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PXI 4472 :

PXI 4472 DSA module: The - ve terminal of the signal seems to have a short with chassis Mains socket earth ( due to PCB mount SMB connector contact with facia panel ) . Is this shorting normal. Does it have any effect on measurement. What happens if the earth voltage were to raise say to 10V ( between Neutral to Earth ) as found in general industrial situation.Will there be any damage to module.

Actually we are feeding the signal from the o/p of Preston IA amplifier to the I/p of DSA module. With the above mentioned shorting the main purpose of isolation is lost.

Pl advise.
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The inputs of the NI-4472 are pseudo differential. The User Manual contains a schematic of the analog input for the 4472 (link to 4472 User Manual) on page 3-3. Because the inputs are pseudo differential, we don't recommend using a ground which may vary by more than +/- 2.5V from the ground of the 4472. This would cause a ground loop to occur which could impair measurement quality or even damage the device.

What is the output of your amplifier? Is it differential? Could you use a single ended amplifier?

-Jack
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Thanks for the reply ,It is very informative ,i went thro the schematic.

The output of the amplifier is Differential..

Any suggestions most welcome.

Regards,

Prasad.
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If all of your sensors have the same ground reference, you could try isolating the sensors/amp's ground from the ground of the 4472.

You could also convert the differential output of the amp to be single ended and reference it to the ground of the 4472.

The ground of the 4472 is the same as the chassis ground of the PXI chassis or PC that it is in.

-Jack
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The sensors will have different ground as they will be drawing the voltage from respective DC power supplies. These DC power suppies in turn draw the supply voltage from UPS o/p. The UPS basically provides Phase and Neutral lines while the earth is taken seperatly from an instrumentation earth pit.

Converting the IA o/p from Differential to Single ended may not be feasible at this juncture as it needs clerences from various agencies. The IA and respective PXI chassis also draw the supply voltages from the UPS o/p and earth from the same instrumentation earth pit.

Regards,

Prasad.
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Hi Prasad. Thank you for contacting National Instruments. You are correct to be concerned about the negative terminal increasing to 10 V; the recommended ground is +/- 2.5 V. However, there is a way to connect the signal to the PCI 4472 in differential mode. Take a look at the following KnowledgeBase found on ni.com and let me know if you have any questions.

http://digital.ni.com/public.nsf/websearch/581574D9C2AEC19B86256E59000CFEDB?OpenDocument

Have a great day!

Marni S.
National Instruments
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