occur in the same frequency band.

Whether the single ended transmission with both devices grounded is acceptable, depends on your precision needs. High frequency equipment such as oscilloscopes use this, because they don't focus on highest voltage precision. If you have a ground loop resistance of 10 mOhm and a return current of 10 mA therein, that gives you a ground difference of 100 µV. You do that calculation with actual numbers in your circuit and then decide if the voltage error is acceptable. If it is acceptable, this is the simplest solution. Connecting to the same outlet lowers the ground resistance somewhat (the mains wiring is in parallel to the shield). Even more efficient would be to install an additional fat ground conductor between both devices.

Option 2 will force the low frequency return currents through the installation wiring. Due to the huge loop area, your signal will very readily pickup EMI, such as mains frequency magnetic fields, which are very well within your signal band. Also, your low frequency return current, still travels between the devices and offsets their grounds, so the signal integrity problem isn't solved.

Option 3 is the only acceptable solution if you have the highest precision needs, but it is more difficult than option 1. Btw, you don't need special twin axial cables and connectors to implement this. A simpler way is to use two BNC cables and twist them. Even a simple stereo phone plug cable will be fine, although not as rugged as BNC.

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