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usb-6009 analog input offset

I'm observing DC offsets on the analog inputs on my USB-6009 in differential mode, even when nothing is connected or when the differential pairs are tied directly together.  The magnitude of the offset varies among the four differential channels: almost nothing on AI0, about -0.5mV on AI1, about +2.1mV on AI2, and (most problematic) almost +10mV on AI3. The effect appears to be independent of sampling rate.  Tying both sides of a pair directly to ground (just for testing, clearly not useful for real scenarios) reduces the offset by about 30%, but does not eliminate it.  Most importantly, these offsets are about the same when I plug my sensors in.

 

Here is an image captured in the DAQmx test panel, while the leads for AI3 (differential) were tied together:

 

voltage offset.jpg

 

Is this within the normal range of factory calibration error for the USB-6009?  Other than this the USB-6009 is now working great for me.

 

This device is brand-new.  Interestingly, I have another (much older) USB-6009, and the 4 differential input channels still show offsets, but they're much lower (about 1mV on each channel, 2 are positive, 2 are negative).  I'd still like to fix it, but it's definitely much less of an issue at 1mV than at 10mV.

 

If it's the case that these offsets are known problems with the USB-6009, but they're expected to be basically consistent from day to day (at typical room temperature), it's an acceptable solution for me to calibrate each of my (two) USB-6009's (by just recording a bunch of data with nothing connected) and store the offsets.  In that case... is there a way for me to use the C API to fetch the serial number of the device, so I can store offsets along with device serial #'s?

 

Thanks!

 

-Dan

 

dan@microsoft.com

http://research.microsoft.com/users/dan

 

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Dan,

Which range are you using on your 6009?  If it is set to the largest range, then that is within the specifications for the absolute accuracy of the card (which can be found on page 25 of the manual: http://www.ni.com/pdf/manuals/371303l.pdf).  For the other ranges, that does seem to be a bit high. Have you tried various different ranges to see how that affects the offset? 

 

It is possible to get the device serial number using the following function:

int32 __CFUNC DAQmxGetDevSerialNum(const char device[], uInt32 *data);

 

-Christina

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I am running into a similar issue. I took the USB-6009 out of the box and was seeing a zero offset of about -4.5mV on AI0. I left the unit running overnight to make sure that there was no unspecified warm-up requirement and am seeing the same offset. There also seems to be an awful lot of noise/instability (the -4.5mV mentioned is actually a range between -3mV and -5.5mV.

 

Measurement range:  +/- 1V

Measurement type:    Differential

Offset AI3:0  (avg):    2mV, 1.5mV, -0.8mV, 4.5mV

 

As Dan had mentioned with his issue, this is not affected by sample rate change, however, when both sides of any differential input are connected to ground the measurement is reduced to a slightly more reasonable +/- 1.1mV (most channels are closer to +/- 0.5mV).

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Hello Max,

 

After reading over your issue, it seems that you are seeing an offset when measuring a differential signal from your 6009. When doing this measurement did you wire the AI+ to AI- together? When you were doing this testing did you do this in Test Panels in MAX?

 

From the values you have sent me, two of the channels offset at within the accuracy specification of 1.54 for the +/- 1 range. The other two I would try to do a self calibrate on the USB-6009 in MAX. In order to do this, right click on the USB-6008 under devices and interfaces and select self-calibrate.

 

If you still notice the noise, I would then suggest providing it a constant voltage from a battery to see if the offset still persists. If the offset is outside the accuracy range still, I would think that the device would need to be repaired.


Jim St
National Instruments
RF Product Support Engineer
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