08-04-2010 08:36 AM - edited 08-04-2010 08:38 AM
I am trying to calibrate 14 voltages that I am reading from a voltage divider into 14 channels of a USB-6210 device. Enclosed is an Excel sheet of data that I am trying to understand. On the bottom of the sheet I have applied the same voltage to all channels and I plot the raw voltage for all channels and the differences between the expected voltage and the raw voltage. I find that those differences are not constant but linear which I don't understand. I then use the baseline linear function to find the factors in the top portion of the spreadsheet. Hence, in the top portion I read the input voltage using a high voltage probe and the Labview voltage that has been adjusted by the baseline linear function in the bottom. Most of the multiplying factors are ~1000 as expected but the voltages for the last 4 channels are significantly different and are not linear for the last two channels. I fitted the two worst voltage channels with polynomial that fits ok but doesn't make any sense. Does this make sense to anyone. I am using Labview 9.0.1 and Windows 7. I have run DAQ Diagnostic utility on the device and it passed all tests. I have looked into the issue of ghosting but the criteria seem invalid here because the voltage steps in neighboring channels is small. I also enclose the VI that takes in raw voltages to show how these functions are used. I feel the results are steady and accurate but I can not explain the discrepancies mentioned above.
Solved! Go to Solution.
08-05-2010 12:32 PM
I fixed the order that the channels were gathered and that seems to have fixed part of the problem so disregard the excel sheet data. I will put an updated spreadsheet If I can not solve the problem.
08-06-2010 07:37 AM
The problem was resolved by changing the sampling frequency in the DAQ assistant. I think the default frequency is 1kHz and I had dropped it to 750 Hz. By lowering the frequency to 150 Hz the capacitor is allowed time to discharge so that no ghosting occurs. In summary the issue was always ghosting and to solve the issue I:
-Removed in the DAQ assistant the channels that were disconnected and therefore unused.
-Change the wiring to the DAQ assistant by sending the lowest voltage to channel 1 and the second lowest voltage to channel 2 etc.
-Changed the sampling frequency from 1kHz to 150 Hz to allow the capacitors to time to discharge.