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When I plug a data-acquisition PCI card to a PC, I'm trying to picture the ground connections. Some following confusion arises:

  1. Is the PC motherboard's ground and PCI's ground common?
  2. Does a PC have analog and digital grounds? Ore one common ground?
  3. If so, is this PC common connected to the chassis earth as well?
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  • \$\begingroup\$ Is there any reason why you can't borrow a meter and measure the resistance of the connection between these points? \$\endgroup\$ – Andy aka Oct 26 '15 at 13:14
  • \$\begingroup\$ The reason Im asking recently Im using a daqboard which measures the offset more than the absolute accuray in the specs. . I find out for zero current the reading(through a shunt resistor) is around -14mV. So the absolute error shouldnt exceed 3mV but it is infact -15mV after the auto calibration. So I contacted to the manufacturer and here I got the following reply: "Maybe you are making a single-ended measurement? In that case, maybe there is an offset between the PCI-DAS6036's ground (that comes from the PC power supply ) and your power supply." \$\endgroup\$ – user16307 Oct 26 '15 at 13:20
  • \$\begingroup\$ And were you making a single ended measurement and are you able to make a differential measurement? \$\endgroup\$ – Andy aka Oct 26 '15 at 13:22
  • \$\begingroup\$ im making 16 channel single ended measurements. can it be that one of those channel inputs' ground causing a problem? But then all channels should have the common ground inside the PCI in single ended. Im confused \$\endgroup\$ – user16307 Oct 26 '15 at 13:26
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    \$\begingroup\$ I totally understand it - if you are remotely measuring a voltage and that voltage is "supposedly" at the same ground potential as local ground then it's quite common to see several milli volts of difference between the two grounds. Very common indeed. \$\endgroup\$ – Andy aka Oct 26 '15 at 13:39
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Yes in a normal desktop PC mains ground is connected to both the chassis and to the ground pins on the power supply output connectors. In turn the motherboard connects both chassis ground and the ground from the power supply to the ground of the expansion slots.

Whether your aquisition card connects the ground of the PCI slot to the ground of it's input connector is a question for NI but I expect it does.

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  • \$\begingroup\$ ok but what do u mean by "PC mains ground"? input of a PC psu has line neutral and earth; and output has +dc and ground. "PC mains ground" refers to which one`? \$\endgroup\$ – user16307 Oct 26 '15 at 14:11
  • \$\begingroup\$ mains ground=mains earth (the former term is usually used in america, the latter in the UK, dunno about other countries) \$\endgroup\$ – Peter Green Oct 26 '15 at 14:26

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