Current offset in Signal Express NI DC power supply VI

[highabove]

hello,

Currently I am implemeting an automated I-V characteristic mesurement for a diode using LabVIEW and PXI chassis, specifically the PXI 4110 DC power supply.  I am using Signal Express DC power supply vi to acquire both Vd and Id.  But I keep having an offset from the current readback made available from the vi.  The current readback was about .2mA.  So I decidied to run the NI DC power supply program driver to see if it has the same offset.  At zero volt, the current readback only show about 2uA offset.  Is there anybody know why the offset are different between the program? Any insight into this is greatly appreciated.     

[Vincent_Carpentier]

Hi HighAbove,

It looks like it is spec'ed at an offset of 0.2mA +0.012%.   If you look at page 2 of the Specifitions: http://www.ni.com/pdf/manuals/371635a.pdf, you will see the resolution/accuracy of current Measurement.

Hope this helps,

[highabove]

Hi Vincent C

Thanks for your insight.  However, I am using 20mA current range.  From the spec sheet, I think the offset should be around 3uA +- .012.  This number is closed to the NI DC power dump interface when I sweeped the diode for the voltages: 0, .1, .2 V.  Nothing suprise here.  But when I use LabVIEW to do the same thing, the offset becomes roughly .2mA.  In addition, if I have just .1V as sweep voltage, the offset becomes close to .2uA.  But when I sweeped it to more than one point eg. 0,.1,.2, the offset becomes in the mA range.  The block diagram I used in LabVIEW was converted from SignalExpress.  It works fine in Signal Express (the offset is correct).  But the converted project to LabVIEW is not. This strikes me.  I really don't know what causes this different.

[Vincent_Carpentier]

Could you post your VI so we can take a look?

Thanks,

[highabove]

http://www.yousendit.com/transfer.php?action=download&ufid=865E7B9178056A48

The above link is my VI.  The main program is diode IV curve tracer irb vs vrb, which you should look.  When you try to run it, it'll ask for a text file which is diode IV 100S.txt.  The graph on the bottom of the front panel is the original plot of diode current versus diode voltage.  The top graph try to reduce the offset, which currently is not correct.  Thanks for your help.

[Vincent_Carpentier]

Hi HighAbove.

I looked at your code and it looks like only Channel 1 is configured to be 20mA in the express VI.  Have you tried configuring all the channels at 20mA and notice any change?  Do you see the offset only on Channel 1 or is it the others or all of them?

Thanks,