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This datalogger manual has a warning about excessive commonmode voltage on its 5th page.

...data acquisition device you have purchased and are about to use is NOT an ISOLATED product. This means that it is susceptible to common mode voltages that could cause damage to the device.

I'm planning to use a metal enclosure which will include some power supplies couple of amplifiers and this logger. Basically two amplifier's will be connected to this logger as single ended. The signal to the amplifiers will come from two transducers 15 meters away from the enclosure by BNC cables. The system already prepared and has its own protection and earthing. I just need to move the system to a new enclosure plus this datalogger.

My questions are following:

1-) If I measure the commonmode voltage as they suggest and warn on 5th page and find out it is alright is there still a risk in another environment for the same setup? I mean I will check comonmode in workplace but some other technicians will install it to a harsh environment for measurements. So should they also check the commonmode voltage there before setting and powering up?

2-) The manual says the common mode voltage might damage the logger. How about if I add 15V zener clamps before the inputs to the logger, wouldn't that protect the logger? Or if there is common mode voltage how to eliminate it?

3-) The transducer amplifier system (the signal source) must be earthed because of lightning protection ect. But logger also in the manual says to be earthed. I'm wondering in that case would there be ground loops. Is this logger's earth connected to its analog ground or analog ground and earth are isolated? Can I verify it and be sure by continuity test?

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  • \$\begingroup\$ What does the supplier recommend? \$\endgroup\$ – Andy aka Jun 2 '17 at 10:48
  • \$\begingroup\$ No more details in manual and I couldnt get an answer and I have hurry. My questions are: Can commonmode voltages differ due to different AC mains of the same setup? Can zeners provide a possible CMV exceeding 15V? Can I check by continuity of earth and analog input are isolated? \$\endgroup\$ – floppy380 Jun 2 '17 at 12:37
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The device is capable of measuring 30VDC with respect to it's 0V, probably chassis. This means that there is no real "common mode" voltage at play here. Unless you're using two inputs as differential, in which case neither of the lines may exceed 30VDC.

I do not see what you're are confused about. The manual is very clear, with pictures how to wire it up, and how not to wire it up.

1) Yes, you should check. But if the source is isolated, it might give ambiguous results, like half mains voltage. If the source is not isolated, and the signals are above 30VDC or 21VAC, additional circuitry might be required.

2) It would protect the logger, but it might damage the source by excess currents. I would assume the logger already has transient protection build-in.

3) Yes there will be ground loops. But the manual already provides a solution for that.ground loops

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  • \$\begingroup\$ But I will use single ended. Can there still be CMV? How should source and loggers earthed that time? What if I earth the source side and do not earth logger side? And if I earth both side why would there be ground loops because logger's earth and logger's ground are isolated or not? \$\endgroup\$ – floppy380 Jun 6 '17 at 8:28
  • \$\begingroup\$ Both sides are always earthed. You just not supposed to connect shield between the two devices in differential mode. Look at the red current path in the image. \$\endgroup\$ – Jeroen3 Jun 6 '17 at 8:30
  • \$\begingroup\$ I plan the single ended signal goes to the logger with a BNC cable. BNC's shield is connected to the earth of the source. Is that okay to wire BNC's + and shield to logger's + and AGND? (BNC's shield not to the logger's earth) Did I get you correct? I would be glad if you can illustrate this for single ended. \$\endgroup\$ – floppy380 Jun 6 '17 at 8:33
  • \$\begingroup\$ @doncarlos It looks like you have no choice in single ended mode. If the ground loop distorts the signal too much, you will have to add an isolator. \$\endgroup\$ – Jeroen3 Jun 6 '17 at 8:52
  • \$\begingroup\$ but isnt in datasheet shows agnd and earth of the data logger isolated? \$\endgroup\$ – floppy380 Jun 6 '17 at 8:56

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