SCXI-1125

[NAUSHICA]

It seems the one of the SCXI-1125 in my setup has some problem.
I have two 1125s and when I measure a signal accross a 50ohm resistor,
(no voltage is applied to this resistor), a DC offset voltage is appeared on the
channels. DC offset values are in the range of  0.1V to 0.3V for all 8 channels.
This doesn't happen with other 1125s(they give almost 0 in every channels).

How can I correct this?

Regards

Naushica

[Logan_K]

What terminal block are you using?

Have you tried shorting the + and - inputs of the channel together? This would tell us if the error is due to current flowing across the resistor, or if it is due to offset errors in the module. If the errors are simply offset errors in the module, those errors can be compensated by using the Auto-Zero functionality of the SCXI-1125 device. You can enable auto-zero module for your task in MAX under the "Device" tab of the task.

How old is your module and your DAQ device? If they are more than a year old and they haven't been calibrated recently, then it is possible that the offset errors are due to the devices being out of calibration.

If those errors are from current, then they indicate a current of up to 6mA across that resistor! This is a very large current - which might happen if you have a ground loop somewhere? Have you made any connections to the Ch+ and Ch- inputs other than the 50ohm resistor?

-Logan

[NAUSHICA]

Hi,

I am using the SCXI-131 as the front mount module. My chassis set up is SCXI-1001 (two SCXI-1125+1313) & PXI-1052
which the DAQ (PXI-6289) is installed. 1125s are configured to be in parallel mode. The resistor I am using is not
connected to any signal sources (just floating). I am observing the terminal voltages with both 1125s but one of them gives a DC offset.

I checked with swapping the 1313s for the wire connections, installed them in PXI-1052 with MUX configuration but same module gives
DC offset. Then I tried with calibration with MAX but it didnt hep. finally I tried auto zero in LabVIEW in channel properties.
I couldnt use them in a loop, but when I enabled just before acquistion the DC offset cleared just once, & in the next iteration thjey were back
as usual.

What do you think about this? Is it a hardware trouble? I bought them abt 6 months back (DAQ +1125s + chasiss')

Rg

Naushad

[SachaE]

Hi,

  apologies for the delay in getting back to you.

Did you try grounding out the signal rather than just floating, whereby it's possible that you could be seeing an offset voltage. If you've tried this with an actualy voltage source (grounded, or a battery), then it sounds as if the module has gone faulty.

If this is the case, can you could please call into the uk office on 01635 572410 and tell the person you speak to that you have a possibly faulty 1125, then we can investigate further and arrange to get the hardware from you.

Thanks

Sacha Emery
National Instruments (UK)

[Logan_K]

I also appologize for the delay. For some reason I didn't get a notification of your reply, which is why I didn't answer right away.

It sounds like auto-zero mode works, which is a good indication that the error is being introduced inside the SCXI-1125. This means that the module's offset errors have either drifted outside of the specifications, or that the module was improperly calibrated at some point. If you don't already have it, you can download and install NI-DAQmx 8.0 or later, which will allow you to perform a self-calibration of the SCXI-1125, which will reduce the offset errors of the module.

http://digital.ni.com/softlib.nsf/websearch/D31D46FFFB745ECA862571490053EAB8?opendocument&node=132060_US

To perform a self calibration, perform these steps:
1) In MAX, find the DAQ device that is connected to the SCXI-1125. Right click on the Device and select "Self-Calibrate".
2) In MAX, find the SCXI-1125 module, and also Right click on it and select "Self-Calibrate".

Now, perform your same tests to see if anything has changed.

If you still have an offset error, then like Sacha said you should definitly try inputing a true voltage source, like a battery, to make sure that the inputs are being scaled properly by the calibration data stored on the SCXI-1125s. If you still see large errors or incorrect readings, then you should probably return the module for repair.

Finally, if Auto-Zero is working, you probably just need to ensure that you are using it properly. The easiest way to enable auto-zero mode is when you create a Task in MAX, you simply go to the Device tab of the task and set Auto-Zero mode to "Once". Then, when you save this task, you can use it in LabVIEW.

I hope this helps.

Regards,
Logan Kunitz