03-12-2012 04:58 PM
I'm trying to read two waveforms, one from the analog input and one from the scope. These waveforms are coming from two sides of a voltage divider and should always be in phase. But I think that there might be a triggering problem. When I change the frequency of the input waveform, my measured waveforms go out of phase.I have programmed each express VI to trigger off of the SYNC.
I can't use analog inputs for both measurements because the sampling rate is too small. I can't use the scope for both inputs because one of the waveforms is a differential measurement.
Any help would be appreciated, thanks.
03-13-2012 05:47 PM
Hi, I looked at the ELVIS Hardware User Manual and noticed that you have 12 channels that can be used as differential, Reference single ended or non reference single ended. Unless you have them all busy I don't see why you can't use the analog inputs. About the sampling rate, it will be determine by the M Series DAQ device that you are using and the source of the signal. It is a good practice to sample, at least, 10 times faster than the frequency of the input signal. The documentation of the card should show what the maximum sampling rate is.
Now, Open the example called "Multi-Device Synch-Analog Input-Cont Acquisition.vi" at the NI Example Finder under Hardware Input and Output»DAQmx»Synchronization»Multi-Device. If you get the same results shown in the picture then you need to analyze if the voltage divider is affecting the phase.
For more information about synchronization check the article called "Synchronization Explained".
03-20-2012 06:41 PM
Sorry about the late reply. Thanks very much for your help. I don't know why, but those vi's don't work with the ELVIS. What I found though was that if I use the general DAQ express VI instead of the oscilloscope VI, the signals sync up nicely.
I can't use the analog inputs alone because the sampling rate is too low when I'm using more than one channel. I need to measure a frequency at 50kHz. But using the rule of thumb you mention I can only go as high as 20kHz.