Changing Power Chord of PXI 1042 chassis giving an offset of 500 mV in Analog Input Channels

[sundarganesh]

Hi,

We have 2 sets of similar PXI system of the following configuration.

PXI 1042 chassis, PXI 6259, PXI 6733, PXI 8431/4, PXI 8430/4 Modules with PXI 8106 controller in one system & PXI 8108 controller in other system.

System I: 1042, 8106, 6259, 6733, 8430/4, 8432/4 with 4 ciruclar connectors

System II: 1042, 8108, 6259, 6733, 8430/4, 8432/4 with 4 circular connectors

The I/Os of these cards through the SCBs are routed to 4 Circular Connectors in each system.

System I is already in operation condition and System II is prepared as a spare.

When we put System II in use instead of System I, we have observed an offset of 500 mV in all the Analog Input channels of 6259 in System II.

To identify where this offset is coming from, we interchanged the components between System I & System II as follows:

i. SCBs, Cables of System I in System II & checked in System II - Same 500 mV offset

ii. SCBs, Cables of System II in System I & checked in System I - no offset

So we thought the internal wiring between SCBs & Circular Connectors might be causing this offset. To identify this too we did the following:

iii. Circular Connectors, SCBs, Cables of System I in System II & checked System II - No offset

iv. Circular Connectors, SCBs, Cables of System II in System I & checked System I - No offset

We thought ground of both the systems could be different. We have taken power from the same source (through UPS) for both the systems and even grounding also made common (ground taken from rear side of chassis).

Finally, we checked with interchange of Power Chords for the chassis. After this, in System I with the Power Chord of System II, the offset of 500 mV started coming even when the other elements are of System I itself. In System II with Power Chord of System I also, the offset is there.

Now in both the System we are observing this offset of 500 mV whatever the combination of the other components.

How can we correct this problem?

Now we are doubting that both the PXI 1042 chassis grounds got affected.

Regards,

Sundar Ganesh

[sumit.sen]

Where are you measuring the voltage? What is the source of the voltage?

[D Robertson]

Sundar,

This sounds like a grounding problem to me.  Unfortunately, the problem could be in many different places, although from what you've said I would be looking on the on the power supply side and not the signal side.  Some things to try:

• Make sure the UPS is giving you a good ground.
• Make sure that this ground is getting all the way from the UPS to the DAQ card.  You can check that be measuring the difference between the ground on the UPS and the one of the ground pins coming out of the PXI 6259.
• Hook up the signals that you want to measure using the differential terminal configuration.  Don't use referenced single ended or non referenced single ended, those configuration ALWAYS have more noise and grounding problems.

Good Luck,

Dave