Wrong Voltages read with USB-6008

[DynaScott]

I am trying to log a 1/4 Hz 5V square wave with NI USB-6008 data logger logging at 1HZ, but after about 10 hours of testing, the data logger begins to read in wrong voltages, such as 1.6 V  when the voltage across the terminals of the differential input measured with a fluke multimeter read 5 V.  I have tried this with 3 different USB-6008 units and all of them exhibit the same behavior after roughly 10 hours.  I have extensively checked to make sure all the grounds were properly connected as well.

I was wondering if perhaps I have a setting wrong, or if I need a firmware update or something like that.

Message Edited by DynaScott on 10-30-2006 04:54 PM

[Salvador_Santolucito]

Hi,

I set this up at my desk just to see if I can reproduce it.  I'll let you know tomorrow morning what happened.  I have a USB-6008 doing a continuous acquisition at 1 Hz, reading 10 pts per DAQmxRead.  I have it set up as differential.  It is reading a 0 - 5 v square wave that is generated by my 6229 M Series.

Both devices are running LabVIEW example programs.

Please let me know if this is what you are doing or if I should modify my setup to more closely replicate your situation.  The sooner I hear back from you the better so that I can have it run overnight.

Thanks,

Sal

[DynaScott]

I think this sounds fairly close to what I was doing.


I ran the test again using both a USB-6008 and in house logger in parrallel, still getting the weird readings with the USB-6008, but the in house logger is working fine.

The only difference is that in front of the in house logger I am running a 2 to 1 voltage divider because our loggers can not handle more than a 3V input.

The 5V wave actually runs at 1/8 Hz because it is an address line to a multiplexer, and the address changes every 4 seconds.  The logger may read different voltages on the first and last of the 4 points it should aquire during each half of the cycle.   The weird points I'm getting are in the middle of the cycle. For example on the rising edge it may read 2.5, 5, 5, 1.9 in a typical sequence, the weird ones are more like 5, 5, 1.4, 5. Or  0, 1.8, 0 0.

I am not sure if I can more fully explain what was happening.

[Salvador_Santolucito]

I tested it overnight and I did not see the behavior that you are getting.  Can you play with the VI's settings and see if the behavior changes?  For example... try changing the sampling rate.  Sample on multiple channels.  Sample a constant voltage.

Let me know how those tests go.

Thanks,
Sal