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There is a data-acquisition device with BNC inputs similar as follows: enter image description here

It is single ended and when run by the software it is multiplexing all 16 adjacent channels.

I connect only first lets say the first three channels to signal sources by BNC cables; and the rest adjacent thirteen BNC inputs are not connected to anything i.e they are floating.

But the data I obtain at the end for the unconnected floating BNC channels have the same signal with the third channel. They copy the last connected channels's voltage signal.

How can this be explained?

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Most likely there is one ADC within the unit and analogue multiplexing is used to sequentially run through all the channels in short time. This means that on the third channel the voltage is sampled and when the multiplexer moves on to the fourth channel, the voltage on the sample and hold capacitor remains unaffected by the open circuit at that input and therefore, the fourth channel appears to be the same as the third channel.

A simple test would be to look at the fourth input with an oscilloscope and see if some portion of the third channel's signal appears during some time period corresponding to when the fourth input channel is "measured".

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  • \$\begingroup\$ From the fourth input upto 16th the BNC inputs are floating, they dont have any input rather than air. Could you elaborate on what you suggest? Do you mean connecting scope's input and the 4th input connector of the daq via a BNC coax cable? \$\endgroup\$
    – user1245
    Aug 24 '17 at 9:33
  • \$\begingroup\$ Btw "the voltage on the sample and hold capacitor remains unaffected by the open circuit " This makes sense but why the fourth channel has no voltage?? Would it read as a random value then? \$\endgroup\$
    – user1245
    Aug 24 '17 at 9:37
  • \$\begingroup\$ Yes, connect that input to the scope and use a 10 Mohm probe. A floating input will neither discharge nor charge the sample and hold capacitor hence it's voltage value does not change. \$\endgroup\$
    – Andy aka
    Aug 24 '17 at 9:37
  • \$\begingroup\$ " A floating input will neither discharge nor charge the sample and hold capacitor" I see what you mean but how about why sometimes I see 50Hz signal if no inputs connected ? Thanks for the answers \$\endgroup\$
    – user1245
    Aug 24 '17 at 9:39
  • \$\begingroup\$ 50 Hz (or 60 Hz in the US etc.) is all around us and it is quite easy to pick up if you use a measurement device with high impedance. \$\endgroup\$
    – Andy aka
    Aug 24 '17 at 9:41

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