NI 9239 offset

[JCahoon]

Hi, 

I'm using a NI 9239 DAQ with (4) BNC terminals, with a cDAQ-9171 Chassis that powers and connects the module to the computer through a single USB.

For the first calibration I capped all 4 BNC AI's with BNC caps and measured voltage readings to see offsets, (averaging over 50,000 readings), and it showed offsets of at smallest 30 uV and at largest 300 uV. 

We bought this module because we needed high-precision measurement and this module has a ~1 uV resolution, so 300 uV is pretty large.

Is there a way to eliminate this offset?

By the way, RMS noises of the measurements from the 4 BNC AI's were all 49 uV with little difference.

Thanks,

[WillyWad]

JCahoon,

I would first say to properly ground the terminal and then check this again. Measuring the value at an input terminal that isn't connected to anything or grounded isn't going to provide any useful information. If you're still seeing this offset when the terminal is properly grounded, or when you're taking a measurement, then it's something we can look into reducing. If you're just capping the BNC terminals such that they're not connected to anything or grounded, whatever we're seeing there is junk data. 

-Will