Unwanted time delay using Labview, NI 9205 and cDAQ-9174

[dkb@wpi]

I am using Labview with a NI 9205 and cDAQ-9174 to calibrate (and eventually take data with) a laser.  The calibration involves four intensity levels: 0%, 100%, and two more in between.  First is 100%, which appears to be working.  Next is 0%.  At the beginning of this step, I am getting the same voltage as at 100%.  Eventually, the voltage will drop down to near zero, where I expect it; maybe 40 - 60 seconds after I start the step.  The next step is one of the intermediate levels and the pattern repeats: starts with the previous step's voltage then after 40 - 60 seconds (sometimes more), the voltage will adjust to what I expect.  This "previous step voltage, adjust to expected" is then repeated for the fourth step.

When I check the channels using Measurement & Automation, I am getting the correct vlotages.  Therefore, I think the problem is with Labview.  The problem is that I've used the same set up in the past to calibrate the laser and haven't had any problems until last week.

Any thoughts?

dkb@wpi

[Alexander_Sobolev]

It can be not labview, but the code. It looks like a very long running average is on.

To be more specific, we need the code.

If it involves a lot of processing, write a simple test VI, for example with daqmx assistant.

If it still gives the same slow voltage change, check input voltage with multimeter.

Also check you are using the same settings, for example single ended/differential. If you are using different connection modes in MAX and Labview, photodiode can see different impedance and change response time from us (50 Ohm) to minute (GOhm from signal to ground)

[dkb@wpi]

OK, thanks, I'll check it out!