signal distortion in Voltage Measurement

[venkatgill]

Hello,

I am using PCI-MIO-16E4 for voltage measurement from a Single phase voltage supply panel. I use CB-68LP as connector cable. When I see the output signal using Scope it is giving exact value and pure sinusoidal, but when it comes to LabView 7.1, the magnitude is correct and the sinusoidal is distorted. The siganal is not continuous and looks like triggered pulse in the middle for every cycle. So, the signal breaks goes up like a sqaure pulse and comes down, sinusoidal continues. I donno if there is any problem with the triggering options which i didnot selected. I am very new to labView. The example file I am using (obtained from example finder/DAQMx/Voltage) is attached below.

Any help is appreciated.

Thanks

[venkatgill]

PLease can someone suggest me a solution for this. I am attaching the sample of a distorted signal here and I need a solution to get rid of this.

Thanks

[sergesl]

Hello V.

What you see in LabVIEW is exactely what is being read. So your signal might be getting distorted at the connector block. The first thing to do is make sure that you wired the signal correctly. The example you are using is measuring in differential, so you need to connect one lead of you signal to channel aix and the other to channel aix+8.
After making sure that the connection is correct, check the signal in MAX: launch Measurement and Automation Explorer, under devices and interfaces, select your device under DAQmx and go to the test panels. Check the signal you wired your signal to on the test panels to make sure that you receive your signal properly. If the signal is still distorted, check with a scope the signal right where it is being input to the DAQ to make sure that distorsion does not occur when you connect your signal to the DAQ.
Let me know what you find out...

[venkatgill]

Hello Serges,

Thanks for your response and suggestions. Let me explain my problem more clearly. I am using channel 6 now for measuring Voltage so I have connected wires to pins 25,58 on CB-68LP from the transducer output. These numberI obtained from LV itself from connection diagram and also from other thesis works and weblinks.

I am getting the right signal intially for sometime when i see both in the scope and in the LV using our DAQ with the above specified connections. It is same for all other channels. I am measuring in the differential mode only (Can you suggest me if this mode is wrong and where to change the setting in 'vi' attached) and verified it in the test panel too. After a couple of minutes the signal is getting distorted in LV as well as in MAX also as attached above.  But in the scope it show the same sinusoidal without any kind of distortion. I feel the problem could be with the Connector block or connecting wire or my LabView file. The DAQ card is really new which we got 1 week back from NI. I will attach the right signal preview with this post.

[venkatgill]

This is the distorted one after a couple of mins.

[venkatgill]

This is the 'vi' I am using

[sergesl]

Hello V.,

Can you try to generate a signal with the daq and route it back in to be acquired using an input channel? This test will allow you to know if the connector block is the problem. If you have the same problem, then I would suggest generating a signal and internally routing it in. You can access internal channels by right-clicking on the physical channel control and selecting IO name filtering and checking the box that indicates internal channels.
this will allow you to generate and read channels without the external connections, isolating the trouble further.

[venkatgill]

HEllo Serges,

I am trying to do the process you said, but as I said I am new to LV. I am using two DAQ Assistant blocks  one for  Analog Output generatin ('ao0' channel) and ther for analog input measurement ('ai0' channel). But The input channel only ask data from connector block. I dont know where are the other options of getting data from output channel.
Can you explain me more clear. I appreciate your suggestions.

Thanks

[Laura F.]

Hello venkatgill,

To do what Serges suggested, use a wire to externally connect ao0 to ai0.  This should be pin 22 to 68. 

Can you find a second connector block to try and narrow down if the problem is stemming from it?

Another thing to try is to input a DC voltage for awhile and see if you get these same spikes in your signal. 

Thanks,

Laura  

[venkatgill]

Hello Laura,

Thanks for your reply. I have another connector block SCB-68 (brand new), but I dont know how to use it. How to place the switch positions and connections.

When I tried by connecting to pins 25 & 68 (for using CH6) it didnot give any signal. Can you help me how to configure this using SCB-68 Borad in differential mode.

Thanks