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scb-68 ground issue
Neleja
Member
10-21-2014 10:52 AM
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Hello,
I am new in LabView world and I would appreciate your help regarding the grounding problem I have.
I use PCIe 6343 and 6341 with 3 scb-68 boards connected. Beside this I have AVR stk600 eval. board with µC connected to the PC via USB-cable and one home-made board which is connected to the power supply.
PCIe cards have their internal ground point and this ground point differs from power supply ground. For this reason the signal from LabView is wrongly interpreted on the home-made and stk600 boards.
When I connect the GND from supply voltage to one of AO GND on the scb-68 I just get another wrong voltage value.
How is internal GND point set in PCIe cards, and what should I do in order to have the same GND ref. point for scb-68 and 2 additional boards? How should I connect them?
The picture of the whole hw setup is attached.
swirl
NI Employee (retired)
10-22-2014 05:17 AM
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The Ground of the PCIe Express cards is refered to the BUS Ground.
So it could be that the GND of your Cards and your Power Supply is on a different Level.
http://digital.ni.com/public.nsf/allkb/10AF3F86965DA0CEC1256BBB005103A2?OpenDocument
I think you can measure your signal with the NRSE (Single-Ended-Nonreferenced) or Differential.
In the Differential Mode, you use two Analog Inputs and the measured Voltage is the difference between these two,
but then you only have half of AI on hand.
In NRSE Mode you connect your external GND to the AI Sense Input. This GND is then used as
your reference GND for your measurement.
I would recommend to use one of these methods to have a common GND in your system.
I think i have some interesting Links for you, where the different methods are explained:
How To Measure Voltage
http://www.ni.com/white-paper/7113/en/
Field Wiring and Noise Considerations for Analog Signals
http://www.ni.com/white-paper/3344/en/
Regards,
swirl
10-22-2014 08:43 AM
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Thank you swirl for your reply. But my problem is not solved.
The problem is that there is always the difference of 50mV between GND from NI-cards( internal GND) and GND from power supply (external GND).
When I connect the external GND to AI SENSE on cards as you suggested, there is still difference between AI GND (e.g. pin 56) and AI SENSE (pin 62) of 50mV on the same card.
Is it possible to connect external GND to NI-cards and to make it equal to internal GND? Or is there any other way to make those two GNDs equal?
