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I want to build a device that samples analog signals from various measurement instruments via their analog outputs. The instruments have their own power supply, and all devices are powered from the mains. I need to measure signals in the range of 0-50Hz with a level of at most 0-10V.

The documentation of the analog outputs mainly talks about the signal voltage levels, but I'm concerned about the common mode. From what I can tell with an oscilloscope, the outputs of two instruments I checked seem to be referred to ground (one line at GND, one above), but even if that is the case, would it be a good idea to simply connect the grounds of all devices?

It seems safest to build the analog input in a way that is fully isolated, but is that really required? I'm not entirely sure how I'd build something like that (switching a capacitor between the input and my ADC perhaps?), so any pointers would be appreciated.

Medo42
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  • You should check to see if each of the measurement instruments are isolated from the mains themselves. If they are, then there's no issue in connecting all of their grounds together. Just don't connect them to the mains ground line (earth ground). If all of the instruments already have isolation from mains, then you don't need any more isolation. An exception to this may be if the instruments output high voltage AC that you're trying to measure. If that's the case, then you may want more isolation. – horta Jul 07 '14 at 14:51
  • As I said in a comment to a (now mysteriously deleted) answer, I expect that all the devices involved will be referenced to earth ground, and the outputs don't seem to be isolated (at least I don't see a 50Hz hum on my scope when I measure the output without connecting the ground line). – Medo42 Jul 07 '14 at 15:27
  • "reads data from various measurement instruments via their analog outputs" - why are you reading data from an analogue output - are you sure you mean "data" or do you mean "signal"? – Andy aka Jul 07 '14 at 17:04
  • If the output side of all of the instruments are all referenced to earth ground, you probably shouldn't be using them at all as they would be dangerous to use. – horta Jul 07 '14 at 17:36
  • I'm not sure you need fully isolated inputs, but true differential inputs may be appropriate. An instrumentation amp or diff amp (depending on the drive impedance) for each of the outputs would likely be the safe approach. You could potentially even use a single diff amp and a differential analog mux to read each of the outputs in turn. – John D Jul 07 '14 at 18:16
  • @horta why are they dangerous to use? Information I found about audio system interconnections suggests that in that area, it is quite typical to have cables which connect the (PE-connected) chassis grounds of two devices via the cable shield, and which carry signals that are not floating vs. ground. What is the difference here? Is the information I found wrong? – Medo42 Jul 08 '14 at 09:17
  • See this previous SE Q&A: http://electronics.stackexchange.com/questions/17496/how-is-using-a-transformer-for-isolation-safer-than-directly-connecting-to-the-p – horta Jul 08 '14 at 14:36
  • @horta, The Q&A describes how an isolating transformer makes a high AC voltage vaguely safer by isolating it from earth potential. I don't understand the relevance of that in my situation. Here, all visible earth-referred voltages are small and referred to earth via PE. – Medo42 Jul 08 '14 at 14:56
  • Ah, well the use of low-voltages was never specified before. Generally if you have a signal in the 0-50Hz, it can't be assumed that there will only be low-voltages. Then again, it can't be assumed anywhere. If you're only using low-voltages, then why do you even have a question related to safety? I don't see why you need any isolation if all you're getting is low voltage. If they're ground referenced as low voltage, then you might as well continue using that same reference. – horta Jul 08 '14 at 15:01
  • I have a question related to safety because the safety has been questioned (both by you and in the now-deleted answer which said I need to use isolation), and I don't want to build any potentially dangerous circuits. I'm kind of paranoid about that so I want to understand all safety concerns that are raised. – Medo42 Jul 08 '14 at 15:23

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