Offset at PCI 6120 if 100kHz antialiasing input filter is active

[hejoba]

The PCI 6120 shows a range dependent offset voltage if the internal 100kHz antialiasing filter is activated. For the +/-1V range the offset is about -11mV, for the +/-10V range the offset is about -138mV, both measured with shorted input. It is the same offset for AC and DC input coupling. Each input channel shows a different offset, but all are far above the specification.
If the filter is turned off, everything is fine. I have performed a self-calibration and I observe a slight change in the offsets after the calibration. It looks like there is no offset adjustment for activated filter in the self-calibration routine.
Any idea?

[Ben_...]

Hello hejoba,

which driver do you use with the pci 6120.
If you are using NI-DAQ 6.9.3f3 and earlier there is a software bug 

If you are using a newer driver please let me know which version and which development environment do you use LabVIEW, LabWindows/CVI the NI-DAQ API.

Have a nice day

Benjamin

[hejoba]

Dear Benjamin,

thank you very much for your reply. Here are my system specs:

LabView 8.2.1 prof. dev. syst.
NI-DAQmx 8.5.0f3

The offset does not appear if I use SignalExpress as a simple data viewer, but with SignalExpress there is no chance to activate the 100kHz filter. If I convert the SignalExpress measurement into LabView code and activate the input filters by use of the DAQmx channel property, the offsets are back.

I have also checked different sampling rates, but its the same. Here are the offset values of all 4 input channels at +/-10V range, AC input coupling, and activated input filters:

ch1: -128mV
ch2: -1mV
ch3: -18mV
ch4: -43mV

The PCI 6120 manual notes, that the DC offset might degrade by 8 LSB with activated filters. 8 LSB at +/-10V range are about 2.4mV. If I add this to the specified maximum offset without filters of 1.7mV, I end up at 4.1mV maximum offset with activated filters. So, only channel 2 meet this spec.

The last external calibration is from May 2007 (that is right before I purchased the card). The calibration temperature was -128°C. Is that correct?

Thanks in advance!

[Ben_...]

Hello hejoba,

i think you can eliminated the issue by doing a self-calibrate in Measurement and Automation Explorer -> Select the PCI 6120 and press selfcalibrate.

Please let me know if the problem consist

Best Regards

Benjamin

[hejoba]

Dear Benjamin,

I have tried the self-calibration several times, but it does not help. There is only a slight change of the offsets after self-calibration. If have tried the self-calibration with open and shorted inputs without success.

Regards, hejoba

[Ben_...]

Hello hejoba,

i will check this her at National Instruments with a PCI 6210.
Do you have a signal connected to the input ?
Or do you have a offset if you connect the input to ground ?

Best Regards

Benjamin

[hejoba]

Dear Benjamin,

the offset is present with open input and with shorted input (i.e. shorted to ground). If I connect an AC or DC signal to the inputs, the offset is there also. So, it really looks like the self-calibration ignores the offset caused by the input filters.

Regards, hejoba

[Ben_...]

Dear hejoba,

could you please download the new DAQmx 8.6 NI-DAQmx 8.6 - Windows 2000/Vista x64/Vista x86/XP because there is a known issue with the antialiasing input filter and DAQmx 8.5

Regards
Benjamin

[hejoba]

Dear Benjamin,

please excuse my late reply but I was really busy. With the new Software everything works fine. The offsets are within the specs (even much better) now. Thank you very much for your help.

Regards, hejoba