usb-6009 noise

[Tappi]

Hi,

  I am trying figure out, how accuracy results I can measure with usb-6009. I attach 4,5 v battery between GND and ai6 and you can see result in attacment (RSE). I found that other people in this forum seems to be same kind of problem. Some where in this forum someone says that accuracy is 5mVrms. So I think that green chart line should bouncing between between 5,760 - 5,765. Not 5,74 - 5,77. Am I right. I even try measure battery in metal enclosure. Both battery and usb-6009 was in metals enclosure. How I solve this problem, buy better hardware?

[Tappi]

I forgot the picture, sorry.

[SnowMule]

What I usually do... in Labview, when I take a reading from the DAQ, I take 100 samples at 1khz rate, then average all those readings. 

Doing that measurement differential is going to improve the accuracy, an RSE measurement is fixed at ±10V regardless of what's on the min/max terminals.  Differential gives you 8 scales from ±1 to ±20V. 

[johnsold]

Tappi,

The system noise is specified at 5 mV rms on the +/-10 V range single ended.  The resolution on the same range is 20/(2^13) = 2.44 mV On the +/-5 V range the resolution would be 1.22 mV.  From your image it looks like the resolution is submillivolt, but the scale precision makes it difficult to tell.  The noise in that image is about 10 times larger than the resolution. That noise is probably on the order of 5 mV rms.  The absolute accuracy at +/-10 V, single ended is specified at 14.7 mV, typical, at 25 degrees and 138 mV maximum over the full temperature range.  This data is on pages 24 and 25 of the USB-6008_9 User Guide.

Certainly the USB-6009 is not designed for high performance measurements.  If you need better accuracy than this, you will need a better device.

Lynn